Powdermetallurgy Of High-Entropy Alloys

As a new concept alloy which contains at least four principle elements,high entropy alloys?HEAs?attracted many researchers'attention due to its extraordinary mechanical properties and some functional properties.In present study,CoCrFeNi and Al_0.3Cr_0.9FeNi_2.5V_0.1.1 HEAs were synthesized by conventional sintering and spark plasma sintering,respectively.The synthesized route,sintering parameters and mechanical properties were researched in this paper.The CoCrFeNi HEAs were successfully prepared by sintering the mixture of element powder,no chemical segregation was founded in sample,despite bits of intermetallic phases of chromium.There is a problem in this method,that is,it is difficult to sinter a sample with near fully-density,crystal grains are coarse and the mechanical properties are not good.In additional,some elements which are easily to form intermetallic phases cannot be used in this method,such as aluminum and nickel.However,this method has many positive characteristics,such as highly applicability with many elements,low time consumption and low cost.Generally,this is a appropriate method to investigate the phase formation rules of HEAs.To synthesis HEAs with high density and good mechanically properties.MA?mechanical alloying?and?spark plasma sintering?were used improve sample density.Al_0.3Cr_0.9FeNi_2.5V_0.1.1 were used in this study.In the section of“powder preparation”,ball-milling were used to synthesis pre-alloyed powder.The pre-alloyed powder with FCC structure were prepared by dry-milling for at least 36 h,however,due to the cold welding effect,productive rate of powder is at a low level.Thus,wet-milling were added after dry-milling,this process also reduces the average size of powders.Finally,the parameter of ball milling process is 48 h dry-milling and 12 h wet-milling was added after dry-milling.In the section of sintering,HEAs can reach highest density?99.7%?under sintering temperature of 1100 ^oC.After aging treatment?600 ^oC for 1 h?,tensile strength of sample is 1648 MPa.Though tensile strength is similar with the sample prepared by arc melting and cold rolling under the same alloy constituent,sample plasticity is too low?<1%?.To explain the poor plasticity,SEM,EBSD,TEM were used to characterize the microstructure of alloys.Although a large amount of precipitations were detected,and increase the strength of materials,the heterogeneous grain size and distribution,brittle BCC phases and micro voids in sample reduces plasticity of materials.