Achieving a unique combination of strength and ductility in CrCoNi medium-entropy alloy via heterogeneous gradient structure

Despite having otherwise outstanding mechanical properties, the practical application of CrCoNi medium-entropy alloy (MEA) is limited by its insufficient room temperature yield strength. Unfortunately, most traditional approaches for increasing strength usually lead to the degradation of ductility, a dilemma known as strength–ductility trade-off. Here, we report a heterogeneous gradient structure (HGS) in this MEA produced by ultrasonic surface rolling processing that can achieve a unique property combination: 3.7 times higher engineering yield strength compared to that of the fully recrystallized microstructure with homogeneous grain size, and at the same time preserving a very high elongation-to-failure of 54.1%. The HGS along depth from the treated surface is characterized by surface nanocrystalline, heterogeneous nanolaminate structure layers and deformed dislocation structure layer. The excellent combination of high strength and good ductility comes from extraordinary work-hardening ability arising from the heterogeneous gradient structure and the abundant extremely thin nanoscale deformation twins.