Research On Microstructures Control Of Fe₄₀Mn₁₀Cr₂₅Ni₂₅ High Entropy Alloy And Deformation Mechanism At Ambient And Cryogenic Temperatures

High entropy alloys(HEAs)have many excellent properties,such as high strength,high hardness,good low temperature performance,good thermal stability,and good corrosion resistance,due to the variable of compositions and microstructures.The HEAs with single-phase face-centered(FCC)structure exhibit excellent tensile ductility,however,their strength is relatively low.So far,the HEAs with FCC structure usually contain expensive transition metal elements,which also limits the application of HEAs in the engineering field.In order to reduce the production cost of FCC HEAs,it is necessary to reasonably design the alloy composition.In order to improve the strength of HEAs with FCC structure,the microstructure should be reasonably adjusted by introducing defects such as dislocations,grain boundaries,and twin grain boundaries.In this paper,several low-cost alloy elements are selected as the components of the current HEA,and a HEA with a composition of Fe₄₀Mn₁₀Cr₂₅Ni₂₅ is designed to reasonably reduce the cost of the alloy.Taking Fe₄₀Mn₁₀Cr₂₅Ni₂₅ HEA as the research object,the microstructures of the alloy are adjusted by cold rolling and then heat treatment process,and the room temperature and liquid nitrogen cryogenic-temperature mechanical properties are studied,the following conclusions are obtained:(1)The as-cast Fe₄₀Mn₁₀Cr₂₅Ni₂₅ HEAs show a single-phase FCC structure.Cold rolled alloys only undergo recovery when annealed at a lower temperature(?700°C).When annealed at intermediate temperature(?750°C),a partial recrystallized structure was obtained.The partial recrystallized alloy consists of coarse non-fully recrystallized grains and fine recrystallized grains.With the increase of annealing temperature(800-900?),completely recrystallization occurred.(2)The yield strength of cold rolled Fe₄₀Mn₁₀Cr₂₅Ni₂₅ HEAs decrease with the increase of annealing temperature.The Fe₄₀Mn₁₀Cr₂₅Ni₂₅ HEAs with particle recrystallization structure show a good combination of higher yield strength(?710 MPa)and tensile elongation(total elongation?18%)at ambient temperature,breaking the strength-ductility trade-off.Based on the contribution of grain boundary strengthening,dislocation strengthening and precipitation strengthening,a theoretical model for predicting the yield strength of Fe₄₀Mn₁₀Cr₂₅Ni₂₅ HEAs were established,and it showed good agreement with the experimental results.(3)Under tensile load,the deformation mechanism of fully recrystallized alloy is dislocation slip,and a small amount of stacking faults occurred in particle recrystallized alloys.The stacking faults reduced the stress concentration caused by dislocations,then,the necking was retarded.(4)The stacking fault energy of Fe₄₀Mn₁₀Cr₂₅Ni₂₅ HEAs at 77 K is 18.1 m J/m² was obtained by molecular dynamics simulation.The Fe₄₀Mn₁₀Cr₂₅Ni₂₅ HEAs with lower stacking fault energy formed a large number of twins under the tensile stress at cryogenic temperature.The formation of deformation twins greatly improves the tensile properties of the Fe₄₀Mn₁₀Cr₂₅Ni₂₅ HEAs compared to the tensile properties at room temperature.The strength increases by about 1.3-1.6 times and ductility increase by about 1.5-2.6 times.(5)Both the grain size and grain orientation affect the twinning of Fe₄₀Mn₁₀Cr₂₅Ni₂₅ HEAs under tensile load at cryogenic temperature.The larger the grain size,the smaller the critical twin stress required.When the flow stress of the alloy reached the critical twin stress,twinning occurred;When the grain orientation in Fe₄₀Mn₁₀Cr₂₅Ni₂₅ HEAs is close to the[111]paralleled to the tensile axis(TD),the grain is more likely to form twins during tensile deformation at cryogenic temperature.When the grain orientation is close to the[001]//TD,the grains are prone to undergo dislocation slip.