Non-equilibrium reaction paths in ferrous austenite decomposition

Several authors in the 1960 Decomposition of Austenite by Diffusional Processes Proceedings remarked about the thermodynamic possibility of austenite decomposition to non-equilibriunt products. These concepts are generalized in terms of the total reaction path traversed during the diffusional decomposition of single phase supercooled austenite from start to finish during its approach toward an equilibrium microstructure. A set of equilibrium reaction path characteristics are proposed. Non-equilibrium reaction paths followed during decomposition of austenite to pearlitic and bainitic products in ternary Fe-C-M steels are evaluated in light of these criteria. Specific examples include divergent pearlite and bay-region bainite formed above and below the TTT diagram start-curve bay. The roles of diffusion, crystallography and carbides are given specific attention.