Parent–martensite interface structure in ferrous systems

Recently, a Topological Model of martensitic transformations has been presented wherein the habit plane is a semi-coherent structure, and the transformation mechanism is shown explicitly to be diffusionless. This approach is used here to model martensitic transformations in ferrous alloys. The habit plane comprises coherent (1 1 1)γ∥(0 1 1)α terraces where the coherency strains are accommodated by a network of dislocations, originating in the martensite phase, and disconnections (transformation dislocations). The disconnections can move conservatively across the interface, thereby effecting the transformation. Since the disconnections exhibit step character, the overall habit plane deviates from the terrace plane. A range of network geometries is predicted corresponding to orientation relationships varying from Nishiyama–Wasserman to Kurdjumov–Sachs. This range of solutions includes habit planes close to {2 9 5}, {5 7 5} and {1 2 1}, in good agreement with experimental observations in various ferrous alloys.