Strain hardening of titanium: role of deformation twinning

The purpose of this study is to investigate the role of deformation twinning in the strain-hardening behavior of high purity. polycrystalline alpha-titanium in a number of different deformation modes. Constant strain rate tests were conducted on this material in simple compression, plane-strain compression and simple shear, and the true stress (sigma)-true strain (epsilon) responses were documented. From the measured data, the strain hardening rates were numerically computed, normalized by the shear modulus (G), and plotted against both normalized stress and epsilon. These normalized strain hardening plots exhibited three distinct stages of strain hardening that were similar to those observed in previous studies on low stacking fault energy fee metals (e.g. 70/30 brass) in which deformation twinning has been known to play an important role. Optical microscopy and Orientation imaging microscopy were conducted on samples deformed to different strain levels in the various deformation paths. It was found that the onset of deformation twinning correlated with a sudden increase in strain hardening rate in compression tests. The falling strain hardening rate correlated with saturation in the twin volume fraction. In shear testing a much lower rate of strain hardening was found, at all strains, and this correlated with a lower density of deformation twinning. (C) 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.