Study On Microstructure And Properties Of FeCr_xNiMnCu_0.5 High Entropy Alloy

The high-entropy alloy has broken the traditional alloy design theory of using a single element as the primary component,and is composed of multiple main elements.FeCr Ni Mn-based HEAs,known for their excellent comprehensive mechanical properties and corrosion resistance,have been extensively studied.In this study,a small amount of Cu was added to FeCr Ni Mn-based HEAs,and FeCr_xNiMnCu_0.5（x=0.75,1.0,1.25）HEAs were prepared by vacuum arc melting furnace.The effects of Cr content and annealing temperature on the microstructure,mechanical properties,and corrosion resistance of the HEAs were investigated through annealing treatments at 600℃,800℃,and 1000℃.The research results show that the microstructure of the as-cast high-entropy alloys with three different Cr contents is dendritic in morphology.With the increase of Cr content,the grain size of the alloy becomes refined,and the structure transforms from single-phase FCC to dual-phase FCC+BCC.The hardness and compressive yield strength of the alloy increase continuously with the increase of Cr content.When x=1.25,the microhardness,compressive yield strength,and compressive strength of the alloy reach 249.5 HV_0.5,963.2 MPa,and 1214.3 MPa,respectively.The formation of BCC phase is the main reason for the improvement of hardness and strength.In tensile tests at room temperature,the strength of the alloy increases while the plasticity decreases with the increase of Cr content.When x=0.75,the yield strength and ultimate tensile strength of the alloy are 397.1 MPa and 675.3 MPa,respectively,with an elongation of 79.4%;when x=1.25,the yield strength and ultimate tensile strength of the alloy are 904.4 MPa and 1106.6 MPa,respectively,with a decrease in elongation to 42.4%.In 1 mol/L H₂SO₄ solution,when x=0.75,the alloy has the highest corrosion potential and the lowest corrosion current density,which are-0.232 V and 2.600×10^-6 A·cm^-2,respectively.The single-phase FCC structure and homogeneous microstructure contribute to its good corrosion resistance.In 3.5 wt.%Na Cl solution,when x=1.25,the alloy has the highest corrosion potential of-0.333 V and the lowest corrosion current density of 5.388×10^-7 A·cm^-2,exhibiting the best corrosion resistance.After annealing treatment of high-entropy alloys with three different Cr contents,it was found that the annealed alloys exhibited either a single FCC structure or a duplex FCC+BCC structure.In the annealed FeCr_1.25NiMnCu_0.5 high-entropy alloy at1000℃,the dendritic structure disappeared and a large amount of short rod-like new phases precipitated,while the other annealed alloys still showed a dendritic structure.The hardness of the annealed alloys increased compared to the as-cast state,with the highest hardness observed after annealing at 800℃for all three high-entropy alloys.Annealing at 1000℃,however,resulted in the decomposition of the FCC or BCC phases inside the alloy due to the excessively high temperature,leading to the formation of voids and a decrease in hardness.After annealing treatment,the strength of the high-entropy alloys with three different Cr contents was significantly improved in compression tests at room temperature,due to the effects of mechanisms such as grain refinement and solid solution strengthening.In 1.0 mol/L H₂SO₄ solution,FeCr_0.75NiMnCu_0.5 exhibited significant passivation behavior after annealing at 600℃,showing the highest passivation potential and the lowest corrosion current density,indicating the best corrosion resistance.FeCr_1.0NiMnCu_0.5 showed the best corrosion resistance in the as-cast state,with corrosion current density one order of magnitude lower than that of the annealed state.After annealing at 800℃,the segregation of elements in FeCr_1.25NiMnCu_0.5 was improved and it exhibited the best corrosion resistance.In 3.5 wt.%Na Cl solution,FeCr_0.75NiMnCu_0.5 annealed at 800℃showed the least corrosion tendency and the lowest corrosion rate,indicating the best corrosion resistance.The corrosion resistance of the as-cast and three different annealed states of FeCr_1.0NiMnCu_0.5 were similar.FeCr_1.25NiMnCu_0.5 exhibited the best corrosion resistance in the as-cast state,but its corrosion resistance decreased after annealing treatment.