Anomalous anisotropy in an additively manufactured solid-solution-strengthened superalloy from room to elevated temperatures

Abstract

Metal additive manufacturing (AM) produces unique grain morphologies owing to the high cooling rates and large temperature gradients, which potentially lead to unexpected mechanical anisotropy. In this study, we unveil an anomalous anisotropic behaviour in a solid-solution-strengthened superalloy with periodic columnar-to-crescent grains fabricated by laser powder bed fusion (LPBF). Specifically, as-built (AB) specimens show higher strength perpendicular to the build direction (BD) than that parallel to the BD at room temperature (RT), while the opposite trend occurs at the elevated temperature (ET, 900°C). Besides, the heat treatment eliminates the anisotropy of strength at both RT and ET. A dislocation-based damage-coupled crystal plasticity finite element (CPFE) model with strain gradients is utilized to understand the origin of the above anomalous anisotropy. It is found that the transition of anisotropy from RT to ET is attributed to the temperature-dependent dislocation annihilation combined with initial dislocations in AB state. In contrast to the heat-treated specimens without anisotropy, the LPBF-induced residual deformation primarily contributes to the anisotropy at RT, whereas the initial dislocations dominate the anomalous anisotropy at ETs for AB specimens. The CPFE model reveals the threshold temperature to be 600°C for the occurrence of anomalous anisotropy, which is experimentally validated. This study presents a comprehensive understanding into temperature-dependent anisotropy of AM superalloys, and in turn guides the regulation of anisotropy by tuning microstructures.