Hierarchical precipitates facilitate the excellent strength-ductility synergy in a CoCrNi-based medium-entropy alloy

An interstitial carbon-doped CoCrNiSi0.3 medium-entropy alloy (MEA) has been developed, which exhibits excellent strength-ductility synergy. This has been achieved through the implementation of a three-level hierarchical-precipitate-microstructure design. The CoCrNiSi0.3C0.048 MEA displays an ultra-high yield strength of 1.34 GPa, accompanied by a uniform tensile elongation of 8.62%, surpassing the performance of most carbon/silicon-doped MEAs. The three-level hierarchical precipitate structure comprises the primary Cr23C6 carbides (2-10 & mu;m), the secondary SiC precipitates (200-500 nm) surrounding the grain boundaries, and the tertiary SiC precipitates (similar to 50 nm) within the grains. The strengthening and toughening behavior of the current MEAs are attributed to multiple mechanisms, including crack branching and blunting in Cr23C6 carbides, dislocation-bypass mechanism around the non-shearable SiC precipitates, hetero-deformation-induced strengthening caused by the interface between matrix/three-level hierarchical precipitates, as well as stacking-fault networks and dense nanotwins activated by precipitates in the matrix. These findings provide valuable insights into the development of high-performance alloys for engineering applications in the future.