Superelasticity and Srain-induced Magnetization Change of Ferromagnetic Co-Ni-Al Shape Memory Alloy

Ferromagnetic shape memory alloys (FSMAs) have received considerable attention as sensor/actuator materials with multi-functional characterizations. Recently, Oikawa et al. have developed Co-Ni-Al β (B2)-based alloys as a new group of FMSAs. A single crystal of the Co-Ni-Al alloy shows a reversible magnetic-induced strain of about 0.06% when slight pre-deformation is applied to the β austenite phase. Furthermore, the Co-Ni-Al alloys with the β+ƒng (A1) two-phase structure show a good ductility and also exhibit shape memory effect (SME) and superelasticity (SE) in polycrystalline state.

In this study, the present authors found that SE properties of the β+g two-phase Co-Ni-Al polycrystalline alloys are enhanced by microstructural controlling and that a large SE strain of over 6% can be obtained in a cyclic tensile test. Furthermore, in the Co-Ni-Al β-based alloys, since magnetic properties parent β phase with B2 structure are different from those of martensite β’ phase with L10 structure at the same composition, this alloy with SE exhibits the reversible strain-induced magnetization change due to the stress-induced martensitic (SIM) transformation and its reverse transformation. This unique magnetic propertiy is expected to be applied to a new type of strain sensors which can detect some strain introduced in the deformed materials through magnetization change.