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

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 Delta 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 Delta 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. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.