The γ → ε-martensitic transformation and its reversion in the FeMnSiCrNi shape-memory alloy

The hcp martensitic transformation and its reversion in an Fe-16.86Mn-4.50Si-10.30Cr-5.29Ni alloy have been studied. The fine structure and morphologies of ε-martensite were systematically investigated using transmission electron microscopy. It is found that, in the overlapped region of the stacking faults, the nucleus of ε-martensite may form by shear and grow along the {111}γ plane, which is not the stacking fault plane. The nucleus may grow into a small, thin plate and may act as the basal structure unit of ε-martensite. When the stress increases, the thin plates continue to grow into an ε-martensite ribbon along this preferred orientation and, during the subsequent recovery annealing, the reverse transformation of these basal structure units occurs. As a result, the ε-martensite plates shrink in both length and thickness.