On the Influence of Alloy Compositions on Martensitic Transformations in NiTi and NiTiCu Shape Memory Alloys

In the present work, we provide precise data sets on compositional effects on phase transformation properties in NiTi and NiTiCu. It is well known that the phase transformation temperatures in binary NiTi shape memory alloys strongly decrease with increasing Ni-content. A large experimental scatter was observed in previous studies and even today, high precision reference data are still missing. NiTi and NiTiCu ingots with different Ni- and Cu-levels were produced using an optimized melting procedure and analyzed by differential scanning calorimetry. We show why it is important to document details of the melting procedure and how precise Ni-concentrations of the NiTi(Cu) matrix can be obtained. We provide clear experimental evidence showing that the predictions of Tang et al. (Acta Mat. 1999) on the increasing deviations from a linear relation between the thermodynamic equilibrium temperature and the Ni-content are correct. We show that increasing Ni-contents not only affect phase transition temperatures but moreover result in decreasing widths of thermal hysteresis and in decreasing latent heats. We rationalize these findings on the basis of crystallographic data by Prokoshkin et al. (Acta Mat. 2004) in the light of the theory of Ball and James (Phil. Trans. Royal Soc. A 1992). For NiTiCu alloys, it was found that – in contrast to literature data – Cu additions only slightly reduce the effect of changes in Ni-level on transformation temperatures.