The effect of β grain coarsening on variant selection and texture evolution in a near-β Ti alloy

Abstract

In the present study, the role of β grain coarsening on α variant selection has been investigated in the near-titanium alloy Ti–21S (Ti–15Mo–3Nb–3Al–0.21Si). The material was first thermomechanically processed in a fully β stabilised condition in order to obtain a fine β grain size before undertaking controlled β grain-coarsening heat treatments. Two different cooling regimes ensured that either all β was retained at room temperature or significant α formation was achieved during cooling with predominant nucleation from β grain boundaries. Detailed electron backscatter diffraction (EBSD) characterisation was carried out on the β quenched and slowly cooled samples in order to compare the predicted α texture based on the β texture measurements assuming no variant selection with the measured α textures. A strong correlation was found between β coarsening and level of variant selection. It was also found that the grain coarsening is driven by the predominant growth of low energy grain boundaries, which strengthen specific β texture components that are part of the 〈1 1 1〉∥ND γ fibre. Finally, it was possible to demonstrate that the strengthened β texture components promote β grain pairs with a common 〈110〉, which is known to enhance variant selection when α nucleates from β grain boundaries.