Induced Martensitic Transformation in Ferromagnetic Co-Ni-Al Shape Memory Alloys

The martensitic transformations induced by temperature variation and uniaxial tension effect have been studied in ferromagnetic Co-Ni-Al shape memory alloys. The characteristic temperatures of martensitic transformations have been established from the temperature dependencies of electrical resistance and low-field magnetic susceptibility. Co_45,4Ni_39,7Al_14,9 (at.%) alloy has sufficiently low temperatures of the structural and magnetic phase transitions and is characterized by low ductility. The superelastic behavior have been investigated at loading in the temperature range as it approaches the structure transition temperature. The phase diagram of formation of stress-assisted martensite on strain temperature have been plotted. According to the Clausius-Clapeyron equation for the first order phase transition there is a tendency to reduce the limit of martensite formation induced by stress as it approaches the martensite transformation temperature, while Young's modulus of the material is virtually unchanged. To visualize the martensite transformation behavior under the stress, the co-registration of cyclic stress-strain curves with simultaneous recording of low-field magnetic susceptibility was proposed.