Microstructure Evolution and Nanomechanical Behavior of Micro-Area in Molten Pool of Selective Laser Melting (CoCrNi)82Al9Ti9 High-Entropy Alloy

Abstract

In this work, the phase evolution mechanism and nanomechanical properties of (CoCrNi)₈₂Al₉Ti₉ high-entropy alloy (HEA) prepared by selective laser melting (SLM) in the molten pool were studied. This HEA contains multiple primary elements and undergoes high-temperature gradient and rapid cooling during SLM. This leads to significant inhomogeneity of nano-scale microstructure characteristics and instability of properties. After optimizing process parameters, the microstructure evolution at the optimal parameter volume energy density of 440 J/mm³ was studied. A phase transition from BCC to FCC occurred in the melt micro-zone. Remelting the micro-area of the melt pool results in a temperature rise and the combustion-induced loss of Al elements. Moreover, the Ni element content increases significantly outside the melt pool. This process enhances the phase stability of FCC and facilitates phase transitions. Additionally, rapid cooling leads to the formation of distinctive ultrafine equiaxial crystals inside the melt pool, accompanied by the generation of intracrystalline needle-like nano-scale phases. Outside the melt pool, the accumulation of energy results in the formation of coarse dendrites. Therefore, the nano-hardness inside the molten pool is remarkably high at 11.79 GPa, while the outside the molten pool is reduced to 9.58 GPa. And the fracture toughness outside the melt pool also decreased. Comparing with inside the melt pool, the residual stress outside the melt pool changed from compressive to tensile stress and decreased from 603.28 to 322.84 MPa.