Effect of nanoscale α2 precipitation on strain localisation in a two-phase Ti-alloy

Many commercial Ti-alloys contain 6 wt% Aluminium and these alloys are prone to precipitation of α₂ (Ti₃Al). Here, we investigate and quantify the effect of α₂ precipitation on strain localisation behaviour for Ti-6Al-4V with an equiaxed microstructure using High Resolution Digital Image Correlation (HR-DIC) in combination with Electron Back Scatter Diffraction (EBSD). HR-DIC has enabled us to quantify strain localisation, which shows that at 1% applied strain the strain heterogeneity in terms of maximum shear strain is about twice in the sample containing α₂ precipitates compared to the α₂-free sample. Theoretical slip trace angles for all possible slip systems were calculated using Electron Back Scatter Diffraction (EBSD) orientation data and cross-correlated with experimental slip trace angles measured from nanoscale shear strain maps recorded by HR-DIC to predict the active slip domain. It has been found that while slip type activity in terms of frequency is strongly dependent on texture in respect to loading direction, the actual shear strain contribution from prismatic slip does increase significantly in the presence of α₂ precipitation. This experimental observation supports previous calculations of Anti-Phase Boundary (APB) energies for α₂ precipitates [1] where widely dissociated partial dislocations on the prismatic plane show a lower APB energy than the APB energy associated with shearing on the basal plane in α₂.