METALS DATA AND CHARACTERIZATION

The performance of metals during use and their behavior during processing can be understood and predicted only with the availability of a detailed body of information on their physical properties and microstructure. The value of this information is greatly enhanced if it is developed within the context of models or theories which describe how the measured properties of a metal will vary with changes in composition, microstructure, temperature, geometry, or other parameters. The Metals Data and Characterization Program includes activities which refine the technology for measuring the properties and behavior of metallic materials, and which correlate these properties and behavior to alloy microstructures.

The large majority of metals are used in applications based on their mechanical properties, with other applications based on electronic, magnetic, optical, or other functional properties forming smaller but nonetheless critical markets. Whatever the application, satisfactory long term performance of metallic components demands chemical and microstructural stability, sometimes in the presence of harsh environments. This program identifies those processing, microstructure, and properties characterizations which are critical to U.S. industry for both the processing and the performance of metals, and carries them out within the context of the NIST mission of providing data and standards. A significant part of the program is the use of advanced microscopy techniques to characterize the microstructures which form the basis of the measured properties.

The measurements of microstructural, mechanical, chemical, and optical properties carried out under this program have an impact in a number of different technology sectors:

• Standard test methods are being developed to support the automotive industry in its effort to improve fuel efficiency by shifting to lighter materials, a shift which has highlighted the critical need for improved understanding and control of sheet metal formability. The Metallurgy Division's effort is being carried out in collaboration with the Manufacturing Engineering Laboratory's National Advanced Manufacturing Testbed (NAMT) program and with ATP- supported consortia of U.S. automakers and several universities.
  
  

• The accuracy of a high speed laser polarimeter technique for measuring the normal spectral emissivity of metals and alloys at high temperature has been demonstrated by measurements on a standard reference material (molybdenum). The millisecond resolution of the existing system is currently being upgraded to microsecond resolution, which should enable measurements extending to temperatures well above the melting point of refractory metals. These techniques provide industry with benchmark high temperature thermophysical properties measurements.
  
  

• Precision measurements of Rockwell Hardness, the primary parameter used to specify the mechanical properties of metals and alloys, are leading to the establishment of traceable national hardness standards. Calibrated test blocks, together with national standards for measurement and calibration procedures, will facilitate the acceptance of a wide range of U.S. products in international markets, as well as minimize product-acceptance disputes in domestic trade.
  
  

• Computational micromagnetic techniques are producing results which are important for understanding magnetization reversal in devices incorporating thin magnetic elements. Micromagnetic head-to-head domain wall structures and energies in thin magnetic strips have been calculated, resulting in a "phase diagram" for transverse and vortex type walls.

Project Title: THERMOPHYSICAL PROPERTIES

Investigators: A. Cezairliyan, J. L. McClure, D. Basak, K. Boboridis, and D. Josell

Objectives:

The objective of this project is to develop and use millisecond- and microsecond-resolution techniques for the accurate measurements of selected thermophysical properties of high- temperature materials in their solid and liquid phases in the range 1300 to 4000 K.

Technical Description:

This project focuses on the development and use of new techniques for the accurate measurement, at high temperatures, of selected thermophysical properties of materials, in both solid and liquid phases, utilizing rapid (millisecond- and microsecond-resolution) pulse- heating (volume and surface) techniques. The properties of interest are: enthalpy, specific heat capacity, thermal expansion, electrical resistivity, hemispherical total emissivity, normal spectral emissivity, melting temperature, heat of fusion, and thermal diffusivity.

Planned Outcome:

State-of-the-art of thermophysical measurements at high temperatures will be advanced. Accurate bench-mark thermophysical data on selected key materials will be generated. High- temperature thermophysical standards will be developed.

External Collaborations:

The laser polarimeter used for the measurement of normal spectral emissivity of the specimen during pulse heating was developed in collaboration with Containerless Research, Inc. The new accurate subsecond technique for the measurement of hemispherical total emissivity was developed in collaboration with a scientist from the National Research Laboratory of Metrology (Japan).

Accomplishments:

Operation of the novel millisecond-resolution laser polarimeter for normal spectral emissivity measurements (which permits determination of true temperature from measured surface radiance temperature) on solids was further validated by performing measurements of specific heat capacity of molybdenum standard reference material in the temperature range 2000 to 2800 K. The present results are in agreement, within 1%, with the certificate values.

Definitive experiments were conducted for the first time to demonstrate applicability of the millisecond-resolution laser polarimetry technique to the non-contact detection of phase transformations in metals and alloys at high temperatures. The measurements included detection of structural phase transformations (cobalt, hafnium, iron, titanium, and zirconium) and melting (molybdenum, nickel, niobium, zirconium, and the alloy 53Nb-47Ti ).

Applicability of the laser polarimetry technique to measurements of normal spectral emissivity of liquid metals was studied. A new system for operation at microsecond speeds was designed and partially constructed in collaboration with the Containerless Research Incorporated.

The new accurate subsecond technique for the measurement of hemispherical total emissivity, developed during the previous year, was used to measure emissivity of niobium, molybdenum, and tungsten at temperatures above 2000 K.

Radiance temperatures (in the wavelength range 530 to 1500 nm) of nickel at its melting point were measured. This work is needed for the establishment of high temperature reference points.

The laser pulse system was used to measure thermal diffusivity of homogeneous as well as multilayered materials. Extensive measurements were performed on specimens consisting of molybdenum and alumina layers and several industrially important multilayered materials. Additional measurements were conducted on pure molybdenum specimens to further assess the operation of the system. Modifications to the system and refinements to the computer programs were made. An accurate knowledge of thermal diffusivity will play an important role in the selection, use, and assessment of thin films and specifically thermal barrier coatings in high temperature applications, such as in jet engine blades.

Impacts:

The high-speed (millisecond resolution) laser polarimeter, developed jointly by NIST and Containerless Research Incorporated (CRI), was commercialized by CRI and was successfully marketed internationally. This novel and unique instrument, which is capable of measuring accurately the normal spectral emissivity of a specimen surface without the requirement of a blackbody configuration, significantly simplifies accurate measurements of high temperatures.

NIST developed and successfully used a new technique for the measurement of

hemispherical total emissivity of metals and alloys in subsecond-duration experiments. This technique will provide, with unprecedented accuracy, hemispherical total emissivity of electrically-conductive materials at temperatures above 1500 K.

Outputs:

Publications:

McClure, J.L. and Cezairliyan, A., "Radiance Temperatures (in the Wavelength Range 525 to 906 nm) of Vanadium at Its Melting Point by a Pulse-Heating Technique," Int. J. Thermophys., 18, 291 (1997).

Josell, D., Cezairliyan, A., van Heerden, D., and Murray, B.T., "Thermal Diffusion Through Multilayer Coatings: Theory and Experiment," Nanostructured Materials, 9, 727 (1997).

Josell, D., Cezairliyan, A., van Heerden, D., and Murray, B.T., "An Integral Solution for Thermal Diffusion in Periodic Multilayer Materials; Application to Iron/Copper Multilayers," Int. J. Thermophys., 18, 865 (1997).

Kaschnitz, E., McClure, J.L., and Cezairliyan, A., "Radiance Temperatures (in the Wavelength Range 530 to 1500 nm) of Iron and Cobalt at Their Melting Points by a Pulse- Heating Technique," High Temp.-High Press., in press.

Cezairliyan, A., Krishnan, S., Basak, D., and McClure, J. L., "Application of Laser Polarimetry to the Measurement of Specific Heat Capacity of Molybdenum in the Range 2000 to 2800 K by a Pulse-Heating Technique," Int. J. Thermophys., in press.

Matsumoto, T. and Cezairliyan, A., "A Combined Transient and Brief Steady-State Technique for Measuring Hemispherical Total Emissivity of Electrical Conductors at High Temperatures: Application to Tantalum," Int. J. Thermophys., in press.

Matsumoto, T., Cezairliyan, A., and Basak, D., "Hemispherical Total Emissivity of Niobium, Molybdenum, and Tungsten at High Temperatures Using a Combined Transient and Brief Steady- State Technique," Int. J. Thermophys., in press.

Kaschnitz, E., McClure, J. L., and Cezairliyan, A., "Radiance Temperatures (in the Wavelength Range 530 to 1500 nm) of Nickel at Its Melting Point by a Pulse-Heating Technique," Int. J. Thermophys., in press.

Josell, D. and Cezairliyan, A., "Thermal Diffusion Through Multilayer Coatings: Theory and Experiment," Int. J. Thermophysics., in press.

Presentations:

Cezairliyan, A., "Measurements of Thermophysical Properties of Metals and Alloys at High Temperatures by Subsecond Pulse-Heating Techniques," Thermophysical Properties - Metallurgical Industry Needs and Resources Symposium, Johnstown, Pennsylvania, October, 1996. (Invited).

McClure, J. L., "Application of Laser Polarimetry to the Measurement of Specific Heat Capacity of Molybdenum in the Range 2000 to 2800 K by a Pulse-Heating Technique," 13th Symposium on Thermophysical Properties, Boulder, Colorado, June, 1997.

Cezairliyan, A., "Effect of Heating Rate on the Melting Behavior of the Alloy 53Nb-47Ti (Mass %) in Rapid Pulse Heating Experiments," 13th Symposium on Thermophysical Properties, Boulder, Colorado, June, 1997.

Basak, D., "Measurements of Selected Thermophysical Properties of the Alloy 53Nb-47Ti (Mass %) in the Range 1500 to 2100 K by a Millisecond-Resolution Pulse-Heating Technique," 13th Symposium on Thermophysical Properties, Boulder, Colorado, June, 1997.

Josell, D., "Thermal Diffusion Through Multilayer Coatings: Theory and Experiment," 13th Symposium on Thermophysical Properties, Boulder, Colorado, June, 1997.

Project Title: MICROSTRUCTURAL STUDIES OF COMPLEX PHASES

Investigator: L. A. Bendersky

Objectives:

The goal of this project is the determination of very complex crystallographic structures and defects in new stable and metastable compounds, especially in systems of importance for microwave wireless applications. Mesoscopic microstructures are being studied to correlate the atomic structure with properties and appear to provide potential for novel magnetic and electrical properties.

Technical Description:

Improved new functional ceramics with better electric and magnetic properties are being sought in different ternary oxide systems. Such materials can be potentially used in a wide variety of electronic devices for microwave wireless communication. Wireless communication technologies are expected to comprise one of the most important growth businesses in the world electronic industry, with the projected market growth of the order of 50% per year.

This work is a close collaboration with Ceramics Division (T. A. Vanderah, R. S. Roth and I. Levin) studying phase equilibria and synthesis of complex oxides, particularly in the BaO:Fe₂O₃:TiO₂ and SrO-Nb₂O₅-TiO₂ systems. Most of the oxides apparently have a new structure type and require complete structural determination. Structural studies by x-ray diffraction often are not successful; therefore, high-resolution transmission electron microscopy (HRTEM) and computer simulation modeling are employed to study these compounds. The work will establish the correlations between structural and physical properties of the studied compounds, and will develop an understanding of the physics behind these correlations. This research is a natural extension of our previous work and our expertise in determining complex metal structures, including quasicrystlline and nanocrystalline alloys.

Planned Outcome:

Identification of a group of BaO:Fe₂O₃:TiO₂ compounds as a new class of materials with a "self-assembled" magnetic multilayer structure having crystallographically flat interfaces. The material may have unusual properties related to the magnetic interactions between layers, similar to the properties of artificial magnetic multilayers, and may therefore be of interest for data storage research.

Understanding of the relationship between structure and dielectric properties of a series of SrO-Nb₂O₅-TiO₂ compounds. The results of this study can be used by laboratories and industries working on devices for microwave applications.

External Collaborations:

Cooperative research program with NKK, Japan (Dr. S. Mitao) to study microstructural stability of gamma titanium-aluminides.

Collaboration with Dr. S. Banerjee, Bhabha Atomic Research Center, Bombay, on microstructural evolution and ordering in Zr-Al-Nb and Ni-Al alloys.

Collaboration with Dr. T. Sigrist, AT&T Bell Labs on structural determination of compounds in the BaO:Fe₂O₃:TiO₂ system.

Collaboration with O.M. Stafsudd, UCLA, on the relationship between structure and dielectric properties of a series of SrO-Nb₂O₅-TiO₂ compounds.

Accomplishments:

The structural characterization of six newly discovered compounds from the BaO-Fe₂O₃-TiO₂ system has been completed. The following results were achieved:

1. All six structures, the E, M, K, N, L and J phases, were shown to belong to the previously unknown class of ordered intergrowth structures built out of two types of alternating slabs, P and H-type. The basic framework of the structures consists of a sequence of close-packed Ba/O layers. The P slab has a perovskite-like structure where Ti cations are accommodated predominantly in the octahedral positions. The H-type slabs have a periodicity triple in its basal plane with respect to the P slab. The structure of the H-type slab is shown to be closely related to the 12:14:15 phase and magnetite-type. The H-type slabs are expected to be rich in Fe (which is accommodated in both octahedral and tetrahedral coordinations) and dilute in Ba.

2. The presence of a one-dimensional structural disorder was observed and explained for the new phases. This unusual disorder phenomena was shown to be related to the lower symmetry of the H-type slabs and reflects the poor spatial correlation between these slabs.

3. A strongly heterogeneous distribution of Fe suggests that all six phases can be considered as natural "self-assembled" magnetic multilayer structures with crystallographically flat interfaces and potentially interesting properties.

Two representative phases of the BaO-Fe₂O₃-TiO₂ compounds, L and M, have been studied (with J. Bonevich) by energy-filtered TEM/EELS imaging to analyze compositional distribution of Fe, Ti and Ba. The imaging shows that H-type slabs have an enhanced Fe concentration and the perovskite slabs are Ba-rich. The distribution of Ti is relatively uniform. This chemical inhomogeneity is consistent with the proposed structural models of the phases.

Experimental work on unknown compounds from the SrO-Nb₂O₅-TiO₂ system has been conducted (with I. Levin and T. Vanderah). A series of structurally related phases A_nB_nO_3n+2 with A=Sr²⁺ and B=(Ti⁴⁺, Nb⁵⁺) have been prepared. Members of the homologous series with n = 4, 5, 6, 7, and non-integer values between 4 and 5 were characterized by HREM/TEM, bulk X-ray powder diffraction, and capacitive measurements of relative permittivities and temperature coefficients. Preliminary capacitive measurements from 100 Hz to 5 MHZ of relative permittivities and temperature coefficients indicate some unusual differences for successive members of the series, despite their crystallographic and chemical similarities.

Experimental and modeling work on unknown compounds with Ca₂Ta₂O₇ stoichiometry has been conducted. Four different, previously unknown polymorphic modifications (3, 6, 7 and 12 layers) were discovered and characterized. Structural models of these compounds were proposed based on the different stacking sequences and shears of complex layers of pyrochlore structure. The modeling work is in progress.

Impact:

The BaO:Fe₂O₃:TiO₂ and SrO-Nb₂O₅-TiO₂ phase diagrams determined by NIST are of immediate interest to U. S. industry involved in the production of ceramics for wireless communications systems, e.g. for microwave circulators and isolators. An understanding of structures and defects of the studied compounds, as well as the structure/properties correlations, will lead to an intelligent approach to tailoring microstructure and properties of such materials.

Outputs:

Publications:

Levin, I., Bendersky, L. A., Brandon, D. G., and Ruhle, M., "Cubic to Monoclinic Phase Transformations in Alumina," Acta Materialia (1997), (in press).

Mitao, S. and Bendersky, L. A., "Morphology and Growth Kinetics of Discontinuous Coarsening in Fully Lamellar Ti - 44 Al (at%) Alloy," Acta Materialia (1997), (in press).

Bendersky, L. A. and Waterstrat, R. M., "Incommensurate Structure of the Phase Zr₃Rh₄," J. Alloys and Compounds, 252, L5, (1997).

Bendersky, L. A., Vanderah, T. A., and Roth, R. S., "Structural Features of "Ba₄Fe₄Ti₃O₁₆" and "Ba₂₆Fe₂₀Ti₂₁O₉₈" as Revealed by High Resolution Electron Microscopy," J. Solid State Chemistry, 125, 281, (1996).

Bendersky, L. A., Vanderah, T. A., and Roth, R. S., "High-resolution electron microscopy of magnetic dielectric oxides in the BaO:Fe₂O₃:TiO₂ system," published in Solid-State Chemistry of Inorganic Materials, Eds. Davis, P. K., Jacobson, A. J., Torardi, C. C., and Vanderah, T. A., MRS, 453, (1997), p. 489.

Mitao, S. and Bendersky, L. A., "Morphology of Discontinuous Coarsening in Fully Lamellar Ti - 44 Al (at%) Alloy," Proceeding of the Second Int. Symposium on Structural Intermetallics, September (1997), (in press).

Presentations:

Bendersky, L. A., Vanderah, T. A., and Roth, R. S., "High-resolution electron microscopy of magnetic dielectric oxides in the BaO:Fe₂O₃:TiO₂ system," MRS 1996, Symposium: Solid-State Chemistry of Inorganic Materials.

Mitao, S. and Bendersky, L. A., "Morphology of Discontinuous Coarsening in Fully Lamellar Ti - 44 (at%) Al Alloy," International Symposium on Structural Intermetallics, Seven Springs, PA., 1997.

Project Title: MECHANICAL AND THERMAL PROPERTIES OF MULTILAYERED MATERIALS

Investigators: D. Josell and T. Foecke

Objectives:

This project will determine the thermal resistance associated with interfaces in multilayer materials. This quantity is tied directly to the effectiveness of these materials as thermal barrier coatings for engine applications being considered by our industrial power generating partners.

This project will also ascertain the relationship between the yield stress and layer thickness of model multilayer materials. Efforts will also be made to determine the operating deformation and fracture mechanisms. In addition, creep properties of model multilayer systems will provide thermodynamic free energies associated with interfaces. The mechanical and thermodynamic properties are required to predict the lifetime and stability of multilayer thin film materials of interest to the United States Air Force.

Technical Description:

Thermal barrier coatings protect engine parts from the elevated temperatures of the combustion process. It has been proposed that the presence of the numerous interfaces in multilayer thermal barrier coatings will decrease their thermal conductivity, making multilayer coatings more effective thermal barriers than the materials from which they are manufactured. Measurements of thermal transport properties of multilayer thermal barrier coatings are therefore being made at elevated temperatures, to simulate operating conditions, using a pulsed laser (<100 ns) heating technique.

Mechanical properties are being determined through analysis of stress-strain curves obtained during room temperature tensile tests and elevated temperature creep tests of multilayer films. A novel sample geometry that allows straining of cross-sectional samples in- situ in the transmission electron microscope is also being used to study deformation and fracture at magnifications of 500,000, permitting direct observation of dislocation motion and crack advance during straining.

Planned Outcome:

The industrial consortium providing the thermal barrier coatings for measurement at NIST will decide whether or not it will further pursue study of these materials, and eventually use them in thermal barrier coatings, based, in part, on the materials properties determined at NIST.

Mechanical and creep properties will be furnished to the Air Force through joint projects with scientists at The Johns Hopkins University and Ohio State University.

External Collaborations:

Daniel Josell's collaboration with the industrial consortium composed of Battelle, Howmet, EPRI, and Solar Turbine continues. The industrial group continues to supply thermal barrier coatings and NIST continues to determine the thermal transport properties of those coatings at potential operating temperatures using the Metallurgy Division's pulsed laser heating system.

A formal collaboration between Daniel Josell and Professor D. Shechtman of the Technion, now up for renewal by the US-Israel Binational Science Foundation for a third year, with Dr. D. van Heerden of Johns Hopkins University, continues to study structural transformations in multilayer materials.

A new collaboration has been formed between Daniel Josell and Professor T. Weihs of the Johns Hopkins University through a joint project funded for four years as of September, 1997 by the Air Force. This work will focus on the effects of interfaces on creep of multilayer materials at high temperatures.

A joint effort between Timothy Foecke and Professor Weihs, funded by AFOSR, to study the mechanical properties and thermal stability of Nb/Nb₅Si₃ microlayered materials, continues.

An ongoing collaboration between Timothy Foecke and Professor P. Anderson of Ohio State University studying dislocations in single crystal metallic nanolaminates has been expanded through the funding of a student by the AFOSR through an AASERT grant. Daniel Josell will participate in this new study of the creep and stability of multilayer coatings in the design and analysis of experiments based upon theory and experiments that he previously published.

Timothy Foecke has initiated a collaboration with Professor S. Barnett of Northwestern University to image the defect structures produced by a microhardness indent in superhard NbN/W single crystal superlattices.

Accomplishments:

The electron beam deposition system for fabrication of multilayer materials has been improved through the inclusion of a new deposition chamber and new process control. Low and high angle x-ray superlattice diffraction peaks from fabricated multilayer coatings indicate excellent system operation.

The effect of annealing on thermal transport through molybdenum/alumina multilayers was studied. The results permit the upper bound for the interface resistance at each interface in the coatings to be placed at a value in agreement with work on metal/metal interfaces, orders of magnitude below the value motivating industrial multilayer thermal barrier coatings research.

The dislocation generation and motion observed during in situ TEM straining experiments were analyzed to determine dislocation pileup and bowing stresses. It was found that stresses in excess of 2.6 GPa were maintained at the head of a pileup containing more than 30 dislocations in a 30 nm Cu /60 nm Ni single crystal multilayer. This value is more than one half the theoretical strength of either constituent material.

Impact:

Observations of dislocation pileups and the effect of interfacial dislocations on glissile dislocations has forced the revision of theoretical treatments of nanolaminate mechanical behavior by most modeling groups. Previously, it was doubted that dislocations could be energetically stable in a nanoscale microstructure, and it was believed that a pileup was impossible.

Thermal transport properties of coatings provided by an industrial consortium (EPRI, Howmet, Battelle, and Solar Turbine) were determined at elevated temperatures using NIST's pulsed laser heating system. The consortium will decide by the end of 1997, based, in part, on these measurements, whether to continue their program on multilayer thermal barrier coatings for engine applications.

Outputs:

Publications:

Foecke, T., "Novel Sample Geometry for In Situ TEM Deformation Experiments," Scripta Materialia (in press).

Foecke, T., "Observation of Generation and Motion of 'Orowan' Bows in a Single Crystal Metallic Nanolaminate," Journal of Materials Research (in press).

Foecke, T., and van Heerden, D., "Deformation Mechanisms in Metallic Nanolaminates," in Proceedings of the Symposium on Chemistry and Physics of Nanostructures and Related Non- Equilibrium Materials, Eds. E. Ma, et al., 193, TMS, Pittsburgh, PA. (1997).

Josell, D., Cezairliyan, A., van Heerden, D., and Murray, B. T., "An Integral Solution for Thermal Diffusion in Periodic Multilayer Materials: Application to Iron/Copper Films," International Journal of Thermophysics 18, 865 (1997).

Josell, D., Cezairliyan, A., van Heerden, D., and Murray, B.T., "Thermal Diffusion Through Multilayer Coatings: Theory and Experiment," Nanostructured Materials 9, 727 (1997).

Josell, D., Cezairliyan, A., and Bonevich, J. E., "Thermal Diffusion Through Multilayer Coatings: Theory and Experiment," International Journal of Thermophysics (in press).

Wang, Z. L., van Heerden, D., Josell, D., and Shapiro, A. J., "Energy Filtered High- Resolution Electron Microscopy for Quantitative Solid State Structure Determination," Journal of Research of the National Institute of Standards and Technology 102, 1 (1997).

Josell, D., and Carter, W. C., "Implications and Applications of Zero Creep Experiments for Multilayer Stability," in Creep and Stress Relaxation in Miniature Structures and Components, Ed. H. D. Merchant, 271 TMS, Warrendale, PA. (1996).

Tepper, T., Shechtman, D., van Heerden, D., and Josell, D., "Fcc Titanium in Titanium/Silver Multilayers," Materials Letters (in press).

Tepper, T., Shechtman, D., van Heerden, D., and Josell, D., "Allotropic Phase Formation in Ti/Zr Multilayers," Materials Letters (in press).

Presentations:

Josell, D., "Elastic, Plastic, and Creep Properties of Multilayered Materials," invited talk at Yale University, September, 1997.

Josell, D., "Interfacial Materials: Microstructural Stability and Thermal Transport Properties of Multilayers," invited talk at Johns Hopkins University, September, 1997.

Josell, D., "Layer Stability and Interfacial Free Energies from the Creep of Multilayers,"

invited talk at 4th Annual International Conference on Composites Engineering, Kona, Hawaii, July, 1997.

Josell, D., "Thermal Diffusion Through Multilayer Coatings: Theory and Experiment,"

13th Symposium on Thermophysical Properties, Boulder, Colorado, June, 1997.

Josell, D., "Implications and Applications of Zero Creep Experiments for Multilayer Stability," invited talk at TMS Fall Meeting, Symposium on Creep and Stress Relaxation in Microstructures, Cincinnati, October, 1996.

Josell, D., "Multilayer Materials: Theory and Experiment for Thermal Transport, Zero Creep and Structural Stability," invited talk at the Technion, Israel, October, 1996.

Josell, D., "Thermal Transport Through Multilayer Materials: Theory and Experiment," invited talk at Joint NSF-NIST Program Review Conference on Nanoparticles, NSF, Arlington, VA. May, 1997.

Foecke, T., "Mechanical Properties of Nanolayered Materials," invited talk at Johns Hopkins University, November, 1996.

Foecke, T., "Deformation Mechanisms Particular to Nanolaminates," invited talk at TMS, Symposium on Chemistry and Physics of Nanostructures and Related Non-Equilibrium Materials, Orlando, February, 1997.

Foecke, T., "Deformation Mechanisms Particular to Nanolaminated Materials," invited talk at Society for Experimental Mechanics Spring Meeting, Bellevue, WA, June, 1997.

Foecke, T., "Fracture Mechanisms in Nanomaterials," invited talk at Joint NSF-NIST Program Review Conference on Nanoparticles, NSF, Arlington, VA. May, 1997.

Foecke, T., "Deformation and Fracture in Metallic Nanolaminates," at Symposium on Fundamentals of Deformation and Fracture, TMS Fall Meeting, Cincinnati, OH, October, 1997.

Project Title: HARDNESS STANDARDS

Investigators: S. R. Low, D. J. Pitchure, W. S. Liggett (ITL), J.-F. Song and T. V. Vorburger (MEL), R. J. Gettings (SRMP/TS), C. D. Faison (NVLAP/TS), and T. R. Shives (under contract to NVLAP)

Objectives:

The primary goals of this project are to provide U.S. industry with the means to make Rockwell hardness measurements with traceability to national standards, and to facilitate acceptability of American hardness measurements worldwide.

Technical Description:

In todays metal products and materials industries, hardness testing is the most widely used mechanical test for quality control and acceptance testing. Even so, worldwide unification and standardization of any hardness scale is yet to be accomplished. Furthermore, prior to the start of this project, no Standard Hardness Reference Scale within the United States was traceable to national standards. Historically, manufacturers of hardness equipment have established their own hardness calibration blocks and internal standard scales, assigning hardness values to each block based only on past performance of similar blocks without traceability to fundamental units of measure. Within the U.S., the consequence of this situation has been that the defined hardness scales of these different calibration laboratories have shown significant variability between laboratories and even within the same laboratory over time. This has led to frequent disputes between materials suppliers and customers and, in some instances, has made U.S. exports unacceptable in other countries.

The level of foreign market business at risk for the U.S.manufacturers of hardness equipment alone is in the $10 - $20M range. However, a much greater concern is that many regulatory agencies in foreign markets are now mandating that, for a product to be acceptable for importation, a well documented chain of measurements must exist from the point of use to the exporters national measurement laboratory. For this reason, U.S. industries that require hardness testing as part of their acceptance criteria may soon experience artificial trade barriers to their products. The most significant impact will be for U.S. industries requiring hardness testing in their product specifications. These industries are essentially any metals manufacturing mill or heat treatment facility, or any manufacturer of products fabricated of a metallic materials, such as fasteners, automobiles, and aircraft. The value of goods affected could be in the billions of dollars.

Starting with the Rockwell hardness scales, the NIST Metallurgy Division in collaboration with the Manufacturing Engineering Laboratory (MEL), Information Technology Laboratory (ITL) and Technology Services (TS) has undertaken to develop or assist in developing the components needed to establish a traceability system for Rockwell hardness measurements in this country. These essential components are: (1) standardized Rockwell hardness scales; (2) certified Rockwell hardness transfer standards; (3) a national laboratory accreditation program; and (4) internationally accepted National test method standards. The standardization of the Rockwell hardness scales and the development of transfer standards will be accomplished through the use of a precision, dead-weight hardness machine which was installed at NIST in 1992. The standardizing machine is essentially free from random and systematic errors in force, force application rate, and displacement, and is based on fundamental units of measurement traceable to NIST. The dead-weight tester also uses geometrically correct indenters certified by the Surface and Microform Metrology Group of MEL.

Standardization of the national Rockwell C hardness scale (HRC), identified as being in greatest demand by U.S. companies, has been accomplished, and HRC transfer standards have been calibrated with the assistance of the statistical expertise of ITL. The test cycle used to standardize the HRC scale was chosen to ensure compatibility with U.S. industrial practice, and to provide an acceptable level of precision and repeatability in the hardness measurements. International HRC scale intercomparisons with countries in Europe and Asia have been made to ensure compatibility with the Rockwell C hardness scales of other countries. Certification of Rockwell hardness indenters as NIST Standard Reference Materials is also currently under development with the assistance of MEL.

A laboratory accreditation program for hardness calibration laboratories is being developed with the assistance of the NVLAP office, and with the cooperation and assistance of ASTM. The hardness calibration laboratories include hardness machine manufacturers, indenter manufacturers, test block standardization laboratories, and companies that perform field calibrations of hardness machines.

NIST is assisting ASTM in revising their current Rockwell hardness Test Method to include requirements for obtaining traceability to the U.S. national hardness scales. This is being accomplished through leadership roles in ASTM and ISO hardness committees.

These efforts are expected to be expanded to the other Rockwell scales and other hardness tests in the coming years. The goal is to create a traceability system for all indentation hardness measurements used in the United States.

Planned Outcome

The short term goals of this project are to standardize each of the Rockwell hardness scales, and provide a means to transfer the national hardness scales to industry through the production and sale of calibrated SRM hardness test blocks and certified indenters. An accreditation program for hardness calibration laboratories will be developed, to be managed by NVLAP. ASTM hardness test method standards will be revised to reflect the use of the NIST SRMs and NVLAP programs. It is anticipated that this program will not end, but will continue to evolve with the changing needs of U.S. industry and with advances in technology.

External Collaborations

The NIST Metallurgy Division collaborates extensively with the U.S. hardness industry and manufacturing industries that use hardness testing in the production of their products. The collaboration is both directly, such as in the procurement of the uncalibrated hardness blocks for SRM production, and also through ASTM, for example in their efforts to revise the test method standards. S. Low chairs three ASTM hardness task groups including Task Groups on Traceable Hardness Standards, and the Technical Advisory Group to ISO on Hardness. He is also the Head of the US Delegation to ISO for Hardness Testing.

Accomplishments:

Approximately 100 test blocks at each of three levels of the Rockwell C scale have been calibrated and delivered to the Standard Reference Materials Program.

An intercomparison using the NIST Rockwell C scale SRM test blocks was conducted between NIST and the National Research Laboratory of Metrology in Japan and found agreement within 0.1 HRC for all three levels. An intercomparison of the HRB scale was also conducted with Japan and found agreement within 0.15 HRB for four hardness levels.

A study was conducted to benchmark the expected shift in the U.S. HRC scale resulting from the release of NIST SRMs.

Impact:

NISTs involvement in the standardization of the U.S. hardness scales can be evidenced by the many industry requests for information concerning the SRM test blocks, the movement of ASTM towards revising the Rockwell hardness test method standard, industry requests to have a hardness calibration laboratory accreditation program developed, and the introduction and expanding use of uncertainty in hardness measurements.

The NIST program has also provided strong support for the ISO decision to adopt test cycles for the standard test method for the Rockwell C Scale which are consistent with industrial practice rather than the longer cycles typically used in hardness research.

Outputs:

Publications:

Song, J.-F., Low, S., Pitchure, D., Germak, A., DeSogus, S., Polzin, T., Yang, H.-Q., Ishida, H., "Establishing a Common Rockwell Hardness Scale Using Geometrically Calibrated Standard Diamond Indenters," Proceedings of XIV IMEKO World Congress, June (1997).

Song, J.-F., Low, S., Pitchure, D., Germak, A., DeSogus, S., Polzin, T., Yang, H.-Q., Ishida, H., "Establishing a Worldwide Rockwell Hardness Scale With Metrology Traceability," Metrologia 34, 4 (1997).

Presentations:

Low, S., "The HRC Test Cycle and Recommendations For Revising ASTM E18," ASTM Committee Week, New Orleans, Louisiana, November 1996.

Low, S., "NIST Hardness Standards," Materials Solutions Conference, ASM International Indianapolis, Indiana, September 1997.

Low, S., "Standardization of Hardness at NIST," Japanese Industrial and Student Visitors at NIST, Gaithersburg, Maryland, September 1997.

SRMs in production:

SRM#2810 Rockwell C Scale Hardness - Low Range

SRM#2811 Rockwell C Scale Hardness - Mid Range

SRM#2812 Rockwell C Scale Hardness - High Range

SRMs under development:

SRM#2809 Rockwell Diamond Indenter

SRM#2814 Rockwell B Scale Hardness - Low Range

SRM#2815 Rockwell B Scale Hardness - Mid Range

SRM#2816 Rockwell B Scale Hardness - High Range

Project Title: MAGNETIC PROPERTIES AND STANDARD REFERENCE MATERIALS

Investigators: L.J. Swartzendruber, R.D. Shull, A.J. Shapiro, D.E. Mathews, R.V. Drew, L.H. Bennett

Objectives:

In cooperation with universities and industry, we are helping to determine methods for characterizing magnetic materials of importance to science and industry and to provide methods and standard reference materials for accurate and traceable magnetic measurements.

Technical Description:

Using the facilities and expertise available at NIST, we determine the magnetic properties of materials important to the scientific and industrial community and develop methods for improved measurements of these properties. We also develop and produce standard reference materials to provide for accurate and traceable calibration of instruments used in the measurement of magnetic properties important to science and industry. In the past NIST has issued SRM772, a nickel sphere, as a magnetic moment standard, and Pt, Pd and aluminum wires and manganese fluoide as magnetic susceptibility standards. The supply of these SRMs has been exhausted for several years and needs to be replenished.

This project determines the parameters necessary to fully characterize magnetic materials; these parameters can then be used in models which correctly predict the behavior of the magnetic material under actual operating conditions.

Planned Outcome:

Companies producing vibrating sample magnetometers and other types of magnetometers rely on NIST to provide standard reference materials for the calibration of these instruments. There is an urgent need to restock the nickel ball standard, SRM 722 and for the provision of new standards more appropriate for newer instruments such as SQUID magnetometers and alternating gradient magnetometers. One planned outcome is a series of SRMs for use in calibrating magnetometers. The first will be a re-issue of SRM772, a nickel sphere with a diameter of 2.5mm and with its absolute magnetic moment certified to 0.3%. A smaller sphere fabricated from single crystal YIG and a thin nickel disk are also planned. In addition to SRMs, improved characterization methods for describing the time decay and accommodation properties of magneitc recording media will be provided to aid the magnetic recording industry in their development of improved materials.

External Collaborations:

External collaborators include Materials Innovation, Inc., Fluxtrol, Inc., Digital Measurement System, and the Institute for Magnetics Research at George Washington University.

Accomplishments:

The equipment for performing the absolute measurements required to certify magnetic moment standard reference materials has been assembled. Final testing and adjustment of this equipment is now underway. Materials suitable for the Ni ball SRM were obtained and sent to a vendor for fabrication into spheres of the required diameter. Improved methods for measurement of the stability of recording materials were devised. These methods were used to characterize accommodation and time decay in some high-density recording materials.

Impact:

A commercial instrument maker, Digital Measurements Systems, Inc., is developing software for the measurement of accommodation in recording materials which is based on cooperative work between NIST and George Washington University.

Output:

Publications:

L.H. Bennett, L.J. Swartzendruber, P. Rugkwamsook, E. Della Torre, and F. Vajda, Experimental Validation of the Preisach Accommodation Properties of Recording Media, J. Appl. Phys. 81, 5227 (1997).

L. J. Swartzendruber, L.H. Bennett, F. Vajda, and E. Della Torre, Relationship Between the Measurement of Accommodation and After-effect, Physica B223, 324 (1997).

Determination of Austenite/Ferrite Ratios in Stainless Steels Using the Mössbauer Effect, L. J. Swartzendruber, G. E. Hicho, F. Biancaniello, R.D. Shull and A.J. Shapiro, Proceedings of the International Conference on Applications of the Mössbauer Effect, September 1997.

Project Title: LIGHTWEIGHT MATERIALS FOR AUTOMOTIVE APPLICATIONS

Investigators: R. B. Clough, R. deWit, R. J. Fields, T. J. Foecke, D. E. Harne, G. E. Hicho, L. E. Levine, E. N. Pugh, F. Bendec and A. Stern (Guest Researchers, Nuclear Research Centre, Negev, Israel) and R. Thomson (Contractor)

Objectives:

The primary objective of this project is to facilitate the introduction of lightweight materials into automobiles in support of the U.S. auto industry's goal to develop automobiles with substantially higher energy efficiency and lower emissions. This will be accomplished by providing models for lightweight metal consolidation and forming, measurements and data for model validation, software that readily transfers the models, and standard test methods for obtaining the data required for implementing the models to the auto companies and their suppliers.

Technical Description:

Major research efforts within the U.S. auto industry are driven by the need to reduce the weight of future vehicles to meet USCAR and PNGV goals. This can most readily be accomplished by the substitution of lightweight materials for the heavy materials currently used. This project consists of two parts: (1) development of a low cost powder processing technology for aluminum alloy and particle reinforced aluminum (PRA) parts, and (2) advancement of formability technology for lightweight sheet metals. In the first part, aluminum alloy and aluminum composite powder metallurgy (PM) materials would be substituted for iron- based PM products. In the second, more formed aluminum or high strength steel sheet would be used in the body of cars, replacing conventional grades of steel sheet. Both of these approaches have been recognized by the auto industry, and the technical barriers to success have been identified.

In the case of PM aluminum and PRA, the cost of existing processing routes is too high, and efforts to produce acceptable parts using press-and-sinter and direct powder forging are underway. The NIST part of this effort is focussed on modeling each step in these consolidation processes from powder to fully dense part. Modeling provides the basis for knowing what to measure about a powder or a process to monitor consistency and to more rapidly design successful processes. Physical modeling of the process can be used with a cost model to make decisions that optimize cost and properties. The modeling is complex and is carried out with significant academic and industrial collaboration. NIST's primary role has been to coordinate the modeling efforts between academia and industry, validate the models, and provide industry with working models and a preliminary data base. In collaboration with MatSys Inc., the modeling is being made available to industry in a user-friendly, commercially supported software package.

The technical barrier to expanding the use of lightweight sheet metals is the limited industrial experience and expertise in forming operations for these materials. The forming of aluminum and high strength steel sheet is significantly different from the forming of conven- tional sheet steel. The expertise developed over many years by tool and die makers for steel does not always apply. To date, only relatively simple shapes, like hoods, have been successfully formed on a commercial basis. The availability of high speed computing and advanced finite element methods (FEM) brings the prediction of forming within reach and provides a way to avoid the trial-and-error approach to metal forming that, while fairly effective with conventional alloys, cannot be efficiently applied to new materials. The automobile industry is currently developing an advanced computer program based on FEM that will predict the forming behavior of materials. NIST is helping industry implement this approach in three ways: improved, physically based models for material behavior during forming, a model for the surface roughening (or smoothing) and consequent changes in die wall/sheet metal friction during forming, and standard test methods for developing data bases of materials deformation behavior under forming conditions. The models provide the equations used in the FEM code, while the test methods provide the precise data for each material that is inserted into and used by these codes.

Planned Outcome:

The NIST powder consolidation modeling effort will result in a validated set of equations that describes the densification of reinforced (or unreinforced) metal powder in terms of the processing conditions. This will result in a commercial software package that accurately models potential processes and that saves U.S. industry time and money which would otherwise have to be spent on trial-and-error investigations.

The NIST forming research will provide new methods for determining the internal defect structure of deformed metals. The information obtained with these methods will be used to establish physically based equations describing the deformation behavior of metals for computer calculations. In addition, a model for the roughening of metals during forming will be developed so that industry can predict the local die surface/sheet metal friction coefficient (a quantity needed for the computer calculations). Lastly, standard test methods will be developed to provide industry with consistent methods for obtaining the needed data base of metal deformation behavior under complex loadings.

External Collaborations:

In the case of the powder consolidation research, a consortium formed by USAMP meets quarterly and the efforts are coordinated at these meetings. The industrial consortium consists of the Big Three, Valimet, Stackpole, and Mascotech. In addition, staff from Ames Lab, ORNL, and University of Michigan are involved. This collaboration consists mainly in the exchange of material and data. NIST also collaborates with MatSys,Inc. and University of Cambridge's Micromechanics Centre (Profs. Fleck and Ashby) to carry out the modeling and the commercialization of the modeling. In addition, a totally new method of compaction, dynamic magnetic compaction, is under investigation as an ATP project. NIST collaborates with IAP, Inc., GM, and Zenith in this effort by providing modeling and measurements of densification by extremely high pressures on powders supplied by the industrial participants.

Formability research has largely been carried out in conjunction with the ATP and NAMT project participants: Chrysler, Ford, General Motors, Budd, Alcoa, US Steel, Livermore Software Technology Corporation and the Autobody Consortium, consisting of 20 OEM's and suppliers to the industry, as well as the University of Ohio, the University of Michigan, and Northwestern University. Collaboration has mainly involved Prof. Ghosh at U. of MI, Prof. Wilson at Northwestern, Alcoa and General Motors. Material and advice on commercial forming processes were supplied by the industrial collaborators.

Accomplishments:

A shape insensitive, in situ density sensor was developed and successfully operated to provide benchmark data for the powder consolidation modeling effort. The reinforcement hardening effect predicted by the Cambridge model was validated for mixtures of 2024 aluminum alloy and SiC powders.

A portion of the IMM workshop on Mechanics and Materials Issues in the Automobile Industry was devoted to exploring the role that NIST should play in improving metal forming technology. While the NIST focus on fundamental and standards related issues was basically supported at this workshop, our research on surface roughening and friction received significant industrial endorsement.

To gauge the state-of-the-science, a focussed session entitled "Dislocations in Deformed Metals and Semiconductor Thin Films and Multilayers" at the March American Physical Society meeting was organized by NIST. In this meeting, it became clear that new approaches to understanding, modeling, and predicting the role of dislocation structures were emerging and that NIST was in the forefront of this development.

A general theory for interpreting Bragg diffraction from dislocations was completed and applied to the specific case of screw dislocations. A set of ultra-small-angle X-ray scattering data was obtained from several single-crystal aluminum samples deformed in situ. High resolution diffraction imaging experiments were also performed on similar, deformed aluminum samples. The results of these measurements were found to be crucial for the correct interpretation of the small-angle scattering data.

A first generation, plane strain tensile test fixture was designed and built. A contract between NIST and University of Michigan was signed to develop a channel forming test to evaluate the utility of the data obtained from the plane strain tensile test under investigation at NIST.

Impacts:

Models of reinforced powder consolidation are now available and are being incorporated in commercially available process modeling software. This software can help industry reduce the amount of trial and error testing required to develop a new process.

A new measurement method has been developed to quantify the dislocation content of deformed metals. When used to develop an improved prediction of metal deformation, coupled with our work on surface roughening and data from standard test methods, this research could save industry at least 50 to 100 million dollars per year.

Outputs:

Publications:

deWit, R., "Diffraction Elastic Constants of a Cubic Polycrystal," J. Appl. Cryst. 30, 510 (1997).

deWit, R., Fields, R. J., Low, S. R., Harne, D. E., and Foecke, T. J., "Fracture Testing of Large-Scale Thin-Sheet Aluminum Alloy," in Fatigue and Fracture Mechanics: 27th Vol., ASTM SP 1296, R. S. Piascik, J. C. Newman, and N. E. Dowling, eds., American Society for Testing and Materials, pp. 451-468 (1997).

Levine, L. E. and Thomson, R., "X-ray Scattering by Dislocations in Crystals: General Theory and Application to Screw Dislocations," Acta Cryst. A53, 590 (1997).

Levine, L. E., Fields, R. J., Black, D. R., Burdette, H., and Long, G. G., "In situ Measurements of Dislocation Structure Evolution in Plastically Deformed Single Crystal Aluminum," Bull. Amer. Physical Soc. 41, 40 (1996).

Levine, L. E., Long, G. G., Black, D. R., and Thomson, R. "In situ Measurements of Dislocation Structure Evolution in Al Single Crystals Deformed in Tension," National Synchrotron Light Source Activity Report (1996).

Levine, L. E., Thomson, R., Long, G. G., Black, and Fields, R. J., "NIST Research in Support of ATP's Springback Predictability and Agile Precision Sheet Metal Stamping Projects," in Proceedings of ATP's MVMT Public Meeting in Ann Arbor (through WERB).

Livne, Z., Munitz, A., Rawers, J. C., and Fields, R. J., "Consolidation of Nanoscale Iron Powders," NISTIR 5990.

Livne, Z., Fields, R. J., and Agulyansky, A., "Evaluation of Press-and-Sinter Parameters for Ta₂O₅ by the Diametral Compression Test," NISTIR 6024.

Munitz, A., Livne, Z., Rawers, J. C., and Fields, R. J., "Cold Compaction of Ball-milled Iron Powders," NISTIR 5991.

Presentations:

Foecke, T. J., "What Should Be the NIST Focus in Metal Forming Research?," Institute of Mechanics and Materials Workshop on Mechanics and Materials Issues in the Automotive Industry, Ann Arbor, Michigan, September, 1997.

Fields, R. J., "Sheet Metal Forming Activities at NIST," Institute of Mechanics and Materials Workshop on Mechanics and Materials Issues in the Automotive Industry, Ann

Arbor, Michigan, September, 1997.

deWit, R., "NIST Research Program on Lightweight Materials for Automotive Applications," North American Deep Drawing Research Group Spring Meeting, Golden, Colorado, May, 1997.

Levine, L. E., "Deformation of Metals: The Inside Story," Washington State University, November, 1996; Lawrence Livermore National Lab., November, 1996; Catholic University, January, 1997; Washington University, February, 1997; and Univ. of Texas at Austin, March, 1997.

Fields, R. J., "NIST's Progress in Modeling PRA Consolidation," USCAR quarterly review meetings in Detroit, Michigan, March, 1997; at Stackpole, Ltd. in Mississauga, Canada, May, 1997; at NCMS in Ann Arbor, Michigan, August, 1997.

Fields, R. J., "In Situ Density Measurements during Isostatic Consolidation of Powders," Metal Powders Industries' International Conference on Powder Metallurgy and Particulate Materials, Chicago, Illinois, July, 1997.

Clough, R. B., "Fundamentals of Metal Powder Consolidation and Deformation," Workshop for ATP Project on Dynamic Magnetic Consolidation, IAP Corp., Dayton, Ohio, March, 1997.

Project Title: PERFORMANCE OF STRUCTURAL MATERIALS

Investigators: R. B. Clough, R. J. Fields, T .Foecke, D. E.Harne, E. N. Pugh, R. E. Ricker, D. A. Shepherd, J. H. Smith, M. R. Stoudt, C. R. Hagwood (Statistical Engineering Division, ITL), and R. C. Cammarata (Guest Researcher, The Johns Hopkins University)

Objectives:

In this project, the expertise and facilities of the Materials Performance Group is used to provide assistance to US industry and other Federal agencies in the broad area related to the service performance of structural metals and alloys. Outputs include test methods and data.

Technical Description:

The cost to U.S. industry of failures of structural materials is extremely large. For example, a study by NIST and Battelle Columbus Laboratories estimated the cost of materials fracture in 1982 dollars to be $119B per year. Because metals are so heavily relied on for structural strength, their failures were found to contribute substantially larger costs than those of non-metals, and much of the cost was associated with the transportation and construction industries (motor vehicles, aircraft and the building of homes and non-residential construction).

In FY 97, work was conducted on six sub-projects:

Stress Rupture of Lead-Free Plumbing Solders

This on-going work is being carried out in collaboration with the Copper Development Association (CDA) and with the B16 Committee of ASME, and is designed to establish permissible pressures for copper tubes joined by lead-free solders. This is driven by the 1986 amendments to the Safe Drinking Water Act which prohibited the use of lead-containing solders in potable water systems. Despite the fact that thirteen lead-free solder alloys are contained in ASTM's Standard Specification for Solder Metal (B32), the current ASME Codes specify joint strengths for only one lead-free solder, Alloy Sb5 (95Sn-5Sb), and these are based on limited data reported by NIST in the early 1940s, so that unusually large safety factors are imposed.

In the NIST studies, stress rupture tests are being conducted on soldered sleeve joints in copper tube for times up to one year at temperatures in the range 100-250 F. In addition to Alloy Sb5, two other lead-free solders are being tested, namely Alloy E (4Cu-0.5Ag-bal.Sn) and Alloy HB (5Sb-0.3Ag-3.5Cu-1Ni-bal.Sn). The testing program is being supported by studies of the mechanism(s) of failure to assist in statistical analysis of the data and in life prediction modeling.

Structural Integrity of High Pressure Gas Cylinders

This work provides technical support for the U.S. Department of Transportation (DOT), which has the responsibility for developing and enforcing the regulations which cover the design, manufacture and testing of cylinders for the transportation of compressed gases. Our focus in this activity is the development of design standards and testing procedures for new cylinders, which currently are constructed of steel, aluminum alloys or composites.

Eta-phase Precipitation and Low Cycle Fatigue in Alloy 706

A major thrust of U.S. heavy manufacturing industry is the development of large land based gas turbine engines for power generation. In support of this, U.S. industry intends to produce large near-net-shape forgings of nickel-based alloy 706 using a novel incremental forging process. Limitations on the cooling rates possible in such large forgings are thought to reduce low cycle fatigue resistance by promoting intergranular precipitation of eta-phase which, in turn, causes the grain boundary regions to be denuded with respect to strengthening niobium- containing precipitates. Controlling the occurrence of this phase has been identified as a primary metallurgical challenge in the near-net-shape forging of Alloy 706.

In FY97, NIST collaborated with ATP recipient Wyman-Gordon Inc. on this problem. Using our electron microscope capabilities, grain boundary microstructures have been characterized in samples of Alloy 706 provided by Wyman-Gordon Inc, and the findings have been correlated with the results of fatigue tests conducted at NIST in air and vacuum at 750 and 900 F. These studies assisted Wyman-Gordon Inc in the identification of heat treatments which minimize the deleterious effects of eta-phase precipitation on fatigue resistance.

Mechanical Properties of Orthorhombic Titanium Aluminides

Orthorhombic Ti-Al-Nb alloys are candidates for use as advanced propulsion and airframe components in future DoD and NASA aerospace programs. Small additions of Mo (less than 2 at %) have been found to significantly improve the tensile and creep properties of these alloys, and this study was undertaken to determine whether the origin of this improvement is primarily microstructural (via phase stabilization) or substructural (via changes in dislocation structure and behavior).It is being conducted on two alloys, Ti-22Al-26Nb and Ti-22Al- 24.5Nb-1.5Mo, both prepared by powder metallurgy rather than by conventional ingot metallurgy, and both heat treated by several different schedules. Samples are being characterized using scanning and transmission electron microscopy and electron probe microanalysis and the resulting microstructural and compositional information is being correlated with data from hot tensile and creep testing.

Influence of Nanolayered Surface Films on Fatigue Initiation

Previous studies at NIST and elsewhere have established that the high density of interfaces in metallic nanolaminates result in significantly greater yield stresses and hardness than those of the component materials. The NIST studies, conducted on monocrystal electrodeposited nanolaminates of Cu-Ni, demonstrated that this effect is due to the interaction of dislocations with the interfaces. This project extends this work to study the effect of thin nanolaminate films on the mechanical properties of the substrate material, specifically on the initiation of fatigue cracks in polycrystal Cu-Ni and other FCC copper alloys. The objective is to evaluate the possibility that such films can significantly increase the service life of critical components.

Metallurgy of the R.M.S. Titanic

A forensic analysis of steel recovered from the wreck of the RMS Titanic is being performed to help answer persistent questions as to why this "unsinkable" ship sank in less than 3 hours after a relatively minor collision with an iceberg. Mechanical tests, including tensile and Charpy V-notch, were performed to establish mechanical properties. The hull steel and rivets were characterized both microstructurally and chemically, deleterious components of the microstructure were identified, and all this information was examined in light of steelmaking practices common to turn-of-the-century Ireland.

Planned Outcomes:

The work on lead-free solders will generate permissible pressure ratings for the applicable ASME Codes for solder joints in copper plumbing tube for use in potable plumbing systems. Initially, ratings will be generated for three solder alloys, but it is probable that industry will request tests on additional alloys.

New technical standards will be developed for the design, manufacture and testing of high strength steel, aluminum alloy, and composite cylinders used in the transportation of high pressure gases.

Correlation between processing and heat treatment parameters, microstructure and high temperature mechanical properties for the orthorhombic Ti-Al alloys will contribute to the science base of the U.S. Air Force Propulsion Initiative (the Integrated High Performance Turbine Engine Technology (IHPTET) Program.

The work on nanolayered surface films will examine the possibility that such films can significantly improve the fatigue resistance of critical structural components.

A determination of whether any metallurgical mistakes were made during the construction of the RMS Titanic will be attempted. Mechanical property data will be available for use in finite element simulations to attempt to explain the sequence of events during the sinking, most importantly the breakup of the ship on the surface.

External Collaborations:

The work on lead-free plumbing solders is being conducted cooperatively with the CDA and with ASME (Committee B16).

In the work on high pressure gas cylinders, there is extensive collaboration with DOT, the Compressed Gas Association and its member companies, and with the international community through the ISO Technical Committee on Gas Cylinder Design (TC58).

The research on Alloy 706 was carried out in close collaboration with engineers at Wyman- Gordon, Inc., who had ATP support for a project entitled "Cost-Effective, Near-Net-Shape, Superalloy Forgings for Power Generation Gas Turbines." A Wyman-Gordon staff member was appointed an Industrial Research Associate at NIST and participated in some of the experimental work.

The studies of orthorhombic Ti-Al-Nb alloys are being conducted in collaboration with the U.S. Air Force Wright Laboratory which is also providing financial and material support, and with the aerospace companies and universities participating in the IHPTET Program.

The research on nanolayered surface films is being performed in cooperation with the Nanoscale Materials Group at The Johns Hopkins University.

The project on the metallurgy of the RMS Titanic is being performed under the auspices of the Discovery Channel and the Society for Naval Architects and Marine Engineers. Collaborators on this project include Prof. Phil Leighly (Univ. of Missouri, Rolla, MO), Dr. Harold Reemsnyder (Homer Labs, Bethlehem Steel, Bethlehem, PA), George Tulloch (RMS Titanic, Inc., New York, NY), Bill Garzke (Gibbs and Cox and SNAME, Arlington, VA), Dr. Jim Matthews (Defense Research Establishment - Atlantic, Halifax, NS), Bob Brigham (CANMET, Ottawa, Quebec), Ed McCutcheon (Cmdr., USCG (Retired), Bethesda, MD), and Prof. Bill Gerberich (Univ. of Minnesota, Minneapolis, MN).

Accomplishments:

In the work on stress rupture of lead-free solders, testing was completed on alloy Sb5. The data were presented to ASME Committee and calculated permissible pressure ratings for plumbing joints are being circulated for letter ballot. Tests on Alloys E and HB are nearing completion. Significant progress was made in both the statistical analysis of the data and the basic studies of the failure mechanisms.

In collaboration with DOT personnel, final specifications were developed for the use of ultrasonic methods for retesting high pressure steel cylinders in place of hydrostatic methods. Also, five ISO draft standards for steel and aluminum cylinders were completed and will be published in FY98.

The NIST studies of Alloy 706 forgings successfully correlated grain boundary precipitation of eta-phase with elevated temperature fatigue resistance, and thus assisted ATP recipient Wyman-Gordon Inc. in the identification of heat treatments which minimized the deleterious effects of eta-phase precipitation on the fatigue resistance of the material.

The deleterious components of the microstructure of both the hull steel and rivets of the RMS Titanic have been identified. For the hull steel, these include large MnS inclusions, large ferrite and pearlite grain size, coarse pearlite lamella, low Mn content, and low Mn/C ratio. In the rivets, which were composed of wrought iron, the slag content was found to be 3 to 4 times that normally found in contemporary material (9.2% versus 2.5%). Also, the direction of the stringers within the rivets was found to change from longitudinal within the shaft to transverse at the intersection of the shaft and the head formed during installation. Given that wrought iron has little transverse ductility, this is postulated as a failure mechanism for lost rivets during the collision. Lost rivets and parted seams were found, in other parts of the overall study, to have been a major component of the flooding of the ship, and thus the sinking.

Impacts:

Significant progress has been made in the process for introducing expanded permissible pressure data for lead-free solders into the ASME Codes. When completed, these data will provide the U.S. building construction industry with far greater choice of solders for joining copper tube in potable water systems and with more realistic safety factors. The impact will be felt primarily in the construction of high-rise and other commercial buildings, where costs will be significantly reduced by replacing brazing or mechanical joining by soldering, and, in some cases, by allowing the reduction of wall thickness of the copper tube.

The new ultrasonic methods adopted for retesting steel high pressure gas cylinders significantly reduce the cost of retesting as well as avoiding the generation of hazardous waste material by the previously used hydrostatic testing. The adoption of the ISO standards for high strength steel and aluminum cylinders will permit U.S. manufacturers to produce cylinders that are accepted for worldwide use.

The NIST research on Alloy 706 helped ATP recipient Wyman-GordonInc. to develop a heat treatment for a large forging which met the fatigue specification of its customer. This was the first large scale forging sold in the US by a domestic company, such forgings being bought previously from overseas suppliers. This advance opens a large market to Wyman-Gordon Inc.

New insights on the sinking of the RMS Titanic have been gained through this investigation, and 85 year-old myths concerning the nature of the damage to the hull have been dispelled.

Outputs:

Publications:

Fields, R. J., "Characterization of Intergranular Eta Phase and Its Effect on the Low Cycle Fatigue Properties of Near-net Shape Alloy 706 Forgings," in the Proceedings of the Public Meeting on Materials Processing for Heavy Manufacturing, Rockville Maryland (September 24, 1996).

Garzke, Jr., W. H., Brown, D. K., Matthias, P. K., Cullimore, R., Wood, D., Livingstone, D., Leighly, Jr., H. P., Foecke, T., and Sandiford, A., "Titanic: Anatomy of a Disaster," accepted Proceedings of The Society of Naval Architects and Marine Engineers Annual Meeting, SNAME, Jersey City, NJ (1997)

Presentations:

Clough, R. B., "Stress-Rupture Data for Lead-Free Solder Joints for Use in Potable Water Systems," Meeting with CDA and ASME staff, New York, NY, August, 1997.

Fields, R. J., "Characterization of Intergranular Eta Phase and its Effect on the Low Cycle Fatigue Properties of Near-net Shape Alloy 706 Forgings," Public Meeting on Materials Processing for Heavy Manufacturing, Rockville, MD, September 24, 1996.

Fields, R. J., "NIST Internal Research in Support of Cost Effective Near-net Shape Superalloy Forgings for Power Generation Gas Turbines," Wyman-Gordon's Annual Project Review for ATP, Worcester, MA, 1996.

Foecke, T., "Metallurgy of the RMS Titanic," University of Minnesota Department Colloquium, July 9, 1997.

Foecke, T., "The History of Fracture Mechanics," Fifth IMM Summer School on the Mechanics-Materials Linkage (banquet keynote address), The Catholic University of America, August 14, 1997.

Foecke, T., Public Affairs presentation to gifted and talented high school students on Titanic, NIST, August 20, 1997.

Project Title: PERFORMANCE OF MATERIALS IN CORROSIVE MEDIA

Investigators: F. Biancaniello,J. L. Fink, E. N. Pugh, R. E. Ricker, M. R. Stoudt, S. D. Ridder, and D. A. Little (Guest Researcher, University of Texas at El Paso)

Objectives:

The primary objective of this project is to develop test methods that enable U.S. industry to produce, market and use materials that resist degradation in corrosive environments.

Technical Description:

An analysis by Battelle Columbus Laboratories (BCL) in 1995 determined that the annual cost of corrosion to the US economy exceeds $300B. Corrosion impacts virtually every industry, but in FY97 our work focussed on corrosion issues in three specific industrial sectors. The first of these focus sectors is the aircraft industry for which the 1995 BCL study estimated corrosion costs at $13B annually. While corrosion rarely causes catastrophic failures in this industry, this cost of corrosion is due to the cost of the repairs and maintenance currently employed to avoid corrosion failures and from the premature retirement of aircraft due to excessive corrosion damage. These costs are expected to rise as the average age of commercial and military aircraft increases. To help reduce the cost of corrosion in aging aircraft, NIST is collaborating with the U.S. Air Force's Wright Laboratory to develop test methods that will enable the development of more corrosion resistant alloys, corrosion prevention technologies, and standards for evaluating the ability of NDE techniques to detect and quantify corrosion damage.

Much of the corrosion damage in aircraft occurs hidden in small crevices and joints where it is difficult to detect, much less measure, corrosion. Another complication arises from the intermittent nature of this hidden corrosion. This results from the fact that the electrolyte responsible for attack is generally condensed moisture contaminated by various salts; the presence of the moisture film depends on the relative humidity, which, in turn, varies with service conditions. NIST responded to this set of problems by developing a test method that utilizes electrochemical techniques, which usually require complete immersion of specimens in an electrolyte, to measure corrosion rates inside a crevice under normal atmospheric conditions. The feasibility of the technique was demonstrated in laboratory tests on simulated crevices in Al alloy 2024 contaminated with different salts; the results established that the corrosion rate varied over four orders of magnitude with changes in relative humidity.

Exfoliation corrosion, a form of intergranular attack, is also a serious problem in Al sheet in aging aircraft, and existing test methods such as the EXCO Test (ASTM G34) do not provide data which enable quantification of the effects of alloy chemistry and heat treatment on susceptibility to this form of attack and sometimes fails to even predict behavior in service. NIST is now evaluating the electrochemical behavior of the relevant Al alloys in the solution used in the EXCO tests and this is expected to lead to the development of a quantitative electrochemical method for measuring the susceptibility.

The second focus sector is the pulp and paper industry, where the processing environments place severe demands on the materials used for critical components. Our current focus is on suction roll shells which cost hundreds of thousands of dollars each and have a service life of only five to eight years because of the combined action of cyclic loading and corrosive attack. Attempts to develop more corrosion resistant alloys are hindered by the lack of standard test methods that accurately predict service performance of the candidate alloys, and consequently NIST is working with CRADA partner Sandusky International, the only U.S. manufacturer of these critical components, to develop such tests. To date, a thorough electro-chemical evaluation has been made of two duplex stainless steels for which service data are available, and work is proceeding on the development of a suitable test method.

The chemical process industry represents the third focus sector. Austenitic stainless steels are used throughout this major US industry primarily due to their superior resistance to pitting corrosion. The alloys examined in this study contain high nitrogen contents, known to increase pitting resistance of stainless steels, and are being developed by Crucible Materials Corporation and NIST under a CRADA, using a powder metallurgy approach. The materials produced in this CRADA were so resistant to pitting corrosion that the existing test methods used by industry such as ASTM G61, G48, and critical pitting temperature tests fail to induce this form of attack. As a result, a test method was needed that would enable comparison of the different alloys produced and identification of the optimum composition and processing condi- tions. To overcome this obstacle, NIST modified an existing standard method by modifying the composition and increasing the temperature of the test solution. In addition, the composition and elevated temperatures of this environment required development of a reference electrode. This test method proved capable of quantifying the pitting resistance of the alloys and, through multiple regression analysis, proved capable of quantifying the relative influence of alloying elements. This test method has now been incorporated into Crucible's alloy development program.

Planned Outcomes:

The outputs of this ongoing project will be primarily test methods of the types described above, but, based on past experience, it is anticipated that future outputs will also include predictive models, data generation and materials characterization.

External Collaborations:

NIST collaborated with the U.S. Air Force and the University of Texas at El Paso on the development of test methods for aging aircraft, with Sandusky International through a CRADA on duplex stainless steels in the pulp and paper processing industry, and with Crucible Materials Corporation through a CRADA on evaluation of the pitting corrosion resistance of nitrogenated stainless steels.

Accomplishments:

This project has resulted in the creation of two new measurement methods during this FY: (1) a method for the measurement of crevice corrosion rates in Al alloys during atmospheric exposure that utilizes a unique sample geometry with electrochemical measurement techniques, and (2) a method for evaluation of the influence of alloying elements on the pitting resistance of nitrogenated stainless steels. The first of these could lead to the development of a new standard test method while an existing standard could be modified to incorporate the second (ASTM G61). In addition, the metallurgical and electrochemical knowledge gained during the development of these methods and during work on the other methods in progress should lead to similar developments and metallurgical advances in the future.

Impacts:

Sandusky International has reported that the results of NIST's research shared with them through the CRADA has already had a significant impact on their research and development planning. Crucible Materials Corp. is incorporating the test method developed for nitrogenated stainless steels into their alloy development program. Following presentation of the crevice corrosion test method at the Aging Aircraft Conference in July, a U.S. Air Force Contractor approached NIST and discussed using this method in their program to develop and evaluate corrosion prevention compounds (corrosion inhibitors) for preventing crevice corrosion in aircraft.

Outputs:

Publications:

Ricker, R. E. and Duquette, D. J., "Corrosion Fatigue of Aluminum Alloys," submitted for publication in the proceedings of the First Joint DoD, FAA, and NASA Conference on Aging Aircraft, Ogden UT (8-10 July 1997).

Ricker, R. E. and Fink, J. L., "Crevice Corrosion Metrology for Aging Aircraft," submitted for publication in the proceedings of the First Joint DoD, FAA, and NASA Conference on Aging Aircraft, Ogden UT (8-10 July 1997).

Biancaniello, F. S., Ricker, R. E., and Ridder, S. D., "Structure and Properties of Gas Atomized HIP-Consolidated High Nitrogen Stainless Steel," submitted for publication in the proceedings of 5th Intl. Conference on Advanced Particulate Materials and Processes," West Palm Beach, FL April 7-9, 1997, Metal Powder Industry Foundation.

Ricker, R. E., "Aqueous Corrosion Resistance of Nickel and Iron Aluminide," Proceedings of International Symposium on Nickel and Iron Aluminides: Processing, Properties and Applications" S. C. Deevi, V. K. Sikka, P. J. Maziasz, R. W. Cahn eds., ASM Intl., Materials Park, OH, pp. 253-263 (1997).

Bandyopadhyay, S., Miller, A. E., Chang, H. C., Banerjee, G., Yuzhakov, V., Yue, D.-F., Ricker, R. E., Jones, S., Eastman, J. A., Baugher, E., and Chandrasekhar, M., "Quasi-periodic Quantum Dot Arrays Produced by Electrochemical Synthesis," Nanotechnology, 7, pp. 360-371 (1996).

Ricker, R. E., Miller, A. E., Yue, D.-F., Banerjee, G., and Bandyopadhyay, S., "Nanofabrication of a Quantum Dot Array: Atomic Force Microscopy of Electropolished Aluminum," J. of Electronic Materials, 25, No. 10, pp. 1585-1592 (1996).

Presentations:

Ricker, R. E., "Corrosion Fatigue of Aluminum Alloys," presented at the First Joint DoD/FAA/NASA Conference on Aging Aircraft, Ogden UT 8-10 July 1997.

Ricker, R. E., "Crevice Corrosion Metrology for Aging Aircraft," presented at the First Joint DoD/FAA/NASA Conference on Aging Aircraft, Ogden UT 8-10 July 1997.

Meetings Arranged:

"Corrosion Deformation Interactions II" A NIST staff member served on the International Scientific Organizing Committee for this meeting and organized financial support for US participants in this meeting held in conjunction with EuroCorr/96, Nice, France.

"Environmental Effects on Ceramic, Intermetallics and Composites," A NIST staff member is serving as co-chair for this symposium to be held during Materials Week 1998.

"Hydrogen Effect in Metals (HEM)/Corrosion Deformation Interactions (CDI)" A NIST staff member is serving as co-chair of the organizing committee for this meeting of these two series of International meetings to be held jointly in Jackson Hole, WY, in Sept. of 2001.

Project Title: MAGNETO-OPTICAL IMAGING

Investigators A.J. Shapiro, R.D. Shull, V.I. Nikitenko*, V.I. Gornakov*

*Guest Scientist, Institute for Solid State Physics, Russian Academy of Sciences.

Objectives:

The objective is to develop techniques for obtaining direct experimental images in real time of magnetization reversal processes for technologically important magnetic materials, such as nanostructured material including multilayers, spin valves, and granular structures. In order to do this a new, nondestructive method, the Magneto-Optical Indicator Film (MOIF) technique, was developed in cooperation between NIST and the Institute for Solid State Physics of the Russian Academy of Sciences. The obtained information, such as static and dynamic magnetization and remagnetization processes and their relationships to thin film characteristics and defects, provides nondestructive characterization of the quality of artifically constructed nanostructures, magnetic devices, and bulk ferromagnets.

Technical Description:

The MOIF technique utilizes a transparent indicator film, a Bi-substituted yttrium iron garnet with in-plane anisotropy, placed on the top of a sample. Polarized light passes through the indicator film and is reflected back by an Al underlayer. Magnetic stray fields with a component perpendicular to the film plane are observed through the magneto-optic Faraday effect created in the garnet film. For example, magnetic stray field images of domain walls of different types and detailed information on the spin rotation processes as well as on the domain wall nucleation and motion during the remagnetization of the magnetic materials can be obtained by the MOIF technique.

Planned Outcome:

The MOIF method is expected to become a standard nondestructive quality control imaging technique for next generation magnetic materials for sensors and storage devices, to contribute to the fundamental understanding of the remagnetization process in artificial magnetic materials, and to allow the investigation of domain wall nucleation and motion in magnetic materials as a function of their microstructure and processing variables.

Accomplishments:

It was shown that epitaxial NiO/NiFe bilayers on single crystal MgO exhibit unidirectional magnetic anisotropy, as revealed by a shift of the hysteresis loop of the ferromagnet along the fields axis. We have studied experimentally the magnetization reversal of epitaxial NiO/NiFe bilayers and extended the model of magnetic exchange biasing to describe the measured enhanced coercivity and asymmetry observed in them. The relative activity of various domain nucleation centers was also observed. For the first time the influence of dislocations on these processes was determined.

Direct experimental examination of the static and dynamic magnetization reversal processes in [Co₆₄Ni₃₁Cu₅ (2nm)/Cu(2nm)]₂₀₀ magnetic multilayers was conducted in real time, proving that the MOIF technique can be utilized as a non-destructive characterization method for quality control.

By means of the MOIF technique the magnetic domain structure of a Si/NiO/Co/Cu/Co/Ta nonsymmetric bottom spin valve was imaged. It was demonstrated that the first stage of magnetization reversal is characterized by the nucleation of many microdomains. With increasing reversed field, the domain walls move over small distances (5-20 micrometers) until annihilation. The magnetic domain size depends on the thickness of the Co layer.

Outputs:

Publications:

V. S. Gornakov, V.I. Nikitenko, . L.H. Bennett, H.L. Brown, M.L. Donahue, W.F. Egelhoff, R.D. McMichael, and A.J. Shapiro. "Experimental study of magnetization reversal processes in nonsymmetric spin valve". J. Appl. Phys. 81 (8), 15 April 1997.

L.H. Bennett, M.J. Donahue, A.J. Shapiro, H.L. Brown, R.D. McMichael, V.S. Gornakov, V.I. Nikitenko, "Investigations of domain-wall formation and motion in magnetic multilayers", Physica B, 223 (1997) 356-364.

Presentations

V.I. Nikitenko, L.M. Dedukh, V.S. Gornakov, Yu. P. Kabanov, L.H. Bennett, M.J. Donahue, L.J. Swartzendruber, A.J. Shapiro, and H.J. Brown. "Spin Reorientation transition and domain structure in magnetic multilayers", presented at INTERMAG 97, New Orleans, Louisiana, Apr 1997.

V.I. Nikitenko, L.M. Dedukh, V.S. Gornakov, Yu. P. Kabanov, L.H. Bennett, M.J. Donahue, L.J. Swartzendruber, A.J. Shapiro, and H.J. Brown." Investigations of spin reorientation transition in antiferromagnetically coupled multilayers.", March Meeting of The Amer.Phys.Society. 1997.

Project Title: DEVELOPMENT OF SCANNING ACOUSTIC MICROSCOPY

Investigator: Eva Drescher-Krasicka

Objectives:

The objective of this project is to develop new, highly sensitivite methods of measuring stresses by scanning acoustic microscopy. This study is in contrast to the multiple descriptions of the dependence of acoustic wave velocity dependance on residual or applied stress. Our efforts are directed toward sensitive stress measurements by monitoring the change of the amplitudes of the polarized modes received from the stressed area. The goal is to show feasibility of the method, and to calibrate acoustic microscopic measurements of internal stresses by applying loads to specimens for which the stress distribution can easily be calculated.

Technical Description:

Many attempts have been made to measure internal stresses ultrasonically by exploiting the third order changes in elastic constants which accompany changes in internal stress. Until recently all of these approaches have employed measurements of the phase velocities of elastic waves. However, these velocities vary only weakly with changes in elastic constants, and attempts to use this approach to measure residual stresses have met with only limited success. A program has been underway at NIST to use other characteristics of ultrasonic waves, which are more sensitive to changes in elastic constants than phase velocity, and which can be detected using an acoustic microscope.

One of these methods makes use of the fact that isotropic materials have two shear wave modes which travel at the same velocity, but which are polarized (direction of particle vibration in the wave) in orthogonal directions. In the presence of an internal stress, the velocities of these waves are altered minutely, as is their polarization. While it is extremely difficult to detect these changes directly, interference effects lead to dramatic intensity variations which correspond to stress levels.

In addition to providing measurements of stresses, this new measurement method promises to provide through thickness information to help in the "tomographic" reconstruction of internal stress fields. To make accurate "tomographic" reconstructions of internal stress fields, inplane information on internal stresses is needed. Leaky-wave ultrasonic modes offer another, novel characteristic of ultrasonic waves which can provide this information, and are extremely sensitive to changes in elastic constants. Mathematically this is seen by expressing the waves in their plane wave or cylindrical form, and matching displacements and normal tractions across the interface by allowing a complex phase velocity for the mode along the interface.

In a wave propagation context these modes appear to 'leak' away from the interface, at an angle which is characteristic of their Poynting vector. The Poynting vector of a leaky mode describes the elastic power flowing out of an arbitrary volume through which the mode is traveling. The 'asymptotic' leakage angle (the macroscopic angle observed at some distance from the interface) and attenuation, as well as the phase velocity, of these waves can be extraordinarily sensitive to the frequency of the impinging beam.

Indeed, the theory of leaky waves has previously been extensively developed by the authors from the Metallurgy Division, and the existence and many properties of such waves have been experimentally confirmed. In this work we quantitatively explore the possibility that the leakage angle for such modes greatly amplifies changes in elastic constants in the materials on either side of the interface, and by measuring this angle using acoustic microscopy, one can measure the in-plane changes of internal stress as a function of depth which cannot easily be obtained by interference effects.

Planned Outcome:

The ultimate goal of this project is to establish a new method, based on acoustic microscope measurements, to evaluate the distribution of the residual stresses inside of a solid sample.

External Collaboration:

The 1997 work was done in close collaboration with Professor John Willis from the Department of Applied Mathematics and Theoretical Physics in Cambridge, U.K., with Dr. John A. Simmons, formerly of the Metallurgy Division and with Professor Ron Kline and his student, Linqiang Jiang, formerly from the University of Oklahoma.

Accomplishments:

FY 1997 work successfully proved the sensitivity of the shear, longitudinal and leaky modes to residual and applied stresses.

Impacts:

A new way of approaching the detection and evaluation of the presence of residual stresses in materials was established. The new method is characterized by much higher than previous (time of flight method) sensitivity to stress measurements.

Outputs:

Publications:

Drescher-Krasicka, E. Willis, J. R., Nature, Vol. 384, November (1996).

Simmons, J. A., Drescher-Krasicka, "Internal Stresses Effects on Leaky Waves at Composite Interfaces, an extended abstract at the 4th International Conference on Composites Engineering," Big Island of Hawaii, July (1997), Conference Proceedings, pp. 911-912.

Presentations:

Simmons, J. A., Drescher-Krasicka, E., paper at the 4th International Conference of Composite Engineering, Big Island of Hawaii, July, 1997.

Project Title: ELECTRON MICROSCOPY

Investigator: J. E. Bonevich

Objectives:

Transmission electron microscopy (TEM) is used to characterize the structure and chemistry of materials at the atomic scale to better understand and improve their properties. New measurement techniques in electron microscopy are developed and applied to materials science research. The Microscopy Facility primarily serves the Metallurgy and Ceramics Divisions as well as other NIST staff and outside collaborative research efforts.

Technical Description:

Atomic structure and compositional characterization of materials can lend crucial insights to their properties. Direct observation of localized structures by transmission electron microscopy (TEM) provides an important information feedback to the optimization of crystal growth and processing techniques. A wide variety of structures may be observed such as crystal structure and orientation, grain size and morphology, defects, stacking faults, twins and grain boundaries, second phase particles -- their structure, composition and internal defect structure, compositional variations and the atomic structure of surfaces and interfaces. To this end, the Metallurgy and Ceramics Divisions TEM facility consists of three transmission electron microscopes, a specimen preparation laboratory, and an image processing/computational laboratory. The state-of-the-art JEOL3010 TEM has atomic scale resolution as well as detectors for analytical characterization of thin foil specimens; a thin window X-ray detector for compositional analysis and an energy selecting imaging filter (IF) for compositional mapping at atomic resolution. Several studies are underway with scientists of the Metallurgy and Ceramics Divisions.

An active collaboration with the Chemical Science and Technology Laboratory (CSTL) continues for Metallurgy Division scientists to develop electron holography techniques with the 300 keV field-emission TEM. The TEM employs a highly coherent electron source enabling quantitative electron holography in addition to the capability of forming ~1 nm probes with 1 nA currents. The hologram records the phase distribution of electron waves interacting with matter and provides a quantitative measure of electromagnetic phenomena such as the magnetic fields inside materials (magnetic nano-composites) and electric fields emanating from pn junctions. Holography also quantitatively measures specimen thickness, surface topography, mean inner potentials of materials, dislocation strain fields, nano-diffraction and electron microscope lens aberrations.

Planned Outcome:

The feedback of structure and compositional information from electron microscopy will serve not only to help optimize existing materials and the processing techniques used to create them, but also to aid in the discovery of new classes of materials. The investigation of materials by electron holography provides quantitative measurements of electro-magnetic fields as well as fundamental data on mean inner potentials.

External Collaborations:

Prof. Searson (Johns Hopkins University) provided electroplated Cu/Ni/Co multilayer nanowires for characterization by holography and high-resolution composition mapping. Prof. Searson also supplied nanoscale ZnO particles for surface morphology and structure measurements. The Naval Research Laboratory supplied nanoscale Au and Pd catalytic particles for mean inner potential measurements. Prof. Pozzi (Bologna University, Italy) collaborated on interpretating holographic observations of superconducting materials.

Accomplishments:

Atomic resolution compositional mapping was applied to a new class of materials with naturally-occurring magnetic multilayers. Phases in the BaFeTiO system were shown to possess an Fe-rich phase separated by a Ba-rich dielectric spacer material. These new materials are expected to find applications in microwave devices.

Compositional mapping was applied to magnetic spin-valve TaO/Co/Cu/Co/NiO materials grown by sputter deposition. Annealing of these materials at standard semiconductor processing temperatures produce a degradation of magnetic properties. The mapping revealed the formation of large metallic Ni islands from the NiO substrate, resulting in the structural disruption of the Co magnetic layers.

High-resolution imaging of BaTiO₃/MgO photonic materials has revealed the formation of amorphous zones in close proximity to the barium titanate/magnesia interface. The zones result from non-stoichiometries in the barium titanate thin films and cause degradation in the desired dielectric properties.

Mean inner potentials and surface morphologies of nanoscale Au/Pd particles were measured by electron holography. The particles were shown to have alloyed compositions with non-equilibrium structures, such as multiple-twinning, and faceted surfaces. Mean inner potentials varied from 17-20 eV.

The theoretical treatment of vortex image contrast in superconductors has been refined. The results show that the London model of a vortex is a useful computational tool for 1-D trace profiles of the electron phase. The extended analytical model correctly interprets the phase profiles resulting from the significant broadening of the magnetic field that occurs in thin film specimens. Good agreement with the experimental data has been achieved.

A division-wide image processing and computation facility was created with workstations, image scanners, and processing software as well as a high resolution photographic quality printer. The facility provides image analysis and quantitative metallography capabilities.

Impact:

The HolograFREE hologram reconstruction software, developed earlier under this project, has been downloaded by research facilities at three corporations (Hitachi, Philips, Exxon) and six universities (Bologna, Stevens Institute, Northwestern, U. C. Berkeley, Wisconsin, Tuebingen). The software provides user-friendly reconstruction of electron holograms.

Outputs:

Publications:

Bendersky, L., and Bonevich, J., "Magnetic Multilayers as a Structural Component in a Series of BaO:TiO₂:Fe₂O₃ Compounds," Phil. Mag. Lett. (in press).

Bonevich, J., Capacci, D., Harada, K., Kasai, H., Matsuda, T., Patti, R., Pozzi G., and Tonomura, A., "Measurement of the London Penetration Depth from Holographic Images of Superconducting Vortices: The Influence of Specimen Thickness," Phys. Rev. B (in press).

Josell, D., Cezairliyan, A., Bonevich, J., "Thermal Diffusion Through Multilayer Coatings: Theory and Experiment," Proc. of the 13th Symp. on Thermophys. Prop., International Journal of Thermophysics (in press).

Bonevich, J., Capacci, D., Harada, K., Kasai, H., Matsuda, T., Patti, R., Pozzi G., and Tonomura, A., "On the Influence of Specimen Thickness in TEM Images of Superconducting Vortices II," Microscopy and Microanalysis, Vol. 3, Suppl. 2, 505 (1997).

Bonevich, J., "Electron Holography of Electromagnetic Fields," Microscopy and Microanalysis, Vol. 3, Suppl. 2, 1059 (1997).

Turner, S., Bonevich, J., Maslar, J., Aquino, M., Zachariah, M., "TEM and Electron Holography of Nanophase TiO₂ Generated in a Flame Burner System," MRS Symp. Proc., Vol. 457, 93 (1997).

Presentations:

Bonevich, J., "Electron Holography - Theory and Applications," Invited talk at Dept. of Materials Science, Johns Hopkins University, Baltimore, Maryland, April, 1997.

Bonevich, J., "Applications of Electron Holography and Phase Imaging," Invited talk at National Institutes of Health, Bethesda, Maryland, May, 1997.

Bonevich, J., "Electron Holography - The Phase Problem," Invited talk at the Meeting of the Microscopy Society of America, Cleveland, Ohio, August, 1997.

U.S. Department of Commerce
Technology Administration
National Institute of Standards and Technology
Materials Science & Engineering Laboratory
Metallurgy Division

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Revised March 09, 1998