Study On Microstructure And Properties Optimization Of AlFeCoNiC High-entropy Alloys

High-entropy alloys?HEAs?,also called multi-component alloys?MCAs?,which consist of three or more principal elements either in equi-or non-equiatomic compositions have attracted increasing attention in recent years.First of all,in this paper,the brittle AlFeCoNi HEA with BCC crystal structure at room temperature was selected as the based alloy to improve its microstructures and mechanical properties by alloying with carbon.Secondly,we studied the effects of Al content on the microstructures and mechanical properties of the Al_xFe_1.5CoNiC_0.12 HEAs.Results show that with the addition of carbon,the microstructures of the AlFeCoNi HEA changed from the coarse equiaxed crystals to the typical dendritic microstructure.It is worth noting that the dendritic regions?DR?were ordered-BCC,i.e.B2 structure,while the interdendritic regions?ID?presented numerous of ordered-FCC?L1₂?cuboidal particles precipitated in the FCC matrix.The volume fraction of the ID regions increased with increasing the carbon content.Interestingly,both the strength and plasticity of alloys greatly increased with the increase of carbon content.Particularly,the AlFeCoNiC_0.08 alloy possessed the most excellent mechanical properties.However,the performance of AlFeCoNiC_0.17 alloy began to decline due to the precipitation of graphite phase.For the Al_xFe_1.5CoNiC_0.12 HEAs,the volume fraction of the ID regions?FCC+L1₂?increased with decreasing the Al content,while the mean size of the precipitates?L1₂?in ID regions decreased with the decrease of Al content.Both the yield strength and plasticity of alloys increased with the decrease of Al content,especially,the plasticity of alloys changed significantly.In addition,the results of the nanoindentation test show that the hardness of the ID regions was significantly higher than that of the DR regions due to the precipitation hardening caused by the superfine/nano-precipitate particles.For the Al_0.8Fe_1.5CoNiC_0.12 alloy,the volume fractions of the DR and ID regions were similar,showing a unique nanostructure-dendrite composite-structure,and the hardness of ID regions in the Al_0.8Fe_1.5CoNiC_0.12 alloy was as high as 8.7GPa.More importantly,the new material exhibited pronounced three-stage work-hardening behavior.It was found that the change of the dominant deformation mechanism,from the dislocation-movement into the deformation-twinning in the nano-structured ID region,could be used to further reasonably explain the multi-stage work-hardening behavior.