Elevated-temperature mechanical properties of precipitates-hardened (NiCoCr)99.25C0.75 at% medium-entropy alloy fabricated by laser powder bed fusion

Abstract

A (NiCoCr)_99.25C_0.75 at% medium entropy alloy (MEA) was fabricated by using a laser powder bed fusion (LPBF) process and a precipitation hardening heat treatment was performed. The microstructure, room temperature to high temperature (25–800 °C) mechanical properties, and deformation behavior of the heat-treated MEA (hereinafter referred to as 0.75 C MEA HT) were investigated. LPBF-built 0.75 C MEA HT comprised a face-centered cubic single-phase solid solution matrix and nano-sized particles (Cr-rich carbides). A high-density dislocation network and a substructure decorated with nanoparticles were observed inside the grains. LPBF-built 0.75 C MEA HT exhibited the best room temperature and elevated temperature properties of LPBF-built NiCoCr MEA reported thus far and higher elevated temperature strength than those of oxide dispersion strengthened (ODS) MA 956 and MA 754, which are typical nuclear power plant steels. LPBF-built 0.75 C MEA HT maintained excellent room temperature and high-temperature mechanical properties because of its heterogeneous structure, high dislocation density, and the reinforcing effect of nanoparticles. On the other hand, the deformation microstructure confirmed deformation twinning and stacking fault even at high temperatures, and some dynamic recrystallization occurred above 700 °C. The deformation and migration of grain boundaries were identified as the main mechanisms for recrystallization, and interaction with nanoparticles was predicted to occur. Based on these results, the room temperature and high-temperature deformation behaviors of LPBF-built 0.75 C MEA HT were discussed in relation to the microstructure.