Microstructure And Mechanical Properties Of Hot Rolled CoCrNi Medium Entropy Alloy Under Sliding Friction And Heat Treatment

Gradient nanocrystallization of metallic materials are expected to coordinate their strength and plasticity,which provides a new idea for preparing new structural materials with high strength and plasticity.Different from traditional alloys dominated by one element,multi-principal alloys exhibit many excellent properties.It has been proved that the low stacking fault energy medium-entropy alloy composed of three principal components Co,Cr and Ni has extraordinary plasticity and damage tolerance.It is a good research object for gradient nanocrystallization.In this paper,the surface of CoCrNi medium-entropy alloy was deformed by sliding friction treatment,so that nano-grains were produced in a certain thickness near the surface of the material,and the plastic zone in the middle of the material was retained to form gradient nano-materials.The phase composition,morphology and mechanical properties of the tested alloy were analyzed by means of X-ray diffraction,metallographic microscopy,scanning electron microscopy,energy dispersive spectrometer,Vickers hardness tester and universal testing machine.The main conclusions were as follows:The coarse grained structure under the surface of CoCrNi alloy was greatly refined to nano-scale,due to the strain gradient generated by sliding friction process,and the hardness increased significantly from 270 HV to 430 HV.The gradient deformation layer with continuous change of structure and properties was formed.The layer with a depth about 250μm was not changed when the thickness of the alloy plate decreased.The results of sliding friction tension of plates with different thicknesses show that: As the thickness of the plate decreases,the yield strength of the alloy increases gradually and the plasticity decreases.When the thickness of the plate is about 3 mm,the plasticity decreased by about 30% and the yield strength increased by nearly two times,which effectively improves the strength-toughness relationship of the CoCrNi alloy.When the thickness of the alloy plate decreases to about 2.3 mm,the alloy shows good strength-plastic after sliding friction treatment.Its yield strength increased significantly from less than 300 MPa before treatment to about 800 MPa,the plastic reduction is less than 40%,and the elongation after fracture is still close to 50%.This excellent strength-toughness relationship is mainly attributed to its unique heterogeneous gradient nanostructures.After heat treatment at intermediate temperature,the structure and the depth of gradient deformation layer of CoCrNi alloy were not changed significantly,but the hardness of gradient deformation layer increased.The surface hardness of CoCrNi alloy increased significantly to about 560 HV.Tensile test results show that the yield of the alloy is further increased to about 1 GPa after heat treatment,but the plasticity is not lost,which breaks the competitive relationship between strength and toughness of the alloy in the past.TEM results show that this is mainly due to the precipitation of nano-second phase and annealing twins in gradient nanolayers.