Microstructure And Mechanical Properties Of CoCrFeNiGd_x High Entropy Alloys Manufactured By Powder Metallurgy

High-entropy alloys(HEAs)have attracted more and more attention in recent years due to its unique alloy design concept and excellent mechanical properties.As one of the earliest studied alloy systems,CoCrFeNihigh-entropy alloy exhibited a single FCC structure with excellent tensile plasticity and low temperature mechanical properties,but its yield strength was very low.In this study,a trace amount of rare earth element Gd was introduced into the CoCrFeNiHEA by mechanical alloying and spark plasma sintering.The rare earth element Gd had a large atomic radius and a strong affinity with Ni.It was expected that a high-strength multiphase high-entropy alloy can be obtained through the introduction of Gd.In this paper,the mechanical alloying and sintering process of HEA powders,phase formation and microstructure evolution of CoCrFeNiGd_0.05 HEA were systematically studied,and the effects of preparation process parameters on its microstructure and mechanical properties were explored.Finally,the microstructure and mechanical properties of CoCrFeNiGd_xhigh-entropy alloys were studied.The CoCrFeNiGd_0.05 HEA powders with a single FCC structure was prepared by mechanical alloying,and its average grain size was reduced from 6.3?m to 17.8 nm.After sintering at 1000?,a fully dense HEA bulk was obtained(relative density is 99.86%),which exhibited a multiphase structure consisted of FCC matrix phase,Gd₂Ni₇ type hexagonal structure(HS)phase riched in Gd-Nielement and and very small amount of Gd₂O₃ oxide phase.The hexagonal structure phase was coherent with the matrix phase and had the following relationship:{(?)}FCC//{(?)}HS.The average grain size of the alloy bulk was375 nm,and the volume fraction of the second phase was about 6.77%.The alloy had excellent comprehensive mechanical properties at room temperature,with a compressive yield strength of 952 MPa,a compressive strength of 3346 MPa and its compressive plasticity was greater than 50%.The tensile yield strength,ultimate strength,fracture elongation and Vickers hardness were 859 MPa,991 MPa,19.4%,and 359.0 Hv respectively.The high strength was mainly due to the grain refinement strengthening,solid solution strengthening and precipitation strengthening caused by the HS phase;SEM result showed that the second phase deformed together with the matrix during compression test,therefore,the alloy had a large work hardening rate,which ultimately led to a large plasticity of the alloy.Ball milling time,sintering temperature and sintering pressure had little effect on the phase composition of the CoCrFeNiGd_0.05 HEA bulk,but had greater impact on its relative density,grain size and second phase,which will have a certain influence on the mechanical properties of the alloy.After ball milling,the distribution of Gd element was more evenly and dispersed.And as the milling time increased from 25 h to 45 h,the size and volume fraction of the second phase particles and the grain size of the alloy decreased constantly,the content of Gd in the matrix phase increased first and then decreased,resulted in the XRD peak position shifted to the left first and then to the right gradually.The yield strength increased and plasticity of sintered bulks decreased.With the increase of sintering temperature,the size and volume fraction of the second phase particles gradually increased,and the relative density and average grain size of the alloy increased too,meanwhile,the strength and hardness decreased and the ductility increased.As the sintering pressure increased,the relative density and average grain size of sintered bulks increased continuously,resulted in a slightly decrease of the strength,and a continuously increase of the plasticity.After milling for 25 h,all the CoCrFeNiGd_x HEAs powders showed single FCC structure.With the increase of Gd content,the time required to complete mechanical alloying and the micro-strain of CoCrFeNiGd_x HEAs powders increased.The average particle size of HEAs powders decreased significantly with the increase of Gd content.After adding Gd,the high entropy alloy changes from single phase FCC to multiphase structure,the densification temperature of Gd-containing alloy powders decreased with the increase of Gd content.With the increase of Gd content,the volume fraction and size of HS phase increased,and the morphology of HS phase gradually changed to strip shape.The grain size of the alloy first decreased significantly and then increased slightly with the increase of Gd content,and the grain size of all the alloys containing Gd was smaller than the alloy without Gd,indicating that the HS phase can effectively suppress the grain growth.Compared with the counterpart alloy prepared by the melting method,the tensile yield strength was increased by 290%for CoCrFeNiHEA.When x increased to 0.05,its strength was greatly increased,while hardly sacrificed its plasticity.For x=0.1,its strength continued to increase while its plasticity significantly decreased.If the content of Gd continued to increase(x>0.1),the strength and plasticity of sintered alloys decreased constantly due to the increase of the grain size and the splitting effect of the second phase to the matrix.The bulky HEA achieved excellent comprehensive mechanical properties at x=0.05.