Study On The Microstructures,Mechanical And Corrosion Properties Of FeCrNiAlTi High-Entropy Alloys With Precipitation Strengthening

High-entropy alloys（HEAs）,Multi-principal-element alloys,often have simple crystal structures,e.g.,body-centered cubic（BCC）,face-centered cubic（FCC）,hexagonal-close-packed（HCP）and ordered derivatives（such as B2（c P2-Cs Cl）,L1₂（c P4-Au Cu₃）L2₁（c F16-Bi F₃））.HEAs have broken the traditional alloy design concept based on one or two principal components,and greatly enlarged the space of the alloy composition design.Remarkably,in Al-LTMs HEAs that Al and/or Ti are added to the LTMs-based HEAs（LTMs:late transition metals,including Mn,Fe,Co,Ni,Cu etc.）,researchers considered that the alloys can improve the toughening owing precipitation strengthening,obtain excellent mechanical properties and have good application potential for constructional materials.However,most of these HEAs contain expensive Co element have high preparation cost,so the application of HEAs is limited.For this reason,this work adopted the replacement method of similar elements to remove the expensive Co element,and Fe Cr Ni Al Ti HEAs were prepared by means of arc melting under an argon atmosphere.To regulate microstructure by several physical parameters of HEAs,Fe Cr Ni Al Ti HEAs could gain the uniform distribution of high-density L2₁ nanoparticles in the BCC matrix.The relationship between microstructure and mechanical properties is revealed,and the cause of microstructure evolution and strengthening mechanisms are also discussed.A systematic study on the composition-structure-corrosion resistance relationship by electrochemical analysis in the 3.5 wt.%Na Cl solution at 30℃,and the relationship among the microstructure,the passive film character and corrosion resistance was established.The conclusions are as follows:According to the framework of VEC andδparameters,the phase-evolution and the phase components in the current HEAs and in many previously-reported HEAs are well predicted.In Fe₄Cr Ni（Al Ti）_x（x=0,0.2,0.6 and 0.8,denoted as（Al Ti）_x）HEAs,the phase components change from the single face-centered cubic（FCC）phase to FCC+BCC+L2₁ phases as the Al and Ti content increases from 0.2 to 0.6 and 0.8,respectively.In Fe_5-xCr_xNi Al_0.3Ti_0.3（x=1.13,1.25,1.36,1.47和1.67,denoted as C13,C25,C36,C47和C67）HEAs,with increasing the Cr/Fe ratio,there is a phase transition from FCC to BCC/L2₁.According to the calculating formula of VEC,increasing the Al and Ti content and the Cr/Fe ratio can decrease VEC value of the alloys,and can be prone to the formation of BCC-related phases in the alloys.By adjusting the microstructures of in the current HEAs with L2₁ nanoprecipitation,and then excellent mechanical properties can be obtained.The true compressive strength of Fe₄Cr Ni Al_0.5Ti_0.5（Fe4）and Fe₆Cr Ni Al_0.5Ti_0.5（Fe6）are 1930 MPa and 1791 MPa,respectively,and the two alloys does not fracture even at a true compressive strain of 70%.Fe4 and Fe6exhibit the best combination of the compressive strength and fracture strain among all the reported BCC HEAs with nanocrystals,which is attributed to the optimized microstructure with high-density L2₁ nanocrystals uniformly distributed in the BCC matrix.C25 has multiphase structure of FCC（23.6%）and BCC/L2₁（76.4%）,and the good combination of ultimate strength（1142 MPa）and plasticity（9.5%）can attribute to the synergetic effects of the microstructure.The major factor in strength increment of Fe Cr Ni Al Ti HEAs with precipitation strengthening:the volume fraction and the particle size of L2₁ nanoparticles,the lattice misfit between the L2₁ and BCC phases.According to the formulas of the precipitate shearing and Orowan bowing mechanisms,in Fe_xCr Ni Al_0.5Ti_0.5（x=2,4,6）HEAs,the actual average sizes of L2₁ nanoparticles are larger than the critical sizes（r₀）,suggesting the Orowan bowing mechanism is dominant.Therefore,the largest strength increment can be obtained by decreasing of the particle size in Fe_xCr Ni Al_0.5Ti_0.5 HEAs.This thesis summarizes that the effects of the composition and microstructure on the corrosion resistance of Fe-Cr-Ni-Al-Ti HEAs.Under the same condition of microstructure,the more Cr,Al and Ti contents in the current HEAs,the stronger the formation ability and the protection capability of the passive film will become,that is,the corrosion resistance of these alloys will become stronger.The alloys with multiphase structure of FCC and BCC/L2₁ can exhibit poor corrosion resistance,such as（Al Ti）_0.6 and C36.According to the equivalent electrical circuit,the surface of Fe-Cr-Ni-Al-Ti HEAs are composed of two parts,the first part of the surface is covered with the protective passive layer,whereas the second part is the vulnerable area where pits prefer to form due to the Cr depletion and/or surface defects.