On the effect of alloy composition on martensite start temperatures and latent heats in Ni–Ti-based shape memory alloys

Abstract In the present work we explain the concentration dependence of the martensite start temperature ( M S ) in Ni–Ti-based shape memory alloys (SMAs). We briefly review the present level of understanding and show that there is a need for further work. We then investigate the strong dependence of M S on alloy composition in binary Ni–Ti, ternary Ni–Ti–X (X = Cr, Cu, Hf, Pd, V, Zr) and quaternary Ni–Ti–Cu–Y (Y = Co, Pd) SMAs. For binary Ni–Ti, we combine differential scanning calorimetry experiments with insight gained through the application of the density functional theory (DFT) to show that heats of transformation Δ H decrease as Ni concentrations increase from 50.0 to 51.2 at.%. This causes a shift in the Gibbs free energy curves of austenite G A ( T ) and martensite G M ( T ), which in turn results in a lower M S temperature. Our DFT results suggest that the strong decrease of Δ H is caused by a stabilization of the B2 phase by structural relaxations around Ni antisite atoms, together with a gradual destabilization of B19′. The martensite start temperatures and the latent heats of transformation for binary, ternary and quaternary Ni–Ti-based SMAs are closely related. We observe smaller latent heats when the geometrical differences between the crystal structures of austenite and martensite decrease.