A Study On Diffusion Of Atoms In CoCrCuFeNi High Entropy Alloy

Recent researches on high-entropy alloys mostly focus on microstructure and performance,while the study of atomic diffusion is relatively scarce.However,the diffusion in high-entropy alloys is not only directly related to the phase transition in the solid state,but also determines the phase stability.Especially for the cases in high-temperature environments,the diffusion of atoms will affect the stability and performance of the microstructures.Therefore,studying the diffusion behavior and mechanism of different atoms in high-entropy alloys is of great significance to understand the changes of microstructure and properties,and further development and of new alloys.In this thesis,Cu-CoCrCuFeNi,Ni-CoCrCuFeNi and Zn-CoCrCuFeNi diffusion couples were prepared by vacuum arc melting and electroplating methods.Annealing treatments at different temperatures and different holding times were carried out for those three kinds of diffusion couple respectively.The microstructure,micro-composition,diffusion layer thickness and micro-hardness of diffusion couple were analyzed.The diffusion mechanism of Cu,Ni and Zn atoms in high-entropy alloy CoCrCuFeNi was discussed.The experimental results show that the Cu atoms fail to inter-diffuse with the other four elements in Cu-CoCrCuFeNi and Ni-CoCrCuFeNi couples,but instead,they usually transport along the grain boundaries after the high-temperature and prolonged diffusion annealing.Interdiffusion of Ni atoms has taken place with other Co,Cr and Fe atoms in high-entropy alloy to form diffusion layer.After annealing at 850?×9h~950?×12h,the diffusion layer depth increases with the increase of temperature and time,but reduces when holding time reaches 15 hours.In the Zn-CoCrCuFeNi diffusion couple,the five components including the Cu atoms in high-entropy alloy all inter-diffuse with Zn atoms.Reactive diffusion occurs when a certain solid solubility limit is reached,and the depth of reaction diffusion layer increases with the holding time.The diffusion of Cu,Ni and Zn atoms in high-entropy alloys all depend on the vacancy mechanism.Only the adaptable vacancies can provide a positions for the migration of these atoms.The more atoms in the high-entropy alloy are fitted to the vacancies,the easier their lattice diffusion.After heat treatment,the hardness of the matrix of these three diffusion couples decreases,but the microhardness of Cu-CoCrCuFeNi and Ni-CoCrCuFeNi diffusive interfaces gradually increases from the coating to the high-entropy alloy.With the increase of annealing temperature and holding time,the hardness of both the matrix and coating increases.However,the microhardness of Zn-CoCrCuFeNi diffusion couple gradually decreases,with little change of hardness occurring at the interface.With the increase of annealing temperature and holding time,the microhardness of the Zn layer slightly increases.