Study On The Microstructure And Properties Of Multi-principal Component CoFeNiMn High Entropy Alloy System

Since the concept of high entropy alloys proposed by Yeh et al,lots of researchers have paid great attention to the development of high entropy alloys.Different from traditional alloys,high entropy alloys exhibit remarkable characteristics,such as high entropy effect,lattice distortion effect,and hysteresis diffusion effect,as well as cocktail effect.Thus,they have special structure and properties,such as high strength and hardness,excellent wear resistance and good magnetic properties.Due to the synergistic effects among the main constituents,addition of alloying elements might significantly alter the microstructure,phase structure and properties of the alloys.Therefore,high entropy alloys with excellent comprehensive properties can be obtained by adjusting the type and content of the alloying elements.In present thesis,the high entropy alloys including CoFe_xNiMn_0.3,CoFe₂NiMn_0.3Al_x,CoFe₂NiMn_0.3Al Cu_x,CoFeNiMnTi_xand CoFe₂NiMn_0.3Ti_xalloys,are successfully prepared by vacuum arc melting method.Firstly,the phase structure and microstructure of the CoFeNiMn high entropy alloy system were analyzed by X-ray diffractometer?XRD?and scanning electron microscopy?SEM?.Secondly,the mechanical and magnetic properties of the alloys were examined by electronic universal testing machine,microhardness tester and vibrating sample magnetometer?VSM?.The main conclusions are drawn as follows:?1?The CoFe_xNiMn_0.3?x=1.0,1.5,2.0,2.5?high entropy alloys with high strength and excellent soft magnetic properties were prepared.With an increase of Fe content,the crystal structure transits from a single FCC phase to a mixture of FCC+BCC phases,and the yield strength,microhardness and soft magnetic properties of the alloys can be also significantly enhanced.The alloy with x=2.5 exhibited the optimized mechanical properties and soft magnetic properties.The yield strength and microhardness reach 300 MPa and 182 HV respectively,and the saturation magnetization and coercivity are 134.2 emu/g and 17.75 Oe,respectively.?2?The addition of Al element not only promotes the formation of BCC phase,but also improves the microhardness and yield strength of the CoFe₂NiMn_0.3Al_x?x=0.25,0.5,0.75,1.0?alloys.It is ascribed to the increased amount of BCC phase and solution strengthening effect.The alloys with x=0.25 and x=0.5 exhibit low yield strength and excellent plasticity.With the increase of Al content,the yield strength of the alloys improves,while their plasticity decreases.For alloys with x=0.75,the yield strength,fracture strength and fracture strain are equal to1100 MPa,2335 MPa,and 33.24%,respectively.As Al content is further increased to x=1.0,both of yield strength and fracture strength decrease and the alloys are brittle in nature.VSM results show that CoFe₂NiMn_0.3Al_xhigh entropy alloys process high saturation magnetization and low coercivity,suggesting that they show excellent soft magnetic properties.With the increase of Al content,their saturation magnetization firstly decreases and then increases.The saturation magnetization reaches the minimum value of 63 emu/g for alloy with x=0.5,while is reaches the maximum one of 148 emu/g for alloy with x=1.0.?3?The semi-hard magnetic materials in the CoFe₂NiMn_0.3Al Cu_x?x=0.25,0.5,0.75,1.0?high entropy alloys were successfully prepared.Cu element is found to improves the strength and plasticity of the alloys,while it decreases the hardness.When the Cu content increases from x=0.25 to x=1.0,the yield strength and fracture strain increase from 682 MPa to 1780 MPa and 4.5%to 23%,respectively,while the hardness decreases from 647 HV to 408 HV.However,Cu addition decreases their magnetic properties.With an increase of Cu content,the saturation magnetization decreases from 131 emu/g to 74 emu/g,and the coercivity decreases from 182Oe to 40.5 Oe.?4?The influences of Ti content on the phase structure,mechanical properties and magnetic properties of CoFeNiMnTi_x?x=0.2,0.4,0.6,0.8,1.0?high entropy alloys are studied.The results show that the addition of Ti element promotes the formation of Laves phase.The crystal structure is a single FCC phase for alloys with 0.2?x<0.6,while it is identified as a mixture of FCC+Laves phases when x?0.6.The alloys with x=0.2 and x=0.4 show excellent plasticity.As the Ti content increases,the plasticity of the alloys decreases gradually but the yield strength increases firstly and then decreases.The yield strength,fracture strength and fracture strain are 1280 MPa,1950 MPa and 20.1%,respectively,for alloys with x=0.6.The hardness increases from 212 HV for alloys x=0.2 to 737 HV for alloys with x=1.0.The magnetic properties show that the CoFeNiMnTi_xhigh entropy alloys have soft magnetic properties,and the saturation magnetization decreases first and then increases with the increase of Ti content.?5?For the CoFe₂NiMn_0.3Ti_x?x=0.2,0.4,0.6,0.8,1.0,1.2?alloys,the crystal structure is a single FCC phase for alloys with 0.2?x<0.6,while it is composed of a mixture of FCC+Laves phases for alloys with x?0.6.As the Ti content increases,the yield strength firstly increases and then decreases,while the plasticity decreases.CoFe₂NiMn_0.3Ti_0.8alloy has the optimized mechanical properties,and their fracture strain,yield strength and fracture strength reach 21.5%,1400 MPa and 2150 MPa,respectively.As Ti content increases from x=0.2 to x=1.2,the hardness steadily increases from 198 HV to 773 HV.