Strong influence of ferromagnetic ordering and internal pressure on the elastic modulus of shape memory alloy

In the present article two effects, which were observed previously in the ferromagnetic shape memory alloys, are explained using the concept of internal pressure influencing the crystal lattice. The first effect is the noticeable change of the temperature dependence of Young's modulus at the Curie point and the second one is the quasi-second-order martensitic transformation of the alloy. It has been shown that the larger the volume change, which accompanies the martensitic transformation of the alloy, the more pronounced the influence of internal pressure on the second- and third-order elastic modulus of the alloy. If the volume change is larger than or equal to 0.5%, the magnetoelastic pressure in the Ni-Mn-Ga alloy is large enough to induce the experimentally observed change of the temperature dependence of Young's modulus at the Curie point. The internal pressure created by the structural defects of the alloy has been considered. It has been shown that in the case where the internal pressure destabilizes the martensitic phase, the martensitic transformation becomes similar to a second-order phase transition. (C) 2012 Elsevier BY. All rights reserved.