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

J.I. Kim, Yinong Liu, S. Miyazaki

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    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.
    Original languageEnglish
    Pages (from-to)487-499
    JournalActa Materialia
    Publication statusPublished - 2004


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