Role of microstructure heterogeneity on deformation behaviour in additive manufactured Ti-6Al-4V

The influence of two different bi-lamellar microstructures, with varied volume fractions of primary alpha lamellae (alpha p) and secondary alpha lamellae (alpha s), on the mechanical performance of laser powder bed fusion fabricated Ti-6Al-4V has been investigated. A low and high alpha p volume fraction variant was created by different solution heat treatment temperatures. Scanning transmission electron microscopy (STEM) was performed on both materials to give a comprehensive assessment of the local microstructure and element partitioning effects. Deformation experiments, combined with high resolution digital image correlation (HRDIC), enabled a detailed comparison of 2D slip patterns at progressive strain increments during tensile loading, with the underlying microstructure collected using electron backscattering diffraction based grain orientation mapping. In both microstrural variants, shear strain was mainly observed in the coarse alpha p lamellae, suggesting that the constituent was softer than the fine alpha s regions. This strength difference is not attributed to potential different levels of Albased solid solution strengthening as a result of element partitioning. Instead, the strength difference was attributed to the fine beta ligaments formed in-between the alpha s during ageing. The low alpha p volume fraction variant, i.e. high-volume fraction of alpha s, displayed a significant increase in yield strength compared to the high alpha p volume fraction variant without compromising ductility through increased heterogeneous plasticity and an enhanced strain hardening rate.