Research On Mechanical Properties Of Fe₄₀Mn₄₀Co₁₀Cr₁₀ High Entropy Alloy Deformed At Cryogenic Temperatures

Differring from the desigh of traditional alloys based on one or two principal elements,multi-principal high entropy alloys generally include multi-principal elements.Because of the high entropy effect,the sluggish diffusion effect,the severe lattice-distortion effect and the cocktail effect,high entropy alloys(HEAs)have excellent physical,chemical and mechanical properties,such as high hardness,high strength,high wear resistance,excellent corrosion resistance,and high low temperature toughness.Therefore,the high entropy alloy possess broad prospects of application and research value.In this work,we research on a single phase Fe40Mn40Co10Cr10 high-entropy alloy with face-centered cubic structure.The alloys are obtained by a series of processing and heat treatment such as homogenization annealing,cold rolling and short-time annealing.The difference of mechanical properties,microstructure and deformation mechanisims at different temperatures have been systematically investigated.In addition,the influence of cryogenic rolling on mechanical properties and microstructure of the Fe40Mn40Co10Cr10 alloy has been investigated for improving the mechainical properties under room temperature.The main conclusions are as follows:Both strength and ductility increase significantly with the probing temperature decreasing.When deformed at 293 K,the FG alloy exhibits the yield strength of 272 MPa,ultimate tensile strength of 567 MPa and maximum elongation of 49%.At 77 K we observed the yield strength of 567 MPa,ultimate tensile strength of1003 MPa and maximum elongation of 65%.At 293 K dislocation slip and mechanical twinning are the prevalent deformation mechanisms.The stacking fault energy decreases and flow stress of fcc matrix increases as the deformation temperature drops is responsible for the formation of martensite under cryogenic deformation.The enhanced work hardening rate is attributed to the variation of deformation mechanisms from dislocation slip to twin deformation and further to phase transformation.The dislocation mean free path is reduced by the profuse fine martensite bundles and the dense twin boundaries,contributing to the alloy’s enhanced strength at cryogenic temperatures.The joint contribution of martensitic transformation and mechanical twinning explains the excellent strength-ductility combination of the alloy when deformed at cryogenic conditions.Such a state where athermal deformation mechanisms can be jointly activated upon loading is achieved by a metastability alloy design approach where the required low stacking fault energy is realized by the reduced Ni and the increased Mn contents relative to the equiatomic FeCoMnCrNi composition.The dual phase structure of γ+ε was abtained by rolling at cryogenic temperature is attributed to improve the mechanical properties of the alloy at room tempareture.When the rolling reduction of 30%at cryogenic temperature and subsequently annealed at 900℃ for 10 s,the yield strength and ultimate tensile strength increased to 580 MPa and 610 MPa respectively and uniform elongation decreased to 14%.However,The strength decreased and ductility increased owing to the phenomenon that ε-martensite transformation to γ-austenite with the annealing time extended.