Microstructures And Mechanical Properties Of Mn-Fe-Co-Ni-Cu High-entropy Alloys

High entropy alloys containing multiple principal elements refers to the alloy system composed of five or more kinds of alloy elements according to the(nearly)equiatomic ratios.A variety of new alloy systems with unique properties have been proposed,in which the FCC alloy system has excellent plasticity and is considered to have well application potential.However,Cr has been a frequently adopted constituent in these alloy systems,such as CrMnFeCoNi.Researches further found that the presence of Cr,which induces precipitation of brittle ?-phase or other compound phases after annealing at intermediate temperatures,which generally deteriorates the mechanical property.For the purpose of eliminating ?-phase precipitation tendency,Cu was used to replace Cr in CrMnFeCoNi in this work.Equiatomic MnFeCoNiCu HEA and its Cu-containing quaternary subsets(Fe Co Ni Cu,Mn Co Ni Cu,MnFeNiCu and Mn Fe Co Cu)were made by induction melting and gravity casting,those five HEAs in as-cast condition show a dendriticlike morphology with dual/multiple phases.The FCC Cu-rich precipitates exist in interdendrite regions of those five alloys with different morphology and distribution.Due to the weak immiscible tendency between Cu and other four elements,Cu tends to be enriched in the liquid phase and forms Cu-rich precipitates in interdendrite regions during solidification.It is notable that the Cu-rich precipitates in Mn Fe Co Cu show irregular flocculent morphology,quite different with other four alloys.The tensile properties show a good plasticity and the elongation of five alloys is all over 30 %,and the MnFeNiCu alloy has the best integrated mechanical properties.In order to eliminate the component segregation,the method of long time annealing at high temperature was chosen to obtain homogenized solid solution.After a long period of annealing at high temperature,the Mn Fe Co Cu alloy retains obvious dendrite structure without any precipitation,and the element segregation was obviously improved compared with the casting state.After annealing,the MnFeCoNiCu,Fe Co Ni Cu,Mn Co Ni Cu and MnFeNiCu alloys all show homogenized isometric crystal with a coarse grain size,and the weeny precipitates were observed in Mn Co Ni Cu alloy.However,due to the coarse grain size of the annealed alloys and the defects in grain boundary after high temperature annealing,their tensile properties were not ideal.Based on the characteristics that Cu tends to induce precipitation of the second phase,the MnFeNiCu alloy was chosen to systematically research the second phase precipitation behavior and its influence on mechanical properties.It has been prepared by arc melting,drop casting,homogenization,cold rolling and recrystallization annealing.After annealing at 800 ?/1 h,the fine Cu-rich precipitates distribute dispersive and homogeneously,and no obvious regional distribution was observed.Its tensile properties at room temperature were significantly improved compared with those in as-cast condition,and the yield strength and ultimate tensile strength reached 825 MPa and 933 MPa,respectively,but the elongation rate decreased to 15 %.The main strengthening mechanism of MnFeNiCu alloy in this state is the precipitation strengthening of Cu-rich precipitates,and the HR-TEM results indicate that the interaction mechanism between dislocation and Cu-rich phases includes shear mechanism.In this research,the Cr-free MnFeCoNiCu HEAs system exhibit well comprehensive mechanical properties.It is expected to get a better performance HEAs system without existence of brittle phase though regulating the precipitation of the Cu-rich phases.This study provides a useful reference and theoretical basis for the development and strengthening of Cr-free high-entropy alloy system.