Effects of composition and ausaging on microstructures and magnetic shape memory response of Fe-Ni-Co-Ti alloys

Abstract

Magnetic Shape-Memory (MSM) materials are a new category of actuator materials with the potential to produce both large actuator strain and rapid response. To explore the possibility of developing MSM materials based on Fe-Ni-Co-Ti alloys, the effects of chemical composition (Co/Ni ratio) and ausaging on microstructure and magnetic shapememory response were studied. A suitable choice of chemical composition and ausaging enabled the formation of martensite at or above room temperature. Most of the observed martensite morphology was lenticular or a mixture of lenticular and thin-plate. A specimen with the lowest martensite start temperature (203 K) exhibited a typical thin plate morphology, which is fully twinned and believed to be required for MSM materials. Ausaging treatments were effective in increasing hardness likely through the precipitation of (Fe,Ni,Co) 3Ti, and higher Co/Ni ratios accelerated the aging kinetics. Ausaging also increased the saturation magnetization (M) through a structural change from austenite to martensite, though the M, of martensite was relatively insensitive to the aging conditions. From these results, it can be expected that Fe-Ni-Co-Ti has the potential to show the same order of strains to Ni2MnGa if the magnitude of transformation strain is the same. An Fe-Ni-Co-Ti specimen with thin-plate martensite exhibited a significant conventional shape-memory effect. However, no significant magnetic field-induced strains were observed, even in a specimen with thin-plate martensite, and this result is thought to be due to the random grain orientation in the polycrystalline materials studied.