Martensitic Transformation in Shape Memory Alloys under Magnetic Field and Hydrostatic Pressure

Abstract

Effects of magnetic field and hydrostatic pressure on martensitic transformation have been examined by using Fe–Pt, Cu–Al–Ni, Ni₂MnGa and Fe–Pd shape memory alloys and Fe–Ni alloys. Following results are obtained; (i) in Fe–Pt, Cu–Al–Ni and Fe–Ni alloys, the experimental magnetic field and/or hydrostatic pressure dependence of martensitic transformation start temperature, M_s, is in good agreement with the dependence calculated by the equation previously proposed by our group to evaluate the relation between M_s and magnetic field and hydrostatic pressure. (ii) Giant magnetostriction has been observed in the martensite state of Ni₂MnGa and Fe–31.2 at%Pd ferromagnetic shape memory alloy single crystals. The values (contraction of 3.8% for Ni₂MnGa and expansion of about 3% for Fe–31.2 at%Pd) are nearly the same values as can be expected from the perfect conversion of variants, i.e., variants are converted to preferable variants whose magnetocrystalline anisotropy energy is minimum among them under the magnetic field.