Thermodynamic mapping of austenite decomposition's approach toward equilibrium in Fe-C-Mn at 700 °C

Transformation of Fe-0.85C-11.56Mn (wt.%) at 700 °C begins by formation of grain boundary cementite in conjunction with carbon and manganese partitioning. Grain boundary cementite formation initiates regional partitioning of manganese from austenite to cementite that continues over months of cementite growth. The large disparity between manganese volume and grain boundary diffusion in austenite highlights why grain boundaries are associated with all stages of the 700 °C phase transformation in this system. Therefore, unlike carbon, manganese partitioning associated with cementite growth is localized to the immediate region near the grain boundary/cementite reaction front. The reaction path involves austenite 'neighborhoods', defined to have manganese chemistries and, consequently, thermodynamic properties different from the majority of the remaining matrix. The remaining matrix does partition carbon but not manganese. Along with differences between grain boundary and matrix manganese diffusivity in austenite, diffusion of carbon is up to 106 times greater than that of manganese resulting in carbon activity in austenite equilibrating at each step of cementite growth. The concept of neighborhood thermodynamics is developed for these altered austenite regions and directed toward producing a transformation pathway analysis. The concept of local equilibrium is also demonstrated not to be viable as a method for phase transformation tracking and thermodynamic mapping as the reaction moves toward equilibrium.