Investigators: William J. Boettinger, Ursula R. Kattner, Sam R. Coriell, Ann Bolcavage

Technical Description:

This project provides a method to predict the fraction solid (and heat content) vs. temperature relationship for multicomponent superalloys. This information is necessary for accurate macroscopic heat flow modeling of castings used to determine soundness. The information also predicts the identity and volume fraction of all phases present in the microstructure. A combination of multicomponent phase diagram calculations with a kinetic analysis of solidification microsegregation is being applied to superalloys. Models will be developed in a form to be implemented in a commercial software code for castings as part of the NIST Consortium on Casting of Aerospace alloys.

Technical Objectives:

• Develop thermodynamic data base for Ni-base superalloys for the calculation of tie- line information required for the solidification models.

• Develop solidification kinetic models for multicomponent superalloys to treat the dendritic aspects of solidification.

• Integrate thermodynamic data base and solidification kinetic models into commercial casting software.

Anticipated Outcome:

• Improved quality of simulation of investment castings by industry.

• More reliable prediction of casting defects.

• Reduction of casting reject rate.

Accomplishments for FY 1995:

• A "multicomponent phase diagram subroutine" of code was prepared that gives the liquidus temperature, solid concentrations, liquidus slopes and the liquid and solid enthalpies as functions of the liquid concentrations for use in various kinetic models. This subroutine employs a thermodynamic data base that describes the free energies of the relevant phases.

• Solidification models were developed that treat liquid diffusion at the dendrite tip and solid diffusion in the dendrite interior for an N-component alloy. These effects reduce the degree of microsegregation predicted by the simple Scheil approach and alter how the latent heat is deposited in the mushy zone of a casting.

• Although good results have been demonstrated using a "preliminary" database employing only binary interactions, work on a refined data base involving the ternary subsystems of the Ni-Fe-Cr-Nb system has been performed. DTA melting experiments on Ni-rich alloys in the Ni-Fe-Cr system have permitted the selection of the literature calculation that best fits the data. Similar experiments on the Ni-Cr-Nb system have provided data necessary for developing ternary interaction energies in this system.

Impacts and Technical Highlights:

• NIST prediction of the fraction solid vs. temperature relationship for the alloy, INCONEL 718, has been incorporated into the thermal model of investment castings at Howmet Corporation. Using this model, significantly improved agreement between measured and predicted temperatures has been obtained. Use of this improved modeling capability will increase the manufacturing yield of large structural castings made from this alloy.

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Last modified: Mon Jan 06 09:46:15 1997 Metallurgy Webmeister