Multiple effects of Ni-rich precipitates in Ni–Ti–Al thin films

Phase transformation and mechanical properties of two distinct Ni-rich Ni–Ti–Al thin films deposited at various temperatures were investigated in this paper. It is found that phase transformation temperatures and hardness of the films obviously increase with growth of Ni-rich precipitates, and their phase transformation hysteresis decreases. Especially, martensitic transformation start temperature of the Ni49.7Ti45.3Al5 films exceeds room temperature and increases by 20 °C, and the Ni44Ti32Al24 Heusler alloy films show near-zero transformation hysteresis. Besides, hardness of the two films increases by 12.8% and 22%, respectively. The evolutions are caused by the growth of Ni-rich precipitates which strengthens the matrix. The precipitation would sharply consume Ni content of the matrix that changes phase transformation behavior from the Ni-rich to Ti-rich films.