Recently developed iron-based shape memory alloys, i.e. Fe-Ni-Co-Al-X and Fe-Mn-Al-Ni, attracted considerable attention due to their unique material properties. Especially, relatively low alloying costs and high reversible transformation strains make them promising candidates for damping applications. Moreover, the critical stress for martensitic transformation of Fe-Mn-Al-Ni shows a low temperature dependency, due to its low Clausius-Clapeyron slope based on a low transformation entropy change between the bcc parent phase and the fcc martensitic phase. However, due to the high anisotropic material behavior, e.g. the orientation dependent transformation strains as well as the varying reversibility of the resulting phase transformation, pseudoelastic performance is strongly affected by microstructural features.
The current study focuses on the cyclic pseudoelastic behavior of Fe-Mn-Al-Ni with adjusted grain boundary decoration using the ductile γ phase induced by different quenching conditions. Samples were subjected to a cyclic heat treatment between 1200°C and 900°C in order to initiate abnormal grain growth. Amounts of γ phase precipitation were varied via different quenching rates after the last cycle. Subsequently, samples were aged at 200°C for 3h. Microstructural features and mechanical properties were evaluated by in situ experiments using optical microscopy, scanning electron microscopy and electron-backscatter diffraction. It was found that a small fraction of the ductile γ phase is sufficient to prevent crack formation without deteriorating the pseudoelastic performance. Post mortem transmission electron microscopy was used in order to identify elementary mechanisms responsible for functional degradation.