Investigation of microstructure and failure mechanisms at room and elevated temperature of Hastelloy X produced by laser powder-bed fusion

Laser powder-bed fusion (LPBF) technique has broad prospects in lightweight structure designing and rapid manufacturing. In this study, solution treatment (ST), hot isostatic pressing (HIP), and HIP+ST were performed on crack-free LPBF Hastelloy X. The corresponding microstructure, precipitates, and mechanical properties after different heat treatments were compared to identify the critical factor affecting the fracture mechanisms at room and elevated temperature. The specimens exhibited a high recrystallization degree under all heat treatment conditions, which had little contribution to the performance differences. However, different holding time and cooling rate led to variations in carbide distribution. Granular M6C carbides were formed in ST specimens, while linear M23C6 carbides were formed in HIP specimens. In contrast, barely no carbides were observed in the HIP+ST specimen. At room temperature, all specimens showed similar tensile strength, while the ranking of elongation was HIP+ST=ST>HIP. The decreased elongation of the HIP specimen was attributed to the linear carbides at grain boundaries. However, the ranking of elongation was presented as HIP>HIP+ST>ST at high temperature. Carbides at grain boundaries coarsened and affected the crack propagation behaviors. The HIP specimen had appropriate carbide size, which restricted the crack propagation within a single grain boundary, forming wedge-shaped cracks. When the carbide was too small or oversized, crack propagated along the grain boundaries and reduced the plasticity. This work demonstrated that the carbide size was a critical factor for the performance control. In addition, the room temperature performance was insufficient for evaluating suitable heat treatment conditions for LPBF Hastelloy X.