The shape recovery conditions for Fe-Mn-Si alloys: An interplay between martensitic transformation and plasticity

This work analyses the improvement of the shape memory properties of Fe-Mn-Si based alloys, and expands upon and deepens recently published developments concerning this topic. We prepared TEM specimens from a Fe-15Mn-5Si-9Cr-5Ni sheet rolled at 800 degrees C and annealed at 650 degrees C-92% shape recovery - and from another sheet annealed at 1000 degrees C-45% shape recovery. Here, we analyse the development of microstructure during the gamma <->epsilon martensitic transformation and correlate it with the shape memory behaviour. Among the observed microstructural features, the introduction of complex dislocation arrays due to plastic deformation appears to be critical for shape memory development, since they can arrest the backward movement of Shockley partial dislocations over the original path. Tailoring the microstructure in order to introduce a high stacking-fault density accompanied by an appropriate dislocation density remains a key issue in the design of these kinds of functional materials.