Site Preference and Alloying Effect of Excess Ni in Ni-Mn-Ga Shape Memory Alloys

The formation energies and electronic structures of Ni-rich Ni-Mn-Ga alloys have been investigated by first-principles calculations using the pseudopotential plane wave method based on density functional theory. The results show that the alloying Ni prefers to occupy the Mn site directly in Ni₉Mn₃Ga₄ and to occupy the Mn site and drive the displaced Mn atom to the Ga site in Ni₉Mn₄Ga₃, which is in accordance with the experimental result. According to the lattice constants and the density of states analyses, these site preference behaviours are closely related to the smaller lattice distortion and the lower-energy electronic structure when the excess Ni occupies the Mn site. The effect of Ni alloying on martensitic transformation is discussed and the enhancement of martensitic transformation temperature by Ni alloying is estimated by the calculated formation energy difference between austenite and martensite phases.