Transformation softening in three titanium alloys

The high temperature flow stress data of Koike et al. (2000) determined on a Ti-5.5 wt% Al-1.5 wt% Fe alloy are reanalyzed in terms of transformation softening. They observed that the harder alpha transforms dynamically into the softer beta phase when deformation is being carried out below the beta transus temperature. These observations are interpreted here as being driven by the thermodynamic requirement to do the least possible work during deformation. Tests were carried out on a near-alpha Ti alloy (IMI-834) in order to test the generality of these results. Here the driving force for transformation is taken as the flow stress difference between the work hardened alpha and the yield stress of the fresh beta that takes its place. This type of analysis is then applied to the experimental results of Xu and Zhu (2010) on C.P. Ti grade 2, which also display marked sub-transus softening. Such behavior is shown to be consistent with the occurrence of transformation softening. (C) 2016 Elsevier Ltd. All rights reserved.