Effect of Ultrasonic Vibration Surface Plastic Deformation Forces on Microstructure and Mechanical Properties of a Medium Entropy Alloy

In this work, a gradient twin structure of a CrCoNi medium entropy alloy (MEA) was constructed by using the ultrasonic vibration surface friction treatment (UV-SFT) under different deformation forces. As for such a gradient twin structure, its overall and specific structures were characterized with the optical microscopy (OM) and transmission electron microscopy (TEM), respectively. The microhardness as well as tensile mechanical properties of such a constructed CrCoNi MEA with gradient twin structures were tested, and its strain hardening ability was evaluated and analyzed. As for a CrCoNi MEA, experimental results show that the UV-SFT successfully leads to its gradient twin structure with similar layers under different deformation forces, which can be divided into three regions: (1) a transition zone, (2) a severe plastic deformation zone, and (3) a nano-grain zone. The UV-SFT process of 300, 450, and 750 N enhances the microhardness value by 2.44 times, 2.55 times, and 2.77 times that of the core and significantly improves the engineering yield strength (from 209.8 ± 5.0 to 450.5 ± 10.0 MPa, 575.1 ± 5.0, and 610.7 ± 5.0 MPa), engineering ultimate tensile strength (from 474.8 ± 3.0 to 649.3 ± 5.0, 713.7 ± 5.0 MPa, and 803.4 ± 5.0), true yield strength (from 214.7 ± 5.0 to 497.7 ± 10.0, 593.5 ± 10.0, and 627.3 ± 10.0 MPa), as well as true ultimate tensile strength (from 965.6 ± 2.0 to 1100.0 ± 5.0, 1175.9 ± 5.0, and 1340.5 ± 5.0 MPa), respectively. The most noteworthy improvement is that the surface plastic deformation of ultrasonic vibration does not reduce the plasticity of the CrCoNi MEA, which ensures its favorable mechanical properties and fracture toughness.