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 (500degreesC). In the present study we show that heterogeneous precipitation is also a dominant microstructural feature when ageing is performed at 400 and 450degreesC 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. (C) 2004 Elsevier B.V. All rights reserved.