The role of anisotropic thermal expansion of shape memory alloys in their functional properties

The functional properties of a ferromagnetic shape memory alloy, which undergoes martensitic transformation of a cubic-tetragonal type, have been analyzed using the Landau theory of phase transitions. A key role of the temperature dependency of the crystal lattice parameters in the formation of the elastic and magnetic properties of the martensitic phase has been determined. To this end the temperature dependencies of the shear elastic moduli and magnetic anisotropy energy densities (MAED) of two quasi-stoichiometric Ni-Mn-Ga alloys have been computed from the experimental temperature dependencies of their lattice parameters. It turned out that the differences in the temperature dependencies of the lattice parameters of these alloys gave rise to drastic differences in their shear moduli and MAED as a function of temperature. The possibility of the spontaneous reorientation of the magnetization vector on cooling of the alloy and a right-angle realignment of this vector by a low magnetic field have been predicted. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.