Cryogenic temperature tensile properties of laser powder bed fused Ti-6Al-4V

Abstract

Unnotched and notched tensile properties of Ti-6Al-4V (Ti64) alloy, additively manufactured using the laser powder bed fusion (LPBF) technique, at cryogenic temperatures of 90, 77 and 20 K were investigated. The LPBF process parameter combination was chosen such that the investigated alloy has not only a minimum in porosity, but also an equiaxed prior β microstructure, which predominantly contains acicular α/α’ lath structure in basket-weave morphology, that results in a high strength and ductility combination as well as insignificant mechanical anisotropy at room temperature (300 K). Tensile tests revealed that the yield (σy) and tensile (σu) strengths of LPBF Ti64 are superior while elongation to failure (ef) is comparable to those of the conventionally manufactured (CM) Ti64 down to 77 K, owing to the fine α/α’ lath microstructure in the former. While the progressive reduction of <a>-basal and prismatic dislocation slip activity with decreasing deformation temperature enables a steep rise in σy, the unnotched specimens fractured catastrophically without macroscopic yielding at 20 K. Detailed postmortem analyses revealed the absence of significant twinning-based deformation in LPBF Ti64, in stark contrast to the CM ones at cryogenic temperatures. Instead, deformation kink bands with varying sizes and misorientation were found to be increasingly active within the prior β grains. A large misorientation at the kink band boundary within the β grain leads to void nucleation followed by crack growth, which eventually results in brittle failure at 20 K. While relatively notch insensitive from 300 to 77 K, LPBF Ti64 was notch brittle at 20 K. These results highlight the importance of the unique hierarchical micro- and meso-structural features of LPBF Ti64 on the tensile mechanical behavior, especially at cryogenic temperatures.