On the influence of heterogeneous precipitation on martensitic transformations in a Ni-rich NiTi shape memory alloy

Recently, a new explanation was given for the evolution of DSC chart features after ageing of Ni-rich NiTi alloys. It was shown that the 2–3–2 evolution of DSC chart features can be fully rationalised on the basis of a microstructural scenario, where metastable Ni-rich Ni4Ti3 particles precipitate on and near grain boundaries in a material, which was solution annealed and subsequently aged. A detailed description of the multiple step martensitic transformation has been given in the literature. However, this was only experimentally validated for one ageing temperature (500 °C). In the present study we show that heterogeneous precipitation is also a dominant microstructural feature when ageing is performed at 400 and 450 °C and that the microstructural scenario described previously also accounts for the phase transformation characteristics after lower temperature ageing. Long-term ageing results in large and widely spaced precipitates which no longer directly affect martensitic transformations.