Study On Strengthening And Toughening Of Gradient CrCoNi Medium-entropy Alloy Prepared By Ultrasonic-assisted Method

Medium Entropy Alloy(MEA)has some mechanical properties that conventional alloys lack,such as high strength(1.0-1.5 GPa)and high plasticity(>50 %),which provide a great scope for exploring the comprehensive performance of materials under complex conditions.The isoatomic ratio CrCoNi medium entropy alloy system shows better properties in strength and plasticity than(High Entropy Alloy,HEA)itself,while ultrasonic vibration-assisted deformation as a new form of plastic deformation has received much attention in recent years.In this paper,CrCoNi MEA with gradient grain/twin structure on the surface was prepared by ultrasonic vibration surface friction treatment(UV-SFT).The microstructure and mechanical properties of the CrCoNi MEA before and after ultrasonic vibratory surface friction treatment with different deformation forces were investigated using optical microscopy(OM),scanning electron microscopy(SEM),electron backscatter diffraction technique(EBSD)and transmission electron microscopy(TEM)to reveal the strengthening mechanism.The main conclusions are as follows:After the CrCoNi MEA was treated by UV-SFT process,not only the gradient grain/twin structure was presented,but also the gradient deformation layer could be divided into three parts:(1)the main features closest to the outer layer were nanocrystalline and twin deformation;(2)the severely plastic deformation layer mainly formed elongated lamellar grains and crossed multi-twin crystal system;(3)the transition layer mainly showed parallel twinning and dislocation motion.The UV-SFT process with three deformation forces of 300,450 and 750 N treated the CrCoNi MEA,and the UV-SFT process with 300,450 and 750 N increased its microhardness by 2.44,2.55 and 2.77 times,respectively,and significantly improved its yield strength and ultimate tensile strength.At the same time,the elongation of CrCoNi MEA slightly decreased and its fracture toughness was significantly enhanced,which mainly depends on its super-strain hardening ability.The nanocrystalline layer closest to the deformation surface of CrCoNi MEA alloy plays a key role in generating high external strain hardening,but the nanocrystalline layer itself has no particularly significant additional hardening.the microhardness of CrCoNi MEA gradually decreases along the depth direction from the surface to the core due to the twin boundaries in the grains that prevent dislocation movement,reduce the average free range of dislocations,and increase the strength and hardness of CrCoNi MEA.In conclusion,the surface plastic deformation by ultrasonic vibration significantly improves the yield strength and ultimate tensile strength of the CrCoNi MEA without losing its plasticity,which exhibits excellent mechanical properties.