Behaviour of Ni-Mn-Ga alloys under mechanical stress

In magnetically controlled shape memory (MSM) materials deformation can be controlled by magnetic field. It is shown that the magnetic field induced strain is directly connected with the twinning stress, i.e. mechanical stress needed to transform the sample to single variant state. In cases where the magnetically-induced stress is larger than the twinning stress, a large field-induced strain can be obtained in the magnetic field. The behaviour under mechanical stress of three single crystalline Ni-Mn-Ga alloys with different compositions was systematically investigated. The twinning stresses and deformations of these alloys and particularly their dependencies on temperatures were determined by compression tests. The present study was carried out by compression tests of the alloys in martensitic phase at different temperature. Results showed that twinning stress decreases with increasing temperature in all studied alloys.