The Cantor’s high entropy alloy (HEA) [1,2] and its congeneric alloys with multiple principle elements [3–9] are known for its high twinnability result from the low stacking fault energy (SFE) [4,10–13], which result in both high strength and good ductility during ambient and cryogenic temperature [14–24]. This project aims at augmenting the global strength of a equiatomic ternary CrCoNi medium entropy alloy while preserving its inherent ductility, by fabricating a composite structure comprises a central core layer with a coarse-grained, deformation-free soft structure, encapsulated by two outer surface layer that exhibit pronounced deformation and display a gradient distribution of grain size and density of diverse defects using Surface Mechanical Attrition Treatment (SMAT).

Surface mechanical attrition treatment (SMAT) technic is a kind of surface nanocrystallization method firstly proposed by Jian LU and Ke LU [25,26] to enhance the global mechanical property of metals and alloys by generating a gradient heterogenous microstructure within the surface layer of target material. This specific heterogenous microstructure could results in synergetic effect of the hard nanocrystallized surface layer and the soft undeformed core matrix layer, therefore strengthening the overall mechanical property of target material without impair its ductility.

The mechanical properties of the CrCoNi MEA after SMAT treatment under both ambient and cryogenic temperatures were evaluated by tensile test. And the gradient heterogenous microstructure on the uppermost surface of specimens was indirectly characterized by the hardness distribution along the depth direction evaluated by microhardness test. EBSD technique was employed to characterize the evolution of microstructure, including overall grain size, grain boundaries, lattice distortion, and twin densities, within CrCoNi MEA after SMAT treatment under both ambient and cryogenic temperatures in mesoscopic view. TEM techniques were used to characterize the microstructure within the uppermost (< 5 𝜇𝑚 in depth) layer of SMAT treated specimens.

During the high strain rate plastic deformation induced by the SMAT technique, twinning and dislocation arrays are the dominant deformation and grain refinement mechanisms of CrCoNi MEA, while at the same time FCC-HCP martensitic phase transformations also observed in both ambient and cryogenic temperature treated specimens. Due to the further enhanced formability of SFs during cryogenic temperature, strain induced HCP martensite phase are more frequently observed in cryogenic SMAT treated specimens. And 4H phase formed by more complex stacking faults activities was also observed in the cryogenic treated specimen. Additionally, the cryogenic treated specimen owns thinner anocrystalline layer (~ 200 nm) but smaller average grain size (17 nm) within this layer, and the room temperature treated specimen exhibit thicker nanocrystalline layer (700 - 1000 nm) but relatively larger average grain size (37 nm), which results in similar yield strength and ductility of both treatment cases, however better ultimate strength in cryogenic treated specimen.  SMAT treatment using various balls was also researched and found that using balls with higher density but lower diameter, like 2 mm WC balls compared with 3 mm ZrO₂ balls, could concentrate more kinetic energy within the top surface layer to facilitate grain refinement in the top surface layer and avoid bulk hardening.