The mechanism of multistage martensitic transformations in aged Ni-rich NiTi shape memory alloys

Usually aged Ni-rich NiTi alloys undergo martensitic transformations on cooling from high temperatures in two steps: B2 to R and then R to B19' (normal behaviour). But under certain ageing conditions, the transformation can also occur in three or more steps (unusual multiple step behaviour). In the present study we use differential scanning calorimetry (DSC) for a systematic investigation of the evolution of transformation behaviour with ageing temperature and time. We demonstrate that during ageing of Ni-rich NiTi alloys, DSC curves exhibit two transformation peaks on cooling after short ageing times. three after intermediate ageing times and finally again two peaks after long ageing times (2-3-2 transformation behaviour). In the present study we propose a new explanation for the 2-3-2 transformation behaviour which consists of two basic elements: (1) The composition inhomogeneity which evolves during ageing as Ni4Ti3 precipitates grow. (2) The difference between nucleation barriers for R-phase (small) and B19' (large). These two elements explain all features of the evolution of DSC charts during ageing including the number of distinct DSC peaks and their positions. (C) 2002 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.