High Temperature Deformation And Welding Behavior Of CoCrFeNi High Entropy Alloy Containing Al,Mo And Ti

High entropy alloys are composed of many elements with equal atomic ratio or near equal atomic ratio,which have unique structural characteristics,so they show unique properties different from traditional alloys.As a typical high entropy alloy,CoCrFeNi has become the focus of attention in related fields because of its excellent fracture toughness at room temperature and low temperature.At present,the research on the mechanical behavior of CoCrFeNi high entropy alloy mainly focuses on the deformation and strengthening and toughening mechanism at room temperature,and there is no systematic research on the deformation mechanism at high temperature and welding behavior of the alloy.In this paper,CoCrFeNi,CoCrFeNi + 4% Al,CoCrFeNi + 4% Mo and CoCrFeNi + 4% Ti high entropy alloys were prepared.By establishing the relationship between microstructure evolution and deformation behavior at high temperature,the micro mechanism of high temperature serration was systematically explored;By comparing the dependence of microstructure characteristics and mechanical properties of four CoCrFeNi based high entropy alloys during welding thermal cycle,the effect of alloy composition on weldability and welding behavior of high entropy alloys was revealed.The main research contents and conclusions are as follows:(1)Based on the static tensile test results at different temperatures,the dependence of mechanical properties of CoCrFeNi based high entropy alloy on deformation temperature was explored.At room temperature,the tensile strength of CoCrFeNi alloy is 544 MPa.After adding Al,Mo and Ti elements,the strength is further improved.With the increase of deformation temperature,the yield strength of CoCrFeNi,CoCrFeNi + 4% Al and CoCrFeNi+ 4% Mo alloys decreases gradually,while the yield strength of CoCrFeNi + 4% Ti alloy decreases gradually below 773 k and increases gradually at 773 ～ 1073 K.(2)By comparing the microstructure evolution characteristics before and after deformation at high temperature,the serration of four CoCrFeNi based high entropy alloys was systematically studied.The serration of CoCrFeNi based high entropy alloy occurs in the range of 573 ～ 973 K,and the serration type is type A at low temperature,and gradually changes to type B and type C with the increase of temperature.(3)Based on the quantitative analysis results of micromechanical model,the dependence of serration on solute atoms in CoCrFeNi high entropy alloy is revealed.There are solutes and solvents in CoCrFeNi high entropy alloys,in which Fe and Cr are more likely to behave as solutes,while Co and Ni can behave as solvents,and Ni is one of the main solvent elements.The diffusion of solute is mainly controlled by pipeline diffusion,and the corresponding concentration is approximately 34%.The addition of Al,Mo and Ti widened the temperature range of serration and increased the average amplitude of serration.(4)By studying the laser welding behavior of four kinds of high entropy alloys,it is found that CoCrFeNi based high entropy alloys have excellent weldability.The mechanical properties of the sample after welding are similar to that of the base metal.There are no solidification cracks and heat affected zone cracks in the structure after welding.The heat affected zone is narrow and there is no obvious segregation in the fusion zone.The addition of Al,Mo and Ti elements effectively improved the weld strength and mechanical properties of CoCrFeNi alloy,and did not aggravate the segregation in the fusion zone.