Microstructure And Properties Of (Fe₅₀Mn₃₀Cr₁₀Co₁₀)_100-XAl_X High Entropy Alloys Prepared By Powder Metallurgy

High-entropy alloys are new types of multi-principal element alloys which have better comprehensive properties than traditional alloys.Fe₅₀Mn₃₀Cr₁₀Co₁₀alloy is a TRIP-assisted dual-phase high-entropy alloy.In this paper,(Fe₅₀Mn₃₀Cr₁₀Co₁₀)_100-xAl_x high-entropy alloys with good comprehensive mechanical properties were prepared by mechanical alloying and spark plasma sintering.The microstructural evolution of Fe₅₀Mn₃₀Cr₁₀Co₁₀during the process of sintering and their mechanical properties were studied.Consequently,effects of Al content on the alloying process,microstructure,mechanical properties and magnetic properties of(Fe₅₀Mn₃₀Cr₁₀Co₁₀)_100-xAl_x high-entropy alloys were studied.Fe₅₀Mn₃₀Cr₁₀Co₁₀HEA powders were prepared by mechanical alloying.It was found that metal powders followed the principle that the metal with lower melting point dissolves more quickly during the process of alloying.The powders obtained single FCC structure with lattice parameter of 3.599?after milling for 40 h.The microstructure of sintered samples at different temperatures consisted of FCC matrix,HCP phase laminate layer and a few oxide particles containing Mn and Cr.FCC and HCP phases had no difference in chemical composition and showed a typical Shoji-Nishiyama?S-N?orientation relationship.When the sintering temperature increased,the grain size of FCC phase increased,fraction of HCP phase,fraction of twin boundaries and the thickness of twins increased,the density of alloy gradually grew up,the yield strength and hardness were reduced,whereas the ductility continually rise.The sintered alloys achieved balanced comprehensive mechanical properties at the sintering temperature of 1000°C,with compressive yield strength of 878 MPa,compressive strength of2013 MPa,plastic strain of 25.0%and Vickers hardness of 315 Hv.The microstructure of sintered samples at different sintering pressures consisted of FCC matrix and oxide particles.With increasing sintering pressure,the average grain size grew up,fractions of twin boundaries increased,the density of alloys increased,the yield strength slightly decreased,but plastic strain obviously increased.(Fe₅₀Mn₃₀Cr₁₀Co₁₀)₈₈Al₁₂ HEA powders showed similar mechanical alloying behavior with Fe₅₀Mn₃₀Cr₁₀Co₁₀HEA powders,but the microstructure consisted of FCC and BCC phase after milling for 40 h.The HEA bulky samples sintered at different temperatures exhibited FCC+BCC dual-phase microstructure and the BCC phases were rich in Al and Cr.With the increasing sintering temperature,the fraction of FCC increased firstly and then decreased.The yield strength of HEA sintered at 1000?was 1228 MPa,which was significantly higher than that of Fe₅₀Mn₃₀Cr₁₀Co₁₀ HEA.(Fe₅₀Mn₃₀Cr₁₀Co₁₀)₈₈Al₁₂ HEA exhibited ferromagnetic characteristic.With the increasing of sintering temperature,the saturation magnetization and coercivity of alloys continually decreased.The(Fe₅₀Mn₃₀Cr₁₀Co₁₀)₈₈Al₁₂ HEA sintered at 1000?is a kind of semi-hard magnetic material with a saturation magnetization of 41.16 Am²/Kg and coercivity of 1511 A/m.When the Al content x increased from 0 to 20,the microstructure of(Fe₅₀Mn₃₀Cr₁₀Co₁₀)_100-xAl_x HEA milled powders changed from FCC to FCC+BCC,and then to a BCC single phase structure.After sintering at 1000?,the microstructure of Al0 alloy?x=0?consisted of FCC matrix and HCP phase laminate layer,the microstructure of Al4 alloy?x=4?consisted of FCC phase,the microstructure of Al8,Al12 and Al16 alloys?x=8,12,16?consisted of FCC+BCC dual phases and the microstructure of Al20 alloy?x=20?consisted of single BCC phase.The grain size was first reduced with the minor addition of Al?x=4?,but gradually increased with the further increase of Al content.The strength and hardness of sintered samples gradually increased and then decreased with the increasing of Al content,ductility continually decreased.The sintered Al16 alloy achieved the highest yield strength of 1916 MPa.The mechanical properties of alloys were related with microstructure of alloys,solid solution strengthening effect of Al,grain sizes and density of alloys.It was found that when x?12 the HEA specimens exhibited ductile fracture characteristics but Al16 and Al20 alloys exhibited brittle fracture characteristic in the fracture morphology.The addition of Al can change the magnetic properties of alloys,Al0 and Al4 alloys exhibited paramagnetism but Al8,Al12,Al16 and Al20 alloys exhibited ferromagnetism.With the increase of Al content,the saturation magnetization of alloys first increased and reached a maximum of 83.07 Am2/Kg in Al16 alloy,then decreased,while the coercivity of HEAs continually decreased.