In-situ nano-reprecipitation enables superior cryogenic mechanical properties in a 3D printable medium-entropy alloy.

Abstract

Precipitates or second-phase particles effectively strengthen metallic materials. Their use in additive manufacturing (AM), however, is constrained by the narrow thermal window of laser processing, which restricts control over precipitate characteristics. Here, we overcome this problem by creating a novel cryogenically strong and ductile alloy by a size-dependent composite powder feedstock design consisting of CoCrNi medium-entropy alloy core powders and decorated TiC shell nanoparticles. We show that these compound particles can be completely dissolved into the alloy during the first laser pulse of AM and then reprecipitate in situ upon subsequent cyclic reheating. Consequently, we introduce a high number density of nanosized TiC precipitates, coherent with the matrix. Their strong resistance to dislocation glide increases tensile strength by ~312 MPa and promotes dislocation accumulation without inducing severe stress heterogeneity or premature cracking, yielding a high strength-ductility product at 87 K. Our in-situ nano-reprecipitation strategy is achieved by laser-induced melting of pre-assembled nanoparticles and their kinetically favorable reprecipitation, which is applicable to other AM alloy systems to enhance their cryogenic mechanical properties.