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 sub structure and consist of thin epsilon-layers separated by layers of un transformed 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 elasti city calculations: it can be described as a periodic array of 30 degre es and 90 degrees Shockley partials in a 2:1 proportion. This model il lustrates the self-accommodation properties of thermal epsilon-martens ite at the finest scale and explains the easy reversion of the hexagon al phase.