Ultrastrong and ductile additively manufactured precipitation-hardening medium-entropy alloy at ambient and cryogenic temperatures

Abstract

Strong and ductile precipitation-hardening face-centered cubic medium-entropy alloys (MEAs) are potential structural material candidates for cryogenic applications, which, however, are rarely reported in the field of additive manufacturing. In this work, we develop a high-performance (CoCrNi)94Al3Ti3 MEA via additive manufacturing and age hardening. Superior tensile strength-ductility combinations, in comparison to other reported additively manufactured face-centered cubic metals, are achieved at both ambient (293 K) and cryogenic (103 K) temperatures. This is attributed to the hierarchical microstructure with a high degree of heterogeneity in terms of grain size, cellular substructure and characteristics of nanoprecipitates (L12 phase). Such microstructure feature leads to a higher ambient-temperature yield strength (921.1 MPa) and ultimate tensile strength (1346.4 MPa) than the as-printed sample that possesses a homogeneous microstructure. Moreover, the precipitate shearing mechanism, hetero-deformation induced hardening effect and deformation-induced stacking faults-based substructure evolution jointly result in a high and persistent strain hardening ability, which ensures a high ductility (27.2% at ambient temperature). The testing at the cryogenic temperature promotes the efficiency of hetero-deformation induced hardening and the formation of stacking faults, leading to an excellent strength-ductility combination (tensile strength 1702.9 MPa and ductility 25.4%). However, unlike the simultaneous increment of strength and ductility for the as-printed sample when the testing temperature decreases from 293 K to 103 K, the ductility of the age-hardened sample at different temperatures only changes slightly. This feature is related to the severe strain/stress concentrations developed within the heterogeneous microstructure of the age-hardened sample at 103 K. Our approach of introducing coherent nanoprecipitates in the additively manufactured microstructure provides a new insight into the development of high-performance MEAs for cryogenic applications.