Microstructure Control And Properties Reserch Of FeMnCrCo Dual-phase High Entropy Alloys Prepared By Vacuum Melting

In this thesis,Fe_65-xMn₂₀Cr₁₅Co_x（x=5,10,15,20）high-entropy alloys were prepared by vacuum melting using four metals of Fe,Mn,Cr,and Co as raw materials,and its structure and performance were systematically analyzed and discussed.Besides,Fe₄₅Mn₂₀Cr₁₅C₂₀ high entropy alloy as the research object,in-depth exploration of the effect of annealing temperature on its structure and performance.The as-cast Fe_65-xMn₂₀Cr₁₅Co_x（x=5,10,15,20）high-entropy alloys are all dual-phase structure ofγaustenite+εmartensite.As the Co content increases,the stacking fault energy of the alloy system gradually decreases,the coarse lath martensite structure is gradually refined,and the two-phase content and phase interface area also change accordingly.The actual composition of the high-entropy alloys prepared by the vacuum melting is almost the same as the nominal composition,and there is no obvious element segregation phenomenon.At the same time,under the influence of the high entropy effect,the number of twin boundaries in the alloy gradually increases,the large-angle boundary gradually decreases,and the grain size gradually decreases.Besides,there is an endothermic peak and an exothermic peak in the as-cast alloy during the heating process,which respectively correspond to theε→γandγ→εphase transitions.And the characteristic temperature of the phase transition gradually shifts towards the high-temperature direction as a whole.With the increase of Co content,the density of the alloy system gradually increases,the hardness and elongation gradually are reduced,the tensile strength is increased first and then decreased,and the tensile strength is up to 849.3 MPa.The magnetism of the alloy gradually decreases and changes from ferromagnetic to paramagnetic.At room temperature,with the increase of strain amplitude,the damping internal friction curve first increases rapidly and then tends to be flat,the maximum value Q^-1_max=0.0595;while the damping internal friction curve under different test frequencies will show three types of changes;the high-entropy alloys of each composition have a highε→γphase transition damping internal friction peak at about200℃.Also,the corrosion resistance of the alloy system increased first and then decreased.Among them,the Fe₅₀Mn₂₀Cr₁₅Co₁₅ high-entropy alloy has the best corrosion resistance.Annealing treatment has a significant effect on the microstructure and properties of Fe₄₅Mn₂₀Cr₁₅C₂₀ high-entropy alloy.The results show that no new phases were formed in the annealed alloy,but the phase fraction content of each phase changed.With the increase of annealing temperature,the content ofγaustenite phase gradually decreases,while the content of theεmartensite phase gradually increases.The number of large-angle grain boundaries gradually increases,and the structure continues to increase.And when annealed at 1000℃,a regularly arranged checkered structure and annealing twins are formed.As the temperature increases,the magnetic properties of the annealed alloy gradually increase and change from paramagnetic to ferromagnetic.The tensile strength and elongation of annealed alloys decrease,while the Vickers hardness improves slightly.The damping performance after annealing has been significantly improved.The Q^-1_max of the alloy annealed at 1000℃is 0.0729,an increase of 29%.When the annealing temperature is further increased,the damping performance drops sharply,which is related to the decrease in the number of damping sources such as twins,stacking faults,and phase interfaces in the alloys,and the increase in internal stress.The corrosion resistance of the annealed alloy has also been significantly improved,with the best corrosion resistance at 900℃.When the annealing temperature is further raised,the internal stress at the grain boundary is increased,which in turn makes its corrosion resistance decrease.