The influence of carbon and oxygen content on the martensitic transformation temperatures and enthalpies of NiTi shape memory alloy

The martensitic transformation temperatures and enthalpies of NiTi shape memory alloy strongly depend on the content of nickel and on carbon and oxygen impurities. Nickel stabilizes the high-temperature phase while carbon and oxygen enrich the surrounding matrix with nickel. In this work it is shown that, as a consequence, the martensitic transformation temperatures and enthalpy changes are lower than specially prepared low-contamination reference samples. Furthermore, the enthalpy changes increase linearly with increasing peak martensitic transformation temperature. This means that the lower the enthalpies, the lower is the peak martensitic transformation temperature suggesting that at some lower temperature no phase change should occur.