Martensitic transformation and magnetic-field-induced strain in Ni-Mn-Ga magnetic shape memory alloys

Abstract
The present paper deals with the thermal- and stress-induced martensitic transformations, effects of bias magnetic field and external compressive stress on transformation- and magnetic-field-induced strain in Ni-Mn-Ga alloys. The effect of the external magnetic field and compressive stress on the reorientation of martensitic variants is discussed. It is found that the stress-induced spear-like martensitic crystal forms at the beginning of loading at the temperature a little higher than Ms. The interfaces between the parent phase and the stress-induced martensite migrate reversibly under the increasing/decreasing stress, showing the characteristics of thermoelastic martensitic transformation. When the Ni ₅₂Mn ₂₄Ga ₂₄ single crystal is cooled down to 200K, an intermartensitie transformation occurs, however there is no appearance of this case for the powder sample of the same alloy with average particle size ofabout 50  m. About 1.8% magnetic-field-induced strain is obtained in the [100] direction of Ni ₂MnGa singte crystal when a 1.5T pulse magnetic field is applied at the M _s temperature.