Deformation twinning in AZ31: Influence on strain hardening and texture evolution

This paper describes the main results from an experimental investigation into the consequences of deformation twinning in AZ31 on various aspects of plastic deformation, including the anisotropic strain-hardening rates, the tension/compression yield asymmetry, and the evolution of crystallographic texture. It was seen that AZ31 exhibited unusually high normalized strain-hardening rates compared to α-Ti that occurred beyond the strain levels where extension twins have completely altered the underlying texture. This observation challenges the validity of the generally accepted notion in the current literature that the high strain-hardening rates in AZ31 are directly caused by extension twins. It is postulated here that the thin contraction twins are very effective in strain hardening of the alloy by restricting the slip length associated with pyramidal 〈c + a〉 slip. This new hypothesis is able to explain the major experimental observations made in this study and in the prior literature. We have also presented a new hypothesis for the physical origin of the observed differences in the thicknesses of the extension and contraction twins. The stress fields in selected matrix-twin configurations were modeled using crystal plasticity finite element models. The contraction twin (01¯11)[01¯12¯] was predicted to form an internal extension twin (011¯2)[01¯11], resulting in the commonly observed “double twin” sequence. The extension twin is suggested to inhibit thickening of this double twin by loss of twin-matrix coherency. Extension twins were predicted to retain their coherency and thus thicken.