Microstructure, Properties, and Metallurgical Defects of an Equimolar CoCrNi Medium Entropy Alloy Additively Manufactured by Selective Laser Melting

Additive manufacturing of an equimolar CoCrNi medium entropy alloy (MEA) by selective laser melting (SLM) was investigated, emphasizing its microstructure, properties, and metallurgical defects. It was found that SLM sample density exhibited a non-monotonic relation with their volume energy density (VED); the density first increased but then decreased while the input VED was gradually increasing. A maximal relative density of 98.9 pct was accessible at a VED of 83.3 J/mm³. X-ray diffraction indicated that the printed CoCrNi MEA exhibited an FCC single-crystal structure where the lattice constant decreased with the increasing VED owing to the evaporation of the Cr element in the SLM process. The average grain size gradually increased with increasing VED, irrespective of whether viewed from the top or the side of the printed part. Similarly, the residual stress increased with increasing VED, which produced more microcracks and deteriorated the tensile preformation of the printed samples, although the yield strength (630 MPa) showed no apparent difference at different VEDs. Owing to the ultrafine cell structure, the mechanical property of the SLM printed sample was twice that of cast or wrought CoCrNi MEA.