Study On Microstructure And Mechanical Properties Of New Ferreous Medium Entropy Alloys

Face-centered cubic(FCC)medium or high entropy alloys show outstanding strength,ductility and cryogenic properties for their unique composition and structure,showing promising application.However,the strain hardening of the FCC alloys is insufficient under high strength or large strain.And,abundant high-cost elements increase the cost of industrialization.In this study,a low-cost ferrous metastable medium entropy alloy with the constituent of(Co20Cr12.5Fe62.5-x Mo3 Nix V2)99.8C0.2(x = 7.5,5,2.5)was designed and fabricated,which are referred to as Ni7.5,Ni5 and Ni2.5,respectively.The effects of composition and heat treatment on microstructure and mechanical properties of the alloys were studied.The influence of matrix stability on deformation behavior and strain hardening mechanism was emphasized,in order to provide reference for the development of medium/high entropy alloys with industrial application prospects.The annealed Ni7.5 alloy is dominated by FCC-phase,and decorated with σ-phase.With the increase of annealing temperature,σ-phase gradually dissolved into matrix.At the initial stage,deformation was dominated by dislocation slip.The deformation induced stacking faults,highly dense dislocation walls and microbands can effectively impede dislocation migration and improve dislocation storage,promoting strain hardening.With the development of deformation,deformation twins generated an obvious twinning induced plasticity(TWIP)effect,which promoted the strain hardening sustaining to the strain of 0.4.After annealing at900 °C for 30 min,the strain hardening rate of Ni7.5 alloy kept stable at ～2000 MPa in the strain range from 0.05 to 0.32,and the total elongation(TE)increased to 53% while the ultimate tensile strength(UTS)remained at 814 MPa,alleviating the strength-ductility tradeoff.The stability of FCC-phase can be reduced by decreasing the Ni content.After annealed at 900 °C for 60 min,the Ni5 alloy was dominated by FCC-phase with lower stability,decorated with numerous nano-sized MC-type and micron-sized M6C-type carbides.During the deformation,the transformation induced plasticity(TRIP)effect occurred by FCC-phase transforming to BCC-phase,which greatly improved the strain hardening.Strain hardening rate kept rising to ～2700 MPa in the strain range from 0.10 to 0.22,and the UTS and TE reached to 800 MPa and 46%,respectively.When the Ni content was further reduced to 2.5 at.%(Ni2.5),the matrix was dominated by BCC-phase decorated with dispersive precipitates and the TRIP effect disappeared.After annealed at 900 °C for 60 min,the Ni2.5 alloy presented a high UST(1148 MPa),but a low TE(3.5%).The TRIP effect and strain hardening capacity can be adjusted with carbon allocation in Ni5 alloy by heat treatment.With the decrease of annealing temperature and time,interstitial carbon atoms gradually precipitated out,forming numerous carbides with uneven distribution.The precipitation of interstitial carbon atoms enhanced the stability of metastable FCC-phase,while the heterogeneous carbides promoted the heterogeneous grains.The allocation of carbon atoms not only stabilizes the metastable FCC-phase from composition,but also promotes the strain-induced phase transformation domain by domain from structure,improving the sustainability of strain hardening and overcoming the strength-ductility trade-off in Ni5 alloy.After annealing at 850°C for 30 min,the peak strain hardening rate of Ni5 alloy reached to～4000 MPa at the strain of 0.37,and the UTS and TE increased to 942 MPa and 59%,respectively,achieving an excellent strength-ductility matching.