Homogeneous and heterogeneous deformation mechanisms in an austenitic polycrystalline Ti-50.8 at.% Ni thin tube under tension. Investigation via temperature and strain fields measurements

An initially austenitic polycrystalline Ti-50.8 at.% Ni thin-walled tube with small grain sizes has been deformed under tension in air at ambient temperature and moderate nominal axial strain rate. Temperature and strain fields were measured using visible-light and infrared digital cameras. In a first apparently elastic deformation stage, both strain and temperature fields are homogeneous and increase in tandem. This stage is followed by initiation, propagation and growth of localized helical bands inside which strain and temperature increases are markedly higher than in the surrounding regions. During the first apparently elastic stage of the unloading, both strain and temperature fields are homogeneous and decrease. The temperature and strain fields evolutions are then analysed in order to determine the deformation mechanisms (types and extents of phase transformations, variants (de)twinning, macroscopic banding) involved during the homogeneous and heterogeneous stages of deformation throughout the whole tube. The findings have significant implications for the understanding and modelling of superelastic behaviour of NiTi shape memory alloys. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.