Hrem study of self-accommodated thermal epsilon-martensite in an Fe-Mn-Si-Cr-Ni shape memory alloy

An Fe-Mn-Si-Cr-Ni shape memory alloy containing 70% of thermal epsilon-martensite has been studied using high resolution electron microscopy. Images recorded along a [011](gamma)parallel to[2110](epsilon) axis show, at the atomic level, that large epsilon-bands have a layered substructure and consist of thin epsilon-layers separated by layers of untransformed austenite. The structure of the epsilon (h.c.p.)/gamma (f.c.c.) interface at the epsilon-layer tips has been determined by using both the analysis of numerically filtered images and isotropic elasticity calculations: it can be described as a periodic array of 30 degrees and 90 degrees Shockley partials in a 2:1 proportion. This model illustrates the self-accommodation properties of thermal epsilon-martensite at the finest scale and explains the easy reversion of the hexagonal phase.