Composition Design And Toughening Of CoFe₂Ni?Mo/V?-M High Entropy Alloys

The high-entropy alloys have great potential to be developed into a new generation of structural materials because of its structure and properties which are not possessed by many traditional alloys.But there are still many critical issues to be addressed.For example,in the application direction of high-temperature structure,body-centered cubic?BCC?high-entropy alloys have excellent high-temperature rheological resistance,but its high cost,poor oxidation resistance and corrosion resistance and other factors restrict its practical application.Face-centered cubic?FCC?high-entropy alloys have excellent toughness and plasticity,but their yield strength is very low and softening effect at high temperature is obvious.The purpose of this project is to further adjust and optimize the structure of FCC type high-entropy alloys by alloying composition design,and to analyze the toughening mechanism of thermo-mechanical processing means,so as to obtain better strength-plasticity matching of high-entropy alloys.On the basis of previous component designs,the CoFe₂NiV_0.5Mo_0.2 high entropy alloy with a single FCC phase structure was selected as the matrix component.By adding different alloying elements,the microstructure was regulated and the alloy components with good comprehensive properties were screened for thermo-mechanical processing.By adding large atomic radius replacement alloying elements represented by Ti,Nb and Zr,we found that(CoFe₂NiV_0.5Mo_0.2)_100-xTi_x series alloy microstructure changed from single-phase FCC structure?x=0,2?to FCC structure solid solution+Fe₂Ti intermetallic compound microstructure?2<x?10?.The addition of Nb/Zr elements makes the microstructure of the alloy change from single-phase FCC solid solution to hypoeutectic structure?x?8?to complete eutectic structure?x=9?and finally to hypereutectic structure.However,the structure of Laves phase was different in the eutectic structure,and the content of alloying elements needed for complete eutectic composition was basically the same.Among them,Nb alloyed high-entropy alloy has better comprehensive mechanical properties,and the lamellar structure of eutectic and near-eutectic is dense,which realizes the effective combination of the second phase strengthening,solid solution strengthening and interface strengthening.The compressive strength and yield strength are higher,and the ductility of the subeutectic alloy is higher.In terms of physical properties,the addition of Nb reduced the corrosion potential of the alloy and significantly increased the passivation capacity of the alloy.In addition,eutectic high entropy alloy has better stability of high temperature phase.By adding small atomic radius interstitial alloying elements represented by C and B,we found that(CoFe₂NiV_0.5Mo_0.2)_100-xC_x?x=0?10?alloy microstructure changed from single-phase FCC structure to eutectic structure?2<x?6??FCC matrix+V₈C₇ carbide fiber?and then to?FCC matrix+coarse V₈C₇+acicular MoC?.With the addition of C element,the yield strength of the alloy is improved obviously and the plasticity is maintained well.When x=6and 8,the yield strength and hardness of the high-entropy alloy can reach 900MPa and 270HV,with excellent comprehensive mechanical properties.The(CoFe₂NiV_0.5Mo_0.2)_100-xB_x?x=0,2,4?alloy structure is composed of FCC matrix and boride precipitation.Where,when x=2,the precipitate presents a dense amplitude modulation form,and the slat interface grows and bends.When x=4,the precipitates will gradually change into the granular shape,and the size will be refined at the same time.The compressive yield strength of the alloy increased with the addition of element B,but the increase in strength was not obvious.Finally,in this study,(CoFe₂NiV_0.5Mo_0.2)₉₈Ti₂ high-entropy alloy?called Ti2 alloy?was selected for homogenization,large-deformation rolling and annealing at different temperatures,and it was found that both Ti2 alloy and matrix high-entropy alloy?called Ti0 alloy?after homogenization were in the structure of a single FCC solid solution phase.In terms of chemistry,Ti and Mo elements of Ti2 alloy are more evenly distributed,eliminating the composition fluctuation between dendrite and dendrite caused by the difference of melting point.The homogenization treatment resulted in the decrease of tensile strength and the increase of plasticity of Ti0 and Ti2 alloys,among which the fracture plasticity of Ti2 exceeded 80%.The annealing treatment at 600??700?caused partial recrystallization of Ti2 alloy,and the isoaxial ultrafine grain structure was first generated in the severely deformed area.Its complete recrystallization was achieved by annealing above 800?.With the increase of annealing temperature,the grain size,strength and plasticity of the annealed alloy become larger.