Microstructure And Properties Of Ultrafine-grained Cr_xMnFeCoNi High Entropy Alloys Prepared By Powder Metallurgy

As one of the earliest studied high-entropy alloys,CrMnFeCoNi HEAs have been deeply and systematically investigated on the relationship between the structure and properties.In this paper,Cr_xMnFeCoNi series high-entropy alloys were prepared by mechanical alloying and spark plasma sintering.Firstly,the microstructures and mechanical properties of quaternary MnFeCoNi alloy were studied.The strengthening mechanism of the alloy was analyzed by theoretical calculation,and the influence of sintering temperature on its microstructures and mechanical properties was explored.On this basis,the effects of the addition and changing content of Cr on the microstructure,mechanical properties and corrosion resistance of MnFeCoNi alloy were studied.It was found that the four-component MnFeCoNi alloy powders exhibited a single FCC structure with an average grain size of 15.6 nm.The microstructures of sintered bulk by SPS(1000?)consisted of a matrix phase with FCC structure and a small amount of second phase with dispersive distribution.The chemical composition of the matrix phase is almost the same as the nominal composition,its volume percentage was 95.1%,and the average grain size was445 nm.The second phase were a variety of oxides containing Mn,the volume percentage was4.98%,and the average particle size was 73 nm.During the EBSD and TEM measurements of MnFeCoNi alloy,a large number of twins was observed.MnFeCoNi alloy exhibits good compressive and tensile properties at room temperature.The yielding strength of compression and tension reached 842 MPa and 756 MPa respectively,fracture strength reached 2714 MPa and 830 MPa respectively,and fracture strain were 55.6%and 17.5%respectively.Its strength is much higher than that of MnFeCoNi alloy prepared by casting method in literatures.A large number of uniformly distributed dimples were observed on the tensile fracture surface,and the second phase particles were distributed in some dimples.It demonstrated that the fracture mechanism of the alloy was ductile fracture.The excellent mechanical properties of the alloy were related to various strengthening mechanisms.Via calculation by theoretical formulas,it is found that the contribution percentages of grain boundary strengthening,dislocation strengthening,second phase strengthening and twin strengthening to the strength of the alloy were 55.4%,6%,10.6%and 16.4%,respectively.Compared with others,the grain boundary strengthening made the biggest contribution.The sintering temperature had little effect on the phase composition of MnFeCoNi alloy,but had obvious effect on the density,grain size and mechanical properties of the alloy bulk.With the increasing of sintering temperature,the density and grain size of the alloy increased,the strength increased and the plasticity decreased.During sintering,the grain would grow up when the sintering temperature rasied,and then,the grain boundry strength accordingly decreased.Therefore,sintering temperature had an indirect effect on the strength and plasticity of the alloy.In addition,there was a significant difference between compressive yield strength and tensile yield strength,which could be attributed to the different sensitivity of strength to pores in the alloys under compressive and tensile condition during mechanical properties testing,and the difference decreased when densities of the alloys increased with the increasing of sintering temperature.When Cr was added to MnFeCoNi alloy,most of Cr was dissolved into the FCC matrix phase.The dissolution of Cr had little effect on the lattice constant and lattice distortion of the FCC matrix phase.In addition,a small part of Cr was dissolved into the second phase to form new compounds containing Cr,Mn and O.The addition of Cr increased the solid solution strengthening effect of the alloy.With the content of Cr increasing,the proportion of twins in the alloys becames larger because of the lower stacking fault energy.The size and distribution of the second phase were also affected by Cr.With the increasing content of Cr,the size of the second phase became larger and the volume percentage increased.Because with the increase of Cr content,the distribution of the second phase was more dispersive,the hindrance to grain growth became smaller during sintering,consequently the grain slightly growed up.Cr also had a significant effect on the mechanical properties of the alloy.With the increase of Cr content in the alloy,the tensile and compressive properties of these alloys both increased in strength and decrease in plasticity.The fracture mechanism of these alloys were ductile fractures.The corrosion potential and pitting corrosion potential of Cr_xMnFeCoNi HEAs increased significantly with the increase of Cr content,and the corrosion current density and passivation current density decreased significantly,which demonstrated that the increasing content of Cr significantly improved the corrosion resistance of Cr_xMnFeCoNi HEAs in NaCl solution.The corrosion morphologies of Cr_xMnFeCoNi HEAs were pitting corrosion.After adding Cr,the number of corrosion pits on the surface of MnFeCoNi alloy decreased obviously.