During alloy design, phase diagrams can provide useful information to predict the stable phase fields as a function of composition and temperature. For example, Turpin et al. (Met. Trans. A, 2005) used Thermo-Calc and the Diffusion Module (DICTRA) to understand the influence of the chemical composition to find the optimal carbon profile in an alloy in order to develop a carburized martensitic stainless steel for applications in the aerospace industry. As a first step, the phase diagram of the steel was calculated using Thermo-Calc.
This recalculated figure shows an isopleth for a Fe-13Cr-5Co-3Ni-2Mo-0.07C martensitic stainless steel. The figure shows that, as the overall carbon content increases, first M23C6 carbides precipitate, then M7C3 carbides appear in the austenitic matrix; if the mass percent of carbon exceeds 3.8, M3C carbides (a structure similar to cementite) will preferentially precipitate at the grain boundaries, which could weaken the microstructure and should thus be avoided.