Study On Microstructure And Heat Treatment Process Of AlCoCrFeNi High Entropy Alloys

In this paper,by controlling the content of one or two elements,a single-phase or multi-phase high entropy alloy with high strength,hardness and a certain degree of plasticity could be designed to meet different mechanical properties.Alx CoCrFeNiy(x:y=1:1,1.5:1,2:1,1.5:1,1:2)high entropy alloys with simple solid solution structures were prepared by vacuum arc furnace smelting and heat treating as the research object.Modern analytic determination methods include SEM,EDS and XRD were used to research the microstructures,phase structures,mechanical properties of these alloys.The effect of phase transition and properties caused by different contents of aluminum,nickel and heat treatment conditions were studied.Observed by scanning electron microscope,the dendritic and inter-dendritic structures were observed in the as-cast AlCoCrFeNi alloy,including BCC and ordered B2 phases.A large number of Fe-Cr rich precipitates and basket-weave like products formed via a spinodal decomposition reaction were distributed among inter-dendritic regions.Al-Ni rich matrix region and areas with a higher concentration of precipitates could be found in the as-cast Al1.5CoCrFeNi alloy,including BCC and ordered B2 phases.The structure and morphology of as-cast Al2 CoCrFeNi alloy were almost the same as that of the former.The gray coarse dendrites and inter-dendritic region with white dendrite and intergranular precipitate phase were detected in the as-cast AlCoCrFeNi1.5 alloy which included FCC and B2 phases.The eutectic structures of slat and pattern layer shapes,coarse dendrites and inter-dendritic structures were found in the as-cast AlCoCrFeNi1.5 alloy,including FCC and B2 phases.The compressive strength and hardness of as-cast alloys were related to microstructure.With the addition of Al content,the hardness of Alx CoCrFeNi alloy increased,while the compressive property decreased.Al2 CoCrFeNi alloy had the highest hardness among them,up to 445.3HV,but had poor compression property.AlCoCrFeNi alloy was harder than Al1.5CoCrFeNi alloy due to its large number of precipitates.With the addition of Ni content,the hardness of AlCoCrFeNiy alloy decreased,while the compressive property increased.AlCoCrFeNi2 alloy had the highest compressive strength among them,up to 3765 MPa,which stain was over 60%,but had the poor hardness of 168.45 HV.After quenching at 600?,the hardness of Alx CoCrFeNiy alloys generally increased,but then decreased as the dendrite coarsened with the increase of temperature.The original inter-dendritic BCC structures and white basket-weave like products transited to new phases of FCC phase in thin strip structure and ? phase in AlCoCrFeNi high entropy alloy at 700 and 800?,and these two phases formed to BCC phase at 900 and 1000?.The hardness shew an increase tendency at first,but decreased when FCC appeared,then increased again as BCC phase formed.The other four alloys would not generate new phase after heat treatment,just the volume ratio of each structure changed,and the alloy below 600 ? would not undergo phase transition.