Microstructure And Mechanical Properties Of FeCoNiCrMn(Nb,Sn)_x HEAs

High entropy alloys?HEA?wre composed of five or more than five elements,and the content of each element should be more than 5 at.%and less than 35 at.%.Because the high entropy effect,the high entropy alloys usually formed with one or more kinds of simple structure solid solution,and the properties of the high entropy alloys are different from the conventional alloys.Recently,in order to meet the requirements of the materials,people began to study the effect of the adding elements and the content of the elements on the microstructure and properties of the alloys.In this thesis,FeCoNiCrMnNbx?x=0,0.1,0.25,0.5,0.75?and FeCoNiCrMnSnx?x=0,0.03,0.07,0.1,0.3,0.5?HEAs were prepared by the arc melting method.FeCoNiCrMnNbx HEAs were heated at 1000?for 10h,then quenched in water?WQ?or cooled down to room temperature in the furnace?FC?,and FeCoNiCrMnSn_x HEAs were heated at different temperature?600??800??1000??for10h,then quenched in water?WQ?.The microstructure and mechanical properties of the as-cast and annealed FeCoNiCrMnNb_x and FeCoNiCrMnSn_x HEAs were investigated using X-ray diffraction?XRD?,optical microscopy?OM?,scanning electron microscopy?SEM?,hardness and room temperature compressive tests.The aim is to study the effect of Nb?Sn and heat treatment on the microstructure,phase consitituents,and properties of the FeCoNiCrMnNb_x and FeCoNiCr MnSn_x HEAs.Meanwhile,the Thermo-Calc program was used to calculate the equilibrium phase,phase mole fraction and concentration changes in each phase at different temperature for the FeCoNiCrMnNb_x HEAs.The main conclusions were as follows:With the Nb content increase,the phase consitituents of as-cast FeCoNiCrMnNb_x HEAs change from single FCC phase?Nb-depleted?to FCC and Laves phases?Nb-rich?,which is consist with the thermodynamic calculation.With x increase,the microstructure varies from the pure FCC solid solution to the typical dendrite structure,the hardness and yield stress are improved,while the fracture strain decrease relevantly.The Nb0.5 alloy gets a good balance of stress and strain,whose fracture stress and fracture strain are 2025 MPaand 28.6%,respectivily?The phase consitituents of as-cast FeCoNiCrMnSn_x?x=0?0.03?0.07?0.1?0.3?0.5?HEAs are influenced by the composition and heat treatment temperature.With the Sn content increase,the phase consitituents have the transformation of FCC?Sn-depleted??FCC+MnNi2Sn?Mn?Ni?Sn-rich?,and the volume fraction of MnNi₂Sn phase increases with Sn increase,the microstructure of FeCoNiCrMnSnx alloys has a structure refinement.The microstructure of FeCoNiCrMnSn_x alloys varies from pure FCC solid solution to the typical dendrite structure,the hardness and yield stress are improved,while the strain is decreased.With the heatreat temperature increase,the hardness?yield strength and the MnNi₂Sn volume fraction in the Fe Co NiCrMnSn_0.03?FeCoNiCrMnSn_0.1?FeCoNiCrMnSn_0.3.3 alloys have the tendency of first increase and then decrease,and they get the max after 800?heatreatment;While,the hardness?yield strength and MnNi₂Sn volume fraction in FeCoNiCrMnSn_0.07?FeCoNiCrMnSn_0.05 alloys have a first decrease?then increase and last decrease tendency with the heatreatment temperature increase,similarly,they get the max after 800? heatreatment.