Study On Phase Control And Strengthening Mechanism Of Laser Additive Manufactured AlxCoCrFeNi High Entropy Alloy

Due to the excellent physical and chemical properties,AlxCoCrFeNi High-Entropy Alloys(HEAs)series became the earliest and most widely studied type of HEAs.As a structural alloy,the combination of strength and ductile is very important.However,the High Entropy Effect of HEAs usually results in a simple phase structure:alloys with FCC single phase are tensile but soft,while single BCC phase alloy is in the opposite.For this reason,the study of dual-phase FCC+BCC alloys became a research hotspot for balancing the strength and toughness of alloys.This article aims at using laser additive manufacturing to rapidly fabricate AlxCoCrFeNi high-entropy alloys with high strength and ductility.Futher with elemental control,process control and post-treatment strengthening methods,the alloy phase ratio,grain size and morphology are adjusted.The structure and properties of the alloy along with the appropriate strengthening methods are studied.The balance of strength and ductility of high-entropy alloys should be realized and the strengthening mechanism are to be explored.The specific research content and results are as follows:(1)Adjust and control the composition of alloy elements.It is found that as the Al content increases,the phase composition changes from FCC(x=0?0.3)to FCC+BCC(x=0.6?0.9),and finally to BCC(x=1.0),the hardness and strength of the alloy increase,while the plasticity decreases.A structure evolution of columnar grains-dendrites-equiaxed grains-equiaxed dendrites happens with Al content rises.The tensile properties of the three alloys with x=0,0.3,0.6 show that the Al_0.6CoCrFeNi HEA can achieve the balance of strength and ductility due to its propriate phase fraction,which has a tensile strength of 900Mpa and a ductility of 25%.(2)By changing the scanning time and fabricating power,it is found that the in-situ heat treatment of the laser additive manufacturing technology will cause a structural evolution of dendrites to columnar grains.The transition conditions are a cyclic heating temperature of 630°C and a heating time of more than 500s,under which the grain boundaries gather and the grain grows.The properties of the alloy changed from a yield strength of 609MPa and an elongation of 23.71%to a yield strength of 500MPa and an elongation of 25.35%.The increase of FCC phase and the coarsening of crystal grains are the main reasons for the increase of plasticity and decrease of strength of DED-Al_0.6CoCrFeNi alloy.(3)After heat treated at 700?,900?,1000?for 4h,it is found that as the temperature increases,the strength of the two alloy structures first increases and then decreases,while the change in plasticity is opposite.The main difference is that after700?heat treatment,the hard and brittle phase?precipitated in the dendrites,therefore its elongation decreased to 9.5%,the yield strength increases to 690MPa.Since the dislocations are cutting the nano-B2 phase when deforming,the yield strength of columnar grains increased to 815MPa.The absence of the?phase and the large-volume FCC maintains the plasticity,therefor the elongation remained at22.08%.In conclusion,the in-situ heat treatment+700?/4h method can effectively improve the strength and toughness of the alloy.