Analytical model for field-induced strain in ferromagnetic shape-memory alloy polycrystals

Abstract

Single crystals of Ni–Mn–Ga have shown promising properties for actuators with field-induced strains up to 6% [Murray et al., Appl. Phys. Lett. 77, 886 (2000)]. There is interest in achieving large field-induced actuation in textured polycrystals as well as in composites of oriented Ni–Mn–Ga particles in a polymer matrix. A simple analytical model has been developed to predict an upper limit to the field-induced strain achievable in ensembles of particles having specified orientations distributions. It is found that in a uniform distribution of orientations (no texture) the strain can reach at most 21% of the single-crystal strain for a given field. Distributions in which the twin plane normal subtends a fixed angle of 55° with the field direction have upper bounds for the strain of 38%. Similarly, orientation distributions in which the normal to the twin plane is the same for all crystals have an upper bound of 50%.