Room- and elevated-temperature mechanical property of selective laser melting-fabricated Hastelloy X with different heat treatments

Abstract

Superalloy alloy Hastelloy X was fabricated by selective laser melting (SLM), and heat treatments have been designed to optimize microstructures for better mechanical property at high temperature. Specimens of as-printed, aging treatment (AT), solution treatment (ST), and wrought conditions are of interests to this study, and their grains, dislocations, grain boundaries, precipitates, twins and defects have been characterized. Compared to the wrought specimens, the as-printed specimens showed sub-micron cellular microstructure, while M23C6-type carbides and Fe2Mo phases were found after AT condition with partial recrystallization. The ST treatment introduced a large number of high angle grain boundaries (HAGBs) and a high twin boundary ratio due to recrystallization. With this, their post-mortem microstructures at both room temperature (RT) and elevated temperature (ET) of 850 °C were easily correlated to the mechanical performance. AT yielded the highest ultimate tensile strength (UTS) of ∼1067 MPa, while ST gave the highest ductility of ∼51% at RT. At ET, the as-printed, AT, and ST SLM specimens showed improved UTS over the wrought counterparts, all with a reduced ductility. With these observations, a comprehensive heat treatment roadmap has been proposed to summarize their microstructure-property relationship, which rigorously included the traditional ST + AT heat treatment and proved SLM-fabricated Hastelloy X's incompatibility. This research not only deepens our understanding of its mechanical behavior after possible heat treatments, but also semi-quantitatively guides its feasible processing even for high-temperature applications.