Experimental Observation of Vacancy-assisted Martensitic Transformation Shift in Ni-Fe-Ga Alloys.

Positron annihilation lifetime spectroscopy is used to experimentally demonstrate the direct relationship between vacancies and the shift of the martensitic transformation temperature in a Ni_{55}Fe_{17}Ga_{28} alloy. The evolution of vacancies assisting the ordering enables shifts of the martensitic transformation up to 50 K. Our results confirm the role that both vacancy concentration and different vacancy dynamics play in samples quenched from the L2_{1} and B2 phases, which dictate the martensitic transformation temperature and its subsequent evolution. Finally, by electron-positron density functional calculations V_{Ni} is identified as the most probable vacancy present in Ni_{55}Fe_{17}Ga_{28}. This work evidences the capability of vacancies for the fine-tuning of the martensitic transformation temperature, paving the way for defect engineering of multifunctional properties.