Investigation On Microstructure And Properties Of CoCrCuFeNi High Entropy Alloys

Based on equimolar ratios and high entropy of mixing, high entropy alloys have beendesigned, which possess simple crystal structure and ease of nano-precipitation. This makeshigh entropy alloys have excellent properties and great potentials in many applications. Ithas become one of hot research topics in the field of materials science and condensedmatter physics. In this paper, comparing mutual solubility, difference of atomic radius,electronegativity, entropy of mixing and other conditions as well as based on Co, Cr, Cu, Feand Ni as the main elements to design different ratio of components. After that, a series ofCoCrCuFeNi high entropy alloys were prepared by high frequency induction meltingfacility. Using SEM, XRD and other kinds of analytical methods, the effect of variation ofelement content on equilibrium as-solidified microstructure and phase structure werestudied systematically. On this basis, CoCrCuFexNi and CoCrCuxFeNi high entropy alloys（x=0.5、1.0、1.5、2.0molar ratio of the atomic element）were undercooled adopting moltenglass purification technology to achieve rapid solidification. The research on themicrostructure evolution of different components and undercooling was carried out deeply.The effect of solidification mode (equilibrium/unequilibrium) on microstructure and phasestructure was studied. The main results are as follows:The microstructure of equilibrium solidified CoCrCuFeNi high entropy alloys aretypical dendrites with Cu element segregating in the interdendritic area where manynano-particles exist. With the change of each content，nano-particles show differentmorphologies and their sizes change a little. When the content of Cu is less than18.18%,Cr elements segregate at the grain boundaries easily. The lower the content of Cu，the moreCr elements segregate. With the increase of Cu content, the the volume of interdendriticphase tend to increase. However, the content of Co、Fe elements is higher in the dendrite.For Cr element, it not only segregates at the grain boundary, but also precipitates asneedle-shape in the interdendritic area. Moreover, the content of Ni elements is higher thanthe other elements slightly in the interdendritic. While, as for CoCrCuFe0.5Ni andCoCrCuFeNi0.5high entropy alloys, the macro segregation of Cu elements appears on thespecimen and the former typical dendrites are not observed. What’s more, the phasestructure of CoCrCuFeNi high entropy alloys is simple FCC solid solution. With thechange of the composition and undercooling, the phase structures are not changed. The hardness values of as-solidified CoCrCuFeNi high entropy alloys are less than200HV. Cr、Fe elements can improve the hardness of the alloys, but Cu elements canreduce the hardness of the alloys. As for Ni、Co elements, both of them have little influenceon the hardness of the alloys.In undercooled CoCrCuFexNi and CoCrCuxFeNi high entropy alloys, two differentmicroscopic have been found: dendrites and liquid phase separation structure. The higherthe Cu content and undercooling, liquid separation can take place easier. When the contentof Cu is11.11%, Cu elements segregation is not observed in the interdendritic region.When the content of Cu is in the range of11.11%～18.18%, dendritics are observed atsmall undercooling, and Cu element is rich in the interdendrite area, wherenano-precipitations are found with size of about100nm or even lower. Compared with theas-cast condition, these nano-particles are uniform and show regular square type. Only inthe case of high undercooling, a small amount of phase separation appears. When thecontent of Cu is larger than or equal to20%, there is a clear liquid phase separationstructure. The higher the undercooling, the more obvious phase separation will be. As foralloys in which liquid phase separation occurs before solidification, the microstructure isconsisted of two regions: Cu depleted region and Cu rich region. Cr elements segregationexists in the two regions and the degree of segregation increases with the increase ofundercooling. Besides, a large number of nano-particles precipitate in the Cu rich region.