The effect of time on the evolution of the martensite structure and strain caused by magnetic field in Ni2MnGa shape memory alloys

Kinetics of the martensitic transformation, change in the preferential martensite variants, field-induced strain and magnetisation under applied magnetic field are studied in-situ. Two Ni2MnGa alloys of the non-stoichiometric composition which reveal large strain (0,5-5%) under magnetic field were investigated using the X-ray diffraction θ-2θ analysis, optical metallography and measurements of the field-induced strain. It is shown that evolution of the martensitic structure under applied magnetic field occurs with time at constant values of the field and temperature. The kinetics of the direct and inverse processes is strongly determined by the relation between the crystallographic direction of easy magnetisation and that of the applied magnetic field. The results of the X-Ray diffraction study are consistent with the data of optical microscopy and measurements of the field-induced strain