Ageing-induced two-stage R-phase transformation in Ti-50.9at.%Ni

This study investigated the effects of low temperature ageing on the transformation behaviour of a Ti - 50.9at.%Ni alloy. It was found that ageing in the temperature range of 473-573 K induced a two-stage R-phase transformation, which was followed by a single-stage martensitic transformation at a lower temperature. The transformation behaviour was analyzed by means of differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). DSC measurements revealed two separate transformation heat peaks, each with a small thermal hysteresis. XRD measurements demonstrated a two-stage evolution of diffraction intensities from the B2 phase and from the R-phase. Transformation temperatures determined from these measurements exhibited good correlation. TEM examinations revealed the presence of fine coherent Ti3Ni4 precipitates. Based on these observations it is identified that the first R-phase transformation on cooling was associated with the formation of the precipitates whereas the second R-phase transformation at a lower temperature was from the matrix away from precipitates. The martensitic transformation was associated with the second R-phase transformation at the lower temperature, i.e., it was an incomplete transformation from the regions away from precipitates. The occurrence of the multiple-stage R-phase transformation is attributed to precipitation-induced inhomogeneity of the matrix, both in terms of composition and of internal stress fields. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.