Microstructure and properties of the welding heat-affected zone in CoCrNi medium-entropy alloy

To analyze the microstructure and mechanical properties of different regions for the welding heat-affected zone (HAZ) of CoCrNi medium-entropy alloy (MEA), the thermal simulations were conducted at peak temperatures of 800 °C, 1000 °C, and 1200 °C to study the microstructure and mechanical properties of the samples after experiencing the welding thermal cycle. The results indicate that during the welding thermal cycle, the atoms in the as-cast CoCrNi MEA rearrange under the influence of temperature, this leads to the diffusion of Cr atoms with larger atomic radius to the lattice positions of the (111) crystal planes, optimizing thecrystal structure. Due to the lattice distortion present in the as-cast CoCrNi MEA, the recrystallization occurs as the peaktemperature of thermal cycle increases. The combined effects of recrystallization and atomic rearrangement shift the coarse crystalembrittlement phenomenon in the high-temperature zone of the traditional alloy welding HAZ towards the low-temperature zone.At a peak temperature of 800 °C, the microstructure is composed of original grains and coarse grains, and "embrittlement"phenomena appear in the welding HAZ. With an increasing of peak temperature, the recrystallization processes take place, and themain XRD diffraction peak shifts to the (111) crystal plane. The atomic rearrangement and recrystallization significantly affectedthe microstructure and properties of the CoCrNi MEA in the welding HAZ, with the impact toughness at 1200 °C increasing to439.79 J/cm 2 , and the tensile strength and elongation increases to 642.9 MPa and 65.5%, respectively.