An Investigation On The Microstructures And Mechanical Properties Of FeCoCrNiMn-based High Entropy Alloys

Fe Co Cr Ni Mn-based high entropy alloys have many excellent properties such as high strength and ductility,high toughness and great corrosion resistance.Owing to great potentials on the industries,the Fe Co Cr Ni Mn-based high entropy alloys have become a hotspot in the field of material science.Based on the Fe Co Cr Ni Mn-based high entropy alloys with fcc structure,several methods were used to promote the properties of alloys,including changing recrystallization fractions,adjusting the proportions of every main elements and adding interstitial carbon atoms.Meanwhile,relevant strengthening and toughening mechanisms have been discussed.The main research conclusions are as follows:（1）Microstructural evolutions of the partial recrystallized Fe Co Cr Ni Mn high entropy alloys during deformation are investigated.It seems that inhomogeneous deformation happens in the high entropy alloys.Strain partition happens in the partial recrystallized structures and strengthens the high entropy alloys,leading to both high strength and ductility during deformation.（2）Dynamics of recrystallization of the Fe Co Cr Ni Mn high entropy alloys are investigated.It can be observed that increase of strength and retardation of ductility happen with the continued heat treatment.The classical trade-off phenomenon consists of a two-stage strength-ductility variation.In the first stage,the yield strength drops fast without apparent ductility compensation.This stage covers the transition of texture and recovery of dislocations structures.In the second stage,the ductility increases remarkably.This stage includes the expansion of the recrystallized regions.（3）Effects of interstitial carbon elements on the dynamics of recrystallized of the Fe Co Cr Ni Mn high entropy alloys are investigated.Compared to the carbon free case,in the early stage the speed of recrystallization is a little bit faster.The reason is that some M₂₃C₆ carbides form in the vicinity of deformation bands and provide ideal sites for nucleation.In this stage the mechanical properties of the high entropy alloy change so fast.In the following stage,however,the speed of recrystallization has slowed down due to the precipitation of carbides.The carbides form stringer structures and pin the recrystallization fronts,leading to the retardation of recrystallization.In this stage the mechanical properties of the high entropy alloy don’t change so much.（4）By adjusting proportions of the main elements,metastable Fe₄₀Co₂₀Cr₂₀Ni₁₀Mn₁₀ HEA is created.Different microstructural evolutions happen in the high entropy alloys with different grain sizes during room temperature tensile tests.The results show that martensitic transformations will be inhibited by smaller grain sizes.The reason is that grain boundaries will hinder the movements of stacking faults therefore retard the growth of HCP nuclei.Effects of strain rates on the mechanical behavior is also clarified.From the prospective of mechanical behaviors,increasing strain rate leads to the increase of strength and decrease of ductility.From the microstructure point of view,high strain rate will inhibit the formation of martensite and launch twinning system for the reason of high temperature induced by the adiabatic phenomenon which inhibits the decomposition of perfect dislocations.In this paper,three kinds of Fe Co Cr Ni Mn-based HEAs were successfully prepared.The relationships between microstructures and deformation behaviors of the high entropy alloys were fully investigated by advanced characterization techniques,and the deformation mechanisms of the alloys were clarified.Therefore,based on these investigations,this paper provides an important theoretical reference for the Fe Co Cr Ni Mn-based HEAs.