THERMODYNAMIC AND KINETIC SIMULATION AND EXPERIMENTAL RESULTS HOMOGENIZING ADVANCED ALLOYS

In nearly all metal alloys, segregation occurs during solidification because some solute elements partition preferentially to the liquid while others partition to the solid.  Many cast articles are subjected to a homogenization heat treatment in order to minimize or eliminate the as-cast segregation and thus improve their hot working characteristics if wrought processed or in-service performance if used as a casting.  Traditionally, the homogenization heat treatment is based either upon past practice on related alloys or uses many time consuming laboratory experiments based on a trial and error approach.  Through the use of thermodynamic and kinetic modeling software, NETL has designed a systematic method that has been successfully developed to design an optimized homogenization heat treatment that eliminates chemical segregation that occurs across secondary dendrite arms within a cast microstructure.  It allows the researcher to homogenize a casting to a level suitable for the application at hand as well as to adjust temperatures and times to fit their in-house equipment (capability, reliability, etc.) or work schedule.  In this approach, the Scheil module within Thermo-Calc is used to predict the as-cast segregation present within an alloy, and then DICTRA (Diffusion Controlled TRAnsformations) is used to model homogenization kinetics as a function of time and temperature.  Thus, in this presentation the computationally designed heat treatment, and the subsequent verification of its success in eliminating the chemical segregation present within the microstructure of real castings, is presented.