Heterostructure Architecture And Enhancement Mechanism Of Strength-ductility In Cu-3.2Ti-0.2Fe-xV Alloy

Cu-Ti alloy is one of the environmental ultra-high strength and high elastic electronic copper alloys,which is used to substitute the Cu-Be alloy,currently,its low elongation is always less than 5%,which can’t meet the property requirement of the ultra-thin and heterogenic elastic component.The ultra-high strength and high ductility is the basement for commercial application of high performance Cu-Ti alloy.Multiphase and micro-nano heterostructure architecture is an effective way to synergistically improve the strength and plasticity.In this project,the volume fraction and stability of Laves were optimized by minor V element in the Cu-3.2Ti-0.2Fe alloy,the continuous and discontinuous precipitate behavior were clarified,the heterostructure were architected by thermo-mechanical treatment,and the mechanism of deformation and synergy enhancement of strength-ductility were revealed.Effect law of V element microalloying on the microstructure of casting and solid solution and phase were researched in the Cu-3.2Ti-0.2Fe alloy.Thermodynamic calculation indicated the volume fraction of Fe₂Ti（Laves）was increased by V element.And the result of first-principles calculations shown the formation energy of the hexagonal Fe₂Ti was decreased when the Fe atoms were substituted by V atoms.The incoherent boundary of Fe₂Ti/Cu matrix was transformed to coherent boundary of（Fe,V）₂Ti/Cu matrix,the casting microstructure was refined and the recrystallized grains size of solid solution was substantially decreased by appropriate V element.V element distinctly affected the precipitation behavior in Cu-3.2Ti-0.2Fe alloy.The hardness of aged alloys were obviously increased while electric conductivity and the response rate of aging slightly decreased with the addition of V element.During the aging process at low temperature（≤450°C）,the activation energy of continuous precipitation in the solution treated alloy was increased by V element,and the wavelength of the modulation was obvious decreased,some FCC structured Ti-rich phases were formed with the mechanism of nucleation-growth in the microstructure.Over-aging was happened with the prolonged time and increased temperature,the nucleation sites was increased as the grain boundary proportion was improved V element,which resulted to the increasing volume fraction of discontinuous precipitation during the earlier stage of over-aging.The discontinuous precipitation of orthorhombicβ-Cu₄Ti phase was inhibited during the later period of over-aging,one reason was the precipitation inhibition of V element,another was the increasing volume fraction of Laves caused by the addition of V element.The multiphase and micro-nano heterostructure was architected by the designation of heating processing,also the precipitation inhibition of V element and retarded recrystallization of Laves.Microstructure composed of coarse grains and fine grains with significant differences in size was formed by short-time annealing（incomplete annealing）in Cu-3.2Ti-0.2Fe-x V alloy,as the difference of recrystallization kinetic caused by the nonuniform distribution of Laves phase,besides,the FCC structured Cu₄Ti phase with sub-micrometers was precipited on the grain boudary and inside the grain,which had good coherent interface relationshipn（δ=2.4%）with the Cu-matrix.After the double-stage aging treatments,the ultimate tensile strength and elongation of Cu-3.2Ti-0.2Fe-0.2V alloy was 976.8 MPa and 18.8%,respectively.The difference between calculated and tested strength was closed to the value of hetero-deformation induced stress that calculated from the loading-unloading-reloading testing.The semi in-situ EBSD and the calculated Schmid factor results shown the deformation mechanism in the alloy with coarse grains was slipping,twinning was only happened when two no-coaxial slipping had comparable Schmid factor and symmetric slip in{110},the deformation mechanism of the fine grains was grain boundary sliding,while the rotation of ultra-fine grains performed good deformation compatibility.To further improve the strength,cold-rolled with the reduction of 5%was performed before the final aging,and the ultimate tensile strength was 1057 MPa and the elongation was10.6%.Heterostructure was obtained in the alloy by fast-heating annealing and its feasibility in industrial production was explored,the ultimate tensile strength and the elongation of the alloys was 995.1 MPa and 9.1%.The recrystallization temperature was decreased to under 750°C by fast-heating annealing,and the inhomogeneity of recovery and recrystallization was resulted to the deformation difference in different micro zones.Significant recrystallization was happened in the heavy deformed area,while it wasn’t in the slight deformed area,and the he nano flakes and nano-twins were formed.Furthermore,fast-heating annealing shortened the dwell-time of precipitation,significantly decreased the precipitation of FCC structured Cu₄Ti phase.The in-suti heated TEM results shown the recovery and the precipitation of FCC structured Ti-rich phase happened simultaneously when the solution-treated and cold-rolled Cu-3.2Ti-0.2Fe-0.2V alloy was heated,then part of the Ti-rich phase coarsened and its Ti content increased with the increasing temperature,FCC structured Cu₄Ti phase was formed ultimately,but it was re-dissolved when the temperature was 750°C and above.