Stress-induced FCC-HCP martensitic transformation of rapid-solidified Co-Ni Foil

Ferromagnetic Co-Ni alloys of less than 35 at%Ni exhibit a phase transformation from γ-phase (FCC) to ε phase (HCP): both phases are stable at higher and lower temperatures, respectively. Since both phases of these alloys exhibit high magnetization and thermoelastic martensitic transformation, it is expected as new ferromagnetic shape memory alloy. In this study, We prepared a rapid-solidified Co-32 at%Ni foil and analyzed thermoelastic and stress-induced martensite phase in temperature range of 77 to 673 K from observation of the texture, structure, magnetic property and Barkhausen noise. From these results, it is found that 1) the thermoelastic martensitic transformation is not reversible and 2) a stress-induced martensite phase is transformed to a austenite phase by heating. The result is caused by the mechanism of the transformation from a γ-phase (HCP) to ε-phase (FCC).