Compositional regulation in additive manufacturing of precipitation-hardening (CoCrNi)₉₄Ti₃Al₃ medium-entropy superalloy : cellular structure stabilization and strength enhancement

Abstract

High or medium-entropy alloys that feature high thermal stability and excellent oxidation resistance are promising candidates for elevated temperature applications. The rapid softening of monolithic high or medium-entropy alloys with single face-centered cubic structure at elevated temperatures, however, is a main weakness. In this paper, we report new high strength γ′-hardened ((CoCrNi)94Ti3Al3)98Nb2 medium-entropy alloy through laser powder-bed fusion (L-PBF) followed by ageing. In particularly, the tensile strengths of the aged ((CoCrNi)94Ti3Al3)98Nb2 alloy at 20 °C and 700 °C can reach up to 1.93 GPa and 1.11 GPa, respectively, 112 % and 122 % stronger than the as-built CoCrNi alloy tested at the same condition. A new strengthening mechanism, i.e., elemental segregation induced the cellular structure stabilization, in tandem with other hierarchical microstructure features, including ultrafine γ′ precipitates, dense twin boundaries, and other types of crystallized defects, co-contribute to the superb tensile strength at room and elevated temperatures. Such a simple alloy design and processing strategy outlines a guideline for designing novel multicomponent alloys and/or composites with superior microstructural stability and mechanical response at room and elevated temperatures.