Multiple-step martensitic transformations in Ni-rich NiTi alloys -: an in-situ transmission electron microscopy investigation

Multiple-step martensitic transformations in Ni-rich NiTi shape memory alloys have so far been rationalized on the basis of dislocation stress fields, coherency stress fields around Ni4Ti3 precipitates and evolving Ni concentrations between precipitates during ageing. The primary objective of the present paper is to show that such transformations can also occur owing to heterogeneous microstructures that form during ageing of solution annealed defect-free materials. These microstructures are characterized by Ni4Ti3 grain-boundary precipitation and by precipitate-free grain interiors. Two microstructures which give rise to two and three distinct differential scanning calorimetry (DSC) peaks on cooling from the B2 regime are subjected to in-situ cooling and heating cycles in the transmission electron microscope. Martensitic transformations are directly studied and the observations provide a new explanation for multiple-step martensitic transformations in Ni-rich NiTi alloys. Most importantly the results of the present study allow us to understand why DSC chart features on cooling from the B2 regime change during ageing, where they lose their one-step character (after solution annealing at 1123 K for 900 s) and evolve from two-step (after ageing at 773 K for 3.6 ks) to three-step (after ageing at 773 for 36 ks K) transformations.