The stress-induced martensitic transformation of aged ferromagnetic shape memory Fe-Ni-Co-Ti-Cu alloys

Under certain conditions, alloys with thermoelastic martensite exhibit superelastic properties. The necessury conditions for the realization of superelastic deformation are a small volume effect DV/V of the martensitic transformation (MT), the presence of a shear strain component, and a high yield strength s^s of matrix phase.

Two polycrystalline Fe-Ni-Co-Ti-Cu alloys, namely Fe-15,2Ni-37,0Co-8,0Ti-6,17Cu, Fe-15,2Ni-36,6Co-6,72Ti-7,62Cu (wt.%) were investigated. The MT temperatures were determined from the temperature dependences of the electrical resistance r(T) and low-field magnetic susceptibility c(T). The influence of austenite ferromagnetism on the volume change DV/V at the starting temperature M_s of forward a«g MT was studied using dilatometric technique. Small elasticity modulus of the austenite below Curie temperature T_c is favoured superelastic behaviour of investigated alloys. The superelastic characteristics are affected also by the duration of austenite annealing and deformation temperature.