Effect Of Alloying On Mechanical Behavior Of Fe-Mn-Cr-Ni Medium Entropy Alloys

Medium entropy alloy is a new type of metal material developed in recent years,which has excellent structural stability and mechanical properties and has great application potential.However,for most of the studied medium entropy alloys containing Co,the cost of raw materials is high,while the strength of medium entropy alloys without Co of single face-centered cubic（FCC）structure is low,such as Fe-Mn-Cr-Ni medium entropy alloys system,which limits their engineering applications.Therefore,in this thesis,low-cost Fe₅₀Mn₂₅Cr₁₅Ni₁₀ medium entropy alloy was selected as the research object,and the effects of alloying on the mechanical behavior of Fe-Mn-Cr-Ni alloys system were investigated.The main research contents and results are as follows:（1）The effect of Co replacing Ni on the mechanical properties of Fe₅₀Mn₂₅Cr₁₅Ni₁₀ was studied,and it is expected to design an alloy with excellent comprehensive properties.The microstructure and mechanical properties of Fe₅₀Mn₂₅Cr₁₅Ni_10-xCo_xalloys were investigated systematically.It was found that with the increase of Co content,the crystalline structure changes from FCC single phase to FCC and hexagonal close-packed（HCP）dual-phase,and the strength of the alloy is improved at both room（298 K）and liquid nitrogen（77 K）temperature.However,because HCP phase is brittle at liquid nitrogen temperature,the ductility of Fe₅₀Mn₂₅Cr₁₅Ni₅Co₅ and Fe₅₀Mn₂₅Cr₁₅Co₁₀ alloys decreases at 77 K,while Fe₅₀Mn₂₅Cr₁₅Ni₁₀ alloy has a good combination of strength and ductility,and exhibits the highest tensile strength-plastic product at liquid nitrogen temperature,showing greater potential for low-temperature applications.（2）The effect of C microalloying on the mechanical properties of Fe₅₀Mn₂₅Cr₁₅Ni₁₀ medium entropy alloy was studied.It was found that C element has a strong solid solution strengthening effect.In order to further clarify the mechanical properties of C-alloyed alloys,the mechanical properties of Fe_49.5Mn₂₅Cr₁₅Ni₁₀C_0.5alloy were compared with those of traditional alloy.Through studying the effect of trace C element on the mechanical behavior of Fe₅₀Mn₂₅Cr₁₅Ni₁₀ medium entropy alloy,it was found that C element significantly improved the yield strength and tensile strength of the alloy.The mechanical properties of Fe_49.5Mn₂₅Cr₁₅Ni₁₀C_0.5 alloy and traditional alloys(Fe_74.5Mn₂₅C_0.5 and Fe_74.5Cr₁₅Ni₁₀C_0.5)were further studied,it was found that the yield strength of Fe_49.5Mn₂₅Cr₁₅Ni₁₀C_0.5 alloy is slightly higher than the other two alloys at room temperature.But the Fe_74.5Cr₁₅Ni₁₀C_0.5alloy has a best tensile strength（911 MPa）.As the temperature decreases from 298 K to 77 K,the strength of Fe_74.5Mn₂₅C_0.5 and Fe_74.5Cr₁₅Ni₁₀C_0.5 alloys increases,but the ductility decreases.However,the Fe_49.5Mn₂₅Cr₁₅Ni₁₀C_0.5 medium entropy alloy has a good combination of strength and ductility at liquid nitrogen temperature,which is mainly attributed to the appearance of deformation twins during low-temperature stretching,that is,twinning-induced plasticity（TWIP effect）.（3）The effects of Al alloying on the microstructure and mechanical behavior of Fe₅₀Mn₂₅Cr₁₅Ni₁₀ medium entropy alloy were studied.It was found that the crystalline structure changes from FCC single phase to FCC and body-centered cubic（BCC）dual-phase with the increase of Al content.The strength of the alloy also increases with the increase of Al content.In particular,when the Al content is 5 at.%,the alloy exhibits excellent mechanical properties with a good combination of tensile strength（1077 MPa）and ductility（～85%）at low temperature.The excellent performance is attributed to the combined effect of Al solid solution strengthening,TWIP effect and dual-phase heterogeneous structure strengthening.