Enhancement And Mechanisms Of Mechanical Properties In Dual-Phase Medium-Entropy Alloys Of Fe_xNi₂Al

In recent years,a significant theoretical breakthrough in developing high-performance metallic structural materials is the concept of multi-principal element alloys,also known as high-entropy alloys（HEA）or medium-entropy alloys（MEA）.Due to their unique composition and microstructure,multi-component alloys have excellent properties such as high strength,high hardness,high oxidation resistance,high corrosion resistance,and high thermal stability,which have broad application prospects.This article studies the microstructure evolution and mechanical properties of Fe_1.5Ni₂Al medium-entropy alloy under different annealing temperature conditions and FeNi₂Al lightweight medium-entropy alloy under different solidification conditions.The phase composition,microstructure,and mechanical properties of the alloys were studied using methods such as EBSD,SEM,XRD,EDS,TEM,hardness,and mechanical performance testing.The main research content and results are as follows:Firstly,the microstructure and mechanical properties of Fe_1.5Ni₂Al entropy alloy are controlled by regulating the heat treatment temperature.Fe_1.5Ni₂Al entropy alloy is composed of Fe-rich L1₂ phase and Al-Ni-rich B2 phase.The as-cast Fe_1.5Ni₂Al MEA has a high tensile strength of 1425.7 MPa and a tensile strain of 4.7%.After annealing at 700℃ and 800℃for2 hours,the L1₂ phase content of the alloy sample significantly decreases,leading to an increase in strength but a decrease in plasticity to around 3%.However,after annealing at 900,1000 and1100℃for 2 hours,the L1₂ phase precipitation in the alloy sample increases to more than 20%,indicating that this alloy system is favorable for the precipitation of L1₂ phase in this temperature range,which leads to an improvement in plasticity to around 6%,but the strength also decreases.As the annealing temperature increases to 1200℃,both the strength and plasticity of the alloy begin to deteriorate due to a sharp decrease in the L1₂ phase content to13.3%in the alloy sample.Therefore,the alloy is more susceptible to failure under tensile stress.After heat treatment,the mechanical properties of the alloy system are excellent below 400℃,but they rapidly deteriorate in a high-temperature environment exceeding 600℃.Secondly,the as-cast FeNi₂Al MEAs were prepared by direct-chilling casting,and the microstructure and mechanical properties of the alloy were controlled by adjusting the temperature of the copper mold.FeNi₂Al HEA is composed of the Fe-Ni rich L1₂ phase and the Al-Ni rich B2 phase.As the casting temperature decreased,the plasticity of the FeNi₂Al MEA significantly improved while the strength and hardness only slightly decreased.Compared with the as-melted alloy,the maximum tensile strain rate of the as-cast alloy prepared at a casting temperature of 290 K increased from 6.1%to 13.1%,while the maximum tensile strength remained above 1.1 GPa.Regarding compression performance,the maximum compression strain of the FeNi₂Al MEA increased from 33.8%to 39.2%,while the maximum compressive strength remained above 2.1 GPa.This is because the significant grain refinement and the change in the L1₂ phase morphology from randomly distributed particles in the matrix to a network structure distributed along the grain boundaries,which result in an increase in the grain boundary and interphase area,which maintains a high strength of the alloy system.The increase in grain refinement,L1₂ phase content,and the appearance of a metastable transition zone between the L1₂ and B2 phase interfaces can effectively relieve stress concentration.The synergy of these mechanisms results in a 7.0%increase in the plasticity of the FeNi₂Al MEA.