Anomalous high-temperature quasi-linear superelasticity of Ni-Fe-Ga-Co shape memory alloy

The superelasticity of shape memory alloys is associated with the martensitic transformation which has been widely used in engineering applications. Here, we clarify quasi-linear superelasticity in Ni-Fe-Ga-Co shape memory alloy exhibiting a recovery strain of 3.58% at 573 K, which is far above the martensitic transformation temperature. Real-time in-situ neutron diffraction measurement was used to explore the underlying quasi-linear superelasticity mechanism via tracing the structural evolution during cyclic compression loading. Neutron diffraction observation showed that the superelasticity is correlated with stress-induced continuous variation of lattice parameter. The in-situ neutron diffraction provides the direct evidence on the anomalous diffraction peak width broadening, which mainly stems from the spatial heterogeneity in strain. The excessive Co doping is responsible for the decrease in stacking-fault energy leading to an increase in stacking faults, which suppresses the martensitic transformation and triggers the nucleation of the weak first-order phase under the external stress. The study provides insights into the interplay between superelasticity and structural transformation in shape memory alloys, and it is also instructive for understanding the anomalous high-temperature quasi-linear superelasticity in functional materials.