Selective Laser Melting-Induced Microstructure and Mechanical Property Change of Hastelloy X at Room and Elevated Temperature

Abstract

The industry-interested superalloy alloy Hastelloy X was fabricated by selective laser melting (SLM), and heat treatments have been designed to optimize the microstructure for high temperature structural applications with better mechanical property. Specimens of as-printed, aging treatment (AT), solution treatment (ST), and wrought conditions are of interests to this study, and their detailed microstructures of grains, dislocations, grain boundaries, precipitates, and twins and defects have been characterized. In comparison to the wrought specimens, the as-printed specimens showed sub-micron cellular microstructure, while the secondary phases of M23C6-type carbides and Fe2Mo were found for AT condition with partial recrystallization. The ST treatment brought a large number of high angle grain boundaries (HAGBs) and a high twin boundary ratio due to dynamic recrystallization (DRX). With this, tensile tests were collected at both room temperature (RT) and elevated temperature (ET) of 850 °C, and their post-mortem microstructures were added to correlate the mechanical performance. The AT condition yielded the highest ultimate tensile strength (UTS) of ~1067 MPa, while the ST treatment gave the highest ductility of ~51% at RT. At ET, all the SLM specimens in as-printed, AT, and ST states showed a little improvement in UTS over the wrought specimens, but their ductility was all less. With these observations, a comprehensive heat treatment roadmap has been proposed to summarize the microstructure-property relationship in SLM and wrought specimens, which even rigorously included the traditional ST+AT heat treatment for Hastelloy X and proved its incompatibility. This research not only deepens our understanding of its mechanical behavior after various feasible heat-treatments but also provides a new semi-quantitative guidance to fabricate Ni superalloys with a good combination of strength and ductility even for high-temperature applications.

Keywords: selective laser melting, Hastelloy X alloy, Microstructure, heat treatment, High-temperature properties