Study On Diffusion Behavior Of Fe,Co,Al And C In AlCoCrFeNi System High Entropy Alloys

High entropy alloy is a new type of metal material developed in recent years,which has different microstructure and properties from traditional alloy.The diffusion behavior of high entropy alloy has hysteresis effect,which makes the high entropy alloy have excellent thermal stability and high temperature oxidation resistance.The formation of nano phase and amorphous phase in high entropy alloy is also closely related to this effect.The diffusion behavior of materials is related to the phenomena of casting,phase transformation and recrystallization.There are few researches on the diffusion behavior of high entropy alloy,and people have not got a good understanding of its mechanism and lack of necessary data accumulation.Therefore,it is of great significance to study the diffusion behavior of high entropy alloy.In this paper,Al6.97Co23.26Cr23.26Fe23.26Ni23.26 high entropy alloy(HEA1)and Al6.97Co31.26Cr23.26Fe15.26Ni23.26 high entropy alloy(HEA2)were prepared by non consumable vacuum arc melting furnace.The Fe-Co quasi binary diffusion couple was formed by fixture method.The effect of temperature and time on the diffusion behavior of Co and Fe in high entropy alloy was studied systematically.Diffusion couple was made of pure A1 and Al6.97Co23.26Cr23.26Fe23.26Ni23.26 high entropy alloy.The effect of different temperature on diffusion reaction layer was studied.The diffusion behavior of C atom in high entropy alloy was studied by solid carburizing.Then,the microstructure of the longitudinal section of the sample was characterized by scanning electron microscopy and the components of the diffusion zone were characterized by electron probe microanalysis.The conclusions of calculation and analysis are as follows:(1)The diffusion coefficients of Fe and Co elements are calculated.It is found that the diffusion velocity of Fe element is always higher than that of Co element at the same temperature in AlCoCrFeNi system high entropy alloy and the difference between the diffusion velocity of Fe element and Co element is larger with the increase of temperature.In traditional metals and alloys,the diffusion coefficients of Fe and Co are always bigger than those of AlCoCrFeNi system high entropy alloy,so it is found that the diffusion of two elements in high entropy alloy is indeed slow.(2)It is found that some intermetallic compounds will precipitate along the grain boundary in the high entropy alloy of AlCoCrFeNi system at low diffusion temperature.By calculating the activation energy of high temperature(1273?1373 K)diffusion and low temperature(1123?1248 K),it is found that the diffusion activation energy of the two temperature ranges is not the same.The reason for this phenomenon is that the diffusion mechanism of different elements in the high entropy alloy has changed at different temperatures.At low temperature,the diffusion path is grain boundary diffusion,supplemented by intragranular diffusion.At high temperature,diffusion is mainly completed by intragranular diffusion.(3)In the diffusion process of Al-HEA1 diffusion couple,there is the Kirkendall effect.With the increase of temperature,the effect is more significant,which eventually leads to the generation of diffusion cracks.In the diffusion process of Al and HEA1,there is a diffusion reaction layer.And different elements have different segregation phenomenon in the diffusion reaction layer.Then the average diffusion of Al element in the diffusion reaction layer is calculated by Heumann method.The diffusion activation energy of Al element is 60.02 kJ/mol calculated by Arrhenius equation.(4)An oxide layer composed of Al and Cr elements appears on the surface of the high entropy alloy after solid carburizing by the redox reaction.Some of the C elements are incorporated into the matrix,the other are precipitated with carbides of Cr.The carbides are mainly distributed in the carburized layer and the grain boundary of the matrix.With the increase of diffusion time,the size of carbides in the carburized layer increases,the number of carbides in the grain boundary increases and the production of NiAl phase is inhibited.