Microstructure and transformation temperatures of Si-added Ni–Ti–Cu and Ni–Ti–Zr shape memory alloys

The effect of Si on the martensitic transformation in Ni₄₄Ti₅₀Cu₆ and Ni₅₀Ti₃₀Zr₂₀ was investigated in the present study. The martensite transformation temperature of the two alloys decreased with the increase of Si content. In particular, a large decrease was observed for Ni₅₀Ti₃₀Zr₂₀. With the addition of 0.5 at% Si, the initial martensite temperature of Ni₅₀Ti₃₀Zr₂₀ decreased by 238.2 K, compared to that of Ni₄₄Ti₅₀Cu₆, which decreased by only 16.4 K. This significant decrease is attributed to the high sensitivity of the valence electron concentration and the decrease in the lattice parameter caused by the addition of Si, leading to the very large increase of stability of B2 with addition of Si in Ni₅₀Ti₃₀Zr₂₀.