Microstructure And Mechanical Properties Of Ultrafine-grained FeCoCrMn(Ni) High Entropy Alloys Prepared By Powder Metallurgy

High entropy alloys?HEAs?,which has a series of promising properties compared with traditional alloys,such as high strength,high hardness,high wear resistance,etc,is an important research direction in the field of metallic materials.It's easy for FeCoNiCrMn HEAs to form the solid solution with the simple FCC structure.In the present work,FeCoCrMn?Ni?HEAs were prepared by powder metallurgy method.The main research contents are as follows:The powder morphology and alloying behavior of quinary FeCoNiCr Mn HEA and quaternary Fe₅₀Co₁₀Cr₁₀Mn₃₀ HEA powders in mechanical alloying process were studied.After ball milling for 40 h,the mechanical alloying processes of two kinds of powders were basically completed and HEA powders with single-phase FCC structure were obtained.Lattice parameter of the two kinds of HEA powders were 3.6047?and 3.6078?,respectively;lattice distortions were 0.63%and 0.58%,respectively;their average grain size has been refined to 15 nm and 16nm,respectively.The DSC result shows that new phases precipitated at 294oC and 435oC in FeCoNiCrMn HEA powder after ball milling for 40 h.Effects of sintering temperature on Microstructure and mechanical properties of FeCoNiCrMn HEA were investigated.Microstructures of the alloys consisted of the disordered FCC matrix phase and a small amount of fine precipitated particles?oxides or carbides containing Cr and Mn?.With the increase of sintering temperature,the relative density of the alloy bulks increased significantly;the average grain size of matrix gradually grew;size of precipitated particles gradually increased;dislocation density in the matrix gradually reduced and the twin lamellar thickness raised.The compressive yield strength and microhardness of the alloys declined gradually,while compressive strength and plastic strain increased significantly with the increase of sintering temperature.The average grain size of the alloys sintered at 900oC was 588 nm;yield strength reached to 1156 MPa;plastic strain arrived 42%;microhardness reached to 360 HV,possessing good mechanical properties.Effects of sintering temperature on Microstructure and mechanical properties of Fe₅₀Co₁₀Cr₁₀Mn₃₀ HEA were studied.Microstructures of the alloys consisted of the disordered FCC matrix phase and tiny amounts of fine precipitated particles?oxides or carbides containing Mn?.With the increase of sintering temperature,the porosity of the alloy bulks reduced greatly;the average grain size of matrix gradually grew;size of precipitated particles gradually increased.The compressive yield strength and microhardness of the alloys declined gradually,while compressive strength and plastic strain increased significantly with the increasing sintering temperature.The fracture mechanism of Fe₅₀Co₁₀Cr₁₀Mn₃₀ HEA was micropore aggregation fracture.With the increase of the sintering temperature,the fracture morphology changed from the equiaxed dimple to the tearing dimple,size of dimples gradually increased and more uniform and deeper.The average grain size of the alloy sintered at 900oC was 285nm;yield strength of it reached to 1299 MPa;compressive strength and plastic strain were 2058MPa and 17%,respectively;microhardness was 399 HV,possessing good mechanical properties.Effects of Fe content?20⁶0%?on microstructure and mechanical properties of FeCoNiCrMn HEA were studied.The increase of Fe content had no obvious effect on phases and microstructures of this alloy.The sintered samples were composed of the single-phase FCC matrix,a Cr-rich phase and the precipitated particle?oxides or carbides containing Cr and Mn?.With the increase of Fe content,the porosity of alloy bulks increased to a certain extent;the average grain size of matrix declined from 588 nm to 401 nm;the volume fraction of Cr-rich phase increased.The compressive yield strength decreased gradually,the compressive plastic strain slightly increased;the tensile yield strength declined gradually;the tensile plastic strain was stable with the increasing sintering temperature.Dimples were observed in the fracture of the sintered alloys.The fracture mechanism of FeCoNi CrMn HEAs was micropore aggregation fracture.