Moving towards the computational simulation of heat treatment with CALPHAD-based tools

In the 100th Column of the “Heat Treat Doctor” published in Industrial Heating magazine, Dan Herring stated that heat treating can best be defined as “the controlled application of time, temperature and atmosphere to produce a predictable change in the internal structure (i.e. the microstructure) of a material.”

Heat treatment, for example normalizing, aging, tempering, surface hardening, etc., is usually a thermochemical diffusion process aiming at improving the physical properties of materials. Such processes, to a large extent, can be simulated by integrating various computational tools and related thermodynamic/kinetic databases based on the CALPHAD methodology which is used to predict the microstructure in terms of phases formed, composition of those phases, etc. Using such an approach it is now possible to apply computational simulations to accelerate the design and optimization of a heat treatment process. In this presentation, we introduce the underlying concepts of the CALPHAD methodology and demonstrate how the Gibbs energy and mobility of each individual bulk phase in a multicomponent, multiphase alloy can be modeled to depend on temperature, pressure, and composition. We draw attention to the importance of interface energy data and other physical property data in the description of the microstructural evolution under different heat treatment conditions. Using examples, based on Thermo-Calc, DICTRA, and TC-PRISMA, we show how CALPHAD-based computational tools can be utilized to simulate the microstructure evolution during various heat treatment processes in steels, Al-based alloys, and Ni-based superalloys. Ongoing challenges toward computational heat treatment will also be discussed.