Research On Microstructure And Mechanical Properties Of Cr_xCoFeNiAl High Entropy Alloys

High entropy alloys are a new type of alloy material that has appeared in recent years.Because it subverts the design concept of traditional alloys,it opens up a new path for material development.At present,the research on high entropy alloys is still in the exploratory stage.Although some progress has been made,there are still some scientific issues that need to be solved urgently,such as the lack of understanding of the mechanism of element doping in the structure of high entropy alloys,the preparation process and process parameters.The effect on the microstructure and properties of high entropy alloys is still unclear.Based on the current problems of high entropy alloys,this paper selected CrxCoFeNiAl high entropy alloys to explore the influence of Cr content on the microstructure and mechanical properties of CrxCoFeNiAl high entropy alloys prepared by arc melting and directional solidification.The effects of different withdrawal rate on the microstructure and mechanical properties of Cr0.8CoFeNiAl high entropy alloys were investigated using process parameters.Finally,the effects of directional solidification process on the microstructure and mechanical properties of Cr0.2CoFeNiAl and Cr0.6CoFeNiAl high entropy alloys were analyzed.The main conclusions are as follows:(1)With the increase of Cr content,the crystal structure of CrxCoFeNiAl high entropy alloys prepared by arc melting was transformed from BCC structure to BCC+FCC mixed structures,and the micro structure of the alloy changed from equiaxed grain structure to dendrite structure.In the dendrite structure,Al and Ni elements were enriched in the dendrite region,while Fe and Cr elements were enriched in the interdendritic region,and Co element was uniformly distributed in the two region.(2)With the increase of the withdrawal rate,the microstructures of the Cr0.8CoFeNi Al high entropy alloys all showed dendritic structures,and their element distribution rules were the same.In the alloy,the dendrite were riched in Ni and Al elements,and the interdendritic were riched in Cr and Fe elements.The compressive strength of the alloy decreased at first and then increased,while the compressive strain decreased.When the withdrawal rate was 10μm/s and 200μm/s,the alloy had a maximum compressive strength of 2760MPa.(3)With the increase of Cr content,the crystal structure of CrxCoFeNiAl high entropy alloy prepared by directional solidification remained unchanged,all of which were BCC structure,and the alloy structure changed from equiaxed to dendrite.The alloy dendrites were riched in Al and Ni elements,and the interdendrites were riched in Fe and Cr elements.(4)The increase of Cr content significantly improved the compressive strength and compressive ductility of arc-melted and directionally solidified alloys.The compressive strength and compressive strain of the alloy prepared by arc melting increased from 1500MPa and 13.56%to 2460MPa and 30.09%,respectively,and the compressive strength of the alloy increased by 64%.The compressive strength and compressive strain of the alloy prepared by directional solidification increased from 875MPa and 30.09%to 2760MPa and 38.59%.(5)After directional solidification,the crystal structures of Cr0.2CoFeNiAl and Cr0.6CoFeNiAl high entropy alloys did not change,and both alloys showed preferential growth in the(200)crystallographic direction.Directional solidification increased the equiaxed grain size of Cr0.2CoFeNiAl high entropy alloy,and the compressive strength and compressive strain of the alloy were decreased.Cr0.6CoFeNiAl high entropy alloy after directional solidification increased the size of dendrites,and the grains were arranged neatly,the compressive strength of the alloy increased from 2270MPa to 2460MPa,while the compressive strain decreased from 28.83%to 26.56%.