Structural Modulation And Properties Of High-entropy Alloys Containing Co-Fe-Ni

Due to their unique atomic structure,high-entropy alloys?HEAs?possess a lot of distinguished properties which are different from traditional alloy design strategy.The research and huge potential application in engineering on HEAs have aroused extensive attention and interest in the international academic community.In this subject,the as-cast CoFeNiCr-based and CoFeNi Al-based HEAs master ingots were prepared by arc melting.In this paper,the phase structures of HEAs are modulated by changing the composition design,subsequent homogenization annealing or cryogenic treatment,which induce the precipitation of the second phase from the solid solution matrix.The mechanical properties,wear resistance and seawater corrosion resistance of the tested HEAs are expected to be improved and further significiantly enhanced by tuning the composition uniformity,size,distribution and existing behavior of the second phases.Our work would provide an experimental basis and technical support of the practical applications for HEAs.The main results are as follows:1.CoCrFeNiMo_x(x=0,0.2,0.5,0.8,1,noted as Mo₀,Mo_0.2,Mo_0.5,Mo_0.8,Mo₁,respectively)HEAs are annealed at 900?for 60 min for homogenization.The HEAs with Mo additionbecome unstable,and the phase separation appears.With increasing the value of x from 0 to 1,the products of homogenized annealing HEAs change from the initial single face-centered cubic?FCC?to FCC+FCC?+?phases,further FCC+?+?phases,then FCC+??+?+?phases.In Mo_0.2.2 HEA,the FCC?phase precipitates from the FCC matrix,while the?phase precipitate from the FCC'phase.Both Mo0.8 and Mo1 HEAs contain a?and new??phase precipitated around the?phase and from the matrix,respectively.The microhardness value of homogenizd CoCrFeNi Mox HEA is significantly higher than that of the as-cast or as-annealed HEAs with the same or similar components.The Mo1 HEA shows a maximum microhardness value of 896 HV,which is more than 4 times that of Mo0 HEA.The synergistic effects of serious lattice distortion and precipitation strengthen are mainly responsible for the increase of microhardness.The precipitation of a large amount of coarse??hard phase leads to the serious embrittlement characteristic of Mo_0.8.8 and Mo₁ HEAs.In addition,Mo_0.5.5 HEA possesses excellent comprehensive corrosion resistance in seawater solution.Appropriate Mo content makes the passive film containing MoO2 denser,which reduces the overall corrosion rate.2.The as-cast Ni₄₀Fe₃₀Co₂₀Al₁₀0 HEA has a single FCC phase structure,showing a dendritic omorphology.The as-cast Ni₄₀Fe₃₀Co₂₀Al₁₀0 HEA exhibits good strength-ductility combination as benchmarked with state-of-the-art as-cast single-FCC HEAs.This is mainly attributed to the remarkable work-hardening behavior,dense dislocation walls formed along the?phase boundary,and the existence of semi-coherent interface.Compared with 304L steel,the as-cast HEA possesses excellent comprehensive corrosion resistance of in seawater solution.The annealing and cryogenic treatments both induce a small amount of unknown phases precipitated from the?matrix of Ni40Fe30Co20Al10 HEA.Compared with other tested samples,the 800?-annealed HEA has larger average microhardness,which is about 19%higher than that of as-cast sample,meanwhile,has the most excellent wear resistance.According to the tensile testing,both annealing and cryogenic treatment are beneficial to improve the?_Y and?_UTSTS of HEA.After annealing at 1200?,the HEA has the enhanced strength without significant reduce of the elongation.Moreover,the cryogenic treatment is more beneficial to improve the synergy of strong and plasticity of HEA.The cryogenic treatment forces the point defects into the grain boundary,furhter resulted in the reduction of vacancies in the crystal lattice,meaning the reduction of crystal defects.So the structure uniformity can be improved,and the residual stress becomes weakened.Compared with the annealed and cryogenic samples,the as-cast HEA possesses excellent comprehensive corrosion resistance in seawater,which is mainly manifested by the most positive Ecorr,the lowest i_corrorr and i_passass value,and a wider?Ep value.3.The as-cast(Ni₄₀Fe₃₀Co₂₀Al₁₀)₉₀Ti₁₀0 HEA exhibits a typical dendritic morphology.The body-centered cubic?BCC?and??phase precipitated from interdendrite?ID?and dendrite?DR?regions of the?matrix,respectively.According to the tensile stress-strain curve,the as-cast sample has obvious brittleness,showing the poor mechanical properties.It is mainly due to the precipitation of??nanoparticles only from the DR region and the lack of continuous size gradient of??phase particles.Compared with 304L steel,the as-cast HEA possesses excellent comprehensive corrosion resistance in seawater solution.In addition to the Fe element,the Ti element in the HEA facilitates reaction with oxygen,which promotes the formation of passivation film.Besides?matrix phase and BCC phase,a small amount of unknown phases are formed under annealing and cryogenic treatment.Compared with the as-cast sample,the annealing and cryogenic treatment significantly reduced the lattice distortion.Appropriate annealing temperature can effectively improve the microhardness and wear resistance of as-cast sample,and 800?-annealed sample has the best ones.The annealing and cryogenic treatments didn't improve the tensile properties of(Ni₄₀Fe₃₀Co₂₀Al₁₀)₉₀Ti₁₀0 HEA,and the brittleness characteristics were obvious.Furthermore,compared with annealing and cryogenic treatments,the as-cast HEA possesses excellent comprehensive corrosion resistance in seawater.