Correlation between atomic order and the characteristics of the structural and magnetic transformations in Ni-Mn-Ga shape memory alloys

The evolution of the degree of L21 long-range atomic order and its influence on the martensitic transformation and Curie temperatures of a near-stoichiometric Ni2MnGa shape memory alloy were studied during several thermal treatments. Both temperatures depend on the quenching temperature and increase on subsequent heating treatments. This increase is concurrent with the presence of an exothermic peak in differential scanning calorimetry measurements. Neutron diffraction experiments were carried out in order to correlate the evolution of the transformation temperatures with the evolution of the L21 order degree, which was estimated through the intensity of the (1 1 1) and (2 0 0) reflections. The results confirm that the quenched-in degree of long-range atomic order depends on the quenching temperature and prove that the evolution of the transformation temperatures is due to the evolution of the atomic order