Phase-field simulation of austenite reversion in a Fe-9.6Ni-7.1Mn (at.%) martensitic steel governed by a coupled diffusional/displacive mechanism

Microstructural features of austenite formation, including the growth morphologies and partitioning of alloying elements, during reverse transformation in a Fe-9.6Ni-7.1Mn (at.%) martensitic steel have been studied by phase field modelling with a coupled diffusional/displacive mechanism. Typical morphological patterns of reversed austenite transformed from lath martensite are revealed by the simulation results, containing acicular austenite (γA) formed along the martensite lath boundaries and globular austenite (γG) formed along the high angle prior austenite grain boundaries. It is also shown that the growth of γA, as governed by a combined displacive and diffusional mechanism, is accompanied by a higher enrichment of alloying elements (Mn and Ni) compared with γG, which is governed by a diffusional mechanism. Meanwhile, γG tends to grow preferentially to one side of the prior austenite grain boundaries, due to the presence of partial non-orientation relationship with adjacent martensite laths during the transformation. At the later stage of the reverse transformation, invasive growth behavior of γG to γA after impingement is found due to the minimization of gradient energy in the system and the concentration difference of alloying elements within the two types of reversed austenite. A possible mechanism for grain refinement during intercritical annealing process is finally proposed.