Microstructures and mechanical properties of laser-directed energy deposited CrCoNi medium-entropy alloy

The distinctive intrinsic heat treatment (IHT) originating from cyclic reheating in the laser-directed energy deposition (LDED) has attracted growing attention in recent years. In this investigation, simulations and experimental characterizations were performed to examine the impact of IHT on the microstructure and mechanical properties of LDED-fabricated CrCoNi medium-entropy alloy (MEA). The results show that the intensity of the IHT is proportional to the utilized laser energy density (LED). As the LED increased, significant dynamic recrystallization and grain refinement occurred within the alloy due to the enhanced intensity of IHT. However, the high LED leads to severe hot cracking within the as-built MEA, resulting in inferior ductility. By decreasing LED, the hot cracking was effectively eliminated. Meanwhile, low LED weakened the intensity of IHT and consequently inhibited kinetic conditions of dynamic recrystallization, resulting in a heterogeneous grain structure characterized by multi-scale-sized grains. This structure provides significant hetero-deformation-induced hardening during plastic deformation, enabling the alloy to have a sustainable work-hardening capacity. We expect that this work will have implications in taking full advantage of the unique IHT of the LDED process to fabricate MEAs with excellent metallurgical quality and mechanical performance.