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 (αp) and secondary alpha lamellae (αs), on the mechanical performance of laser powder bed fusion fabricated Ti-6Al-4V has been investigated. A low and high αp volume fraction variant was created by different solution heat treatment temperatures. Scanning transmission electron microscope (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 αp lamellae, suggesting that the constituent was softer than the fine αs regions. This strength difference is not attributed to potential different levels of Al-based solid solution strengthening as a result of element partitioning. Instead, the strength difference was attributed to the fine β ligaments formed in-between the αs during ageing. The low αp volume fraction variant, i.e. high-volume fraction of αs, displayed a significant increase in yield strength compared to the high αp volume fraction variant without compromising ductility through increased heterogeneous plasticity and an enhanced strain hardening rate.