Sc And Zr Microalloying Effects And Microscopic Mechanisms In Al Alloys

To satisfy the application requirements of high strength and corrosion-resitant aluminum alloys in the development of modern equipment,microalloying was introduced and the corresponding process was designed,aiming to clarify the effects Sc and Zr microalloying on the multi-scale microstructure and properties of Al-Zn-Mg-X alloys,and optimize the mechanical properties and corrosion resitance of alloys.Under this background,the effects of heat treatment,alloy composition and deformation on the microstructure evolution and performance response of Al-Sc-Zr?-Er?alloy have been systematically based on the characterization of the precipitation behavior of Al-Sc-Zr?-Er?alloy,and the effects of Sc,Zr and Er microalloying were revealed.Then,to meet the requirements of alloys with high strength and good corrosion resistance,the benefit effects of Sc and Zr micro-alloying were introduced into the design of novel Al-Zn-MgCu alloy,several novel Al-Zn-Mg-Cu-Sc-Zr alloys and different hot processing methods and heat treatments have been designed.The microstructure evolution,mechanical properties and corrosion resistance of alloys under different treatments have been systematically studied.Effects of Sc and Zr additions on the formation and evol ution of microstructure,and mechanisms of performance responses of Al-Zn-Mg alloys have been revealed based on a systematic characterization of microstructur e,including grains,second phases,major alloying elements and precipitates.The microstructure evolution in the Al-Sc-Zr?-Er?alloy during aging has been characterized by TEM and 3DAP.It was found that core/shell configuration of Al₃?Sc,Zr?precipitate formed with a Sc-rich core and a Zr-rich shell has been developed during aging in the Al-Sc-Zr alloy,and core/double-shell structural configuration of Al₃?Er,Sc,Zr?precipitates formed with an Er-rich core,a Sc-rich inner shell and a Zr-rich outer shell has been developed in the Al-Sc-Zr-Er alloy.When aging at 300 ?,it is mainly Sc atoms that precipitate out in Al-Sc-Zr alloy,and both Sc and Er atoms precipitate out in Al-Sc-Zr-Er alloy,Zr atoms mainly start to precipitate out at 400 ?.Sc,Zr and Er atoms can precipitate out during two-stage aging.The microhardness evolution of Al-Sc-Zr?-Er?alloys under different aging treatments suggests that the peak microhardness of AlSc-Zr alloy decreases as the increase of aging temperature,and the time to peak aging decreases with increasing temperature during one-step aging.While the Al-Sc-Zr-Er alloy has a faster aging hardening response and a higher peak microhardness at 400 ? than that at 300 ?.Both two-stage aging treatment and hot extrusion can improve the mechanical properties of Al-Sc-Zr?-Er?alloys.The peak microhardness of Al-0.06Sc-0.23Zr?wt.%?and Al-0.07Sc-0.2Zr-0.11Er?wt.%?alloys after the two-stage aging?300 ?/25 h+400 ?/50 h?were both up to 552 MPa.The Er addition to Al-Sc-Zr alloy can improve the microhardness of alloy and affect its precipitation behavior during aging,Er addition can increase the nucleation rate of precipitate and accelerate the age-hardening response of alloy.This is mainly attributed to the strong Er-Er,Er-Sc and ErZr interactions and the high Er-vacancy binding energy that lead to an accelerated precipitation of solute atoms.In addition,Al-Sc-Zr?-Er?alloys show high electrical conductivity after solution and aging treatments,making them demonstrate apparent potential as heat-resistant aluminum alloys that can be used in electrical wires.Processing technologies combining hot extrusion,cold drawing and aging treatments can greatly improve the mechanical properties and electrical conductivity of alloy.The treated Al-0.06Sc-0.23Zr?wt.%?sample shows the best properties with a tensile strength of around 194 MPa and an electrical conductivity of around 61 %IACS,which are higher than the requirements for aluminum wire material in IEC standard 32004-2007.Micro-alloying with Sc and Zr has a great effect on the microstructure of Al-Zn-MgCu alloy.Results show that micro-sized primary Al₃?Sc,Zr?precipitate and nano-sized secondary Al₃?Sc,Zr?precipitate formed in the alloy with Sc and Zr additions.The primary Al₃?Sc,Zr?precipitates can significantly refine the grains,and the secondary Al₃?Sc,Zr?precipitate can strongly pin the dislocations,hinder the dislocation movement and migration of subgrain boundaries,thus inhibit the recrystallization.In addition,the Al₃?Sc,Zr?precipitates can influence the distribution of major alloying elements during aging.It was found that Zn atoms can segregate into the Zr-rich layer of the Al₃?Sc,Zr?precipitate and substitute the Al sites,and the Zn segregation intensifies with increasing aging time.Mg and Cu atoms segregate to the Zr-rich shell of the Al₃?Sc,Zr?precipitates during the early stages of aging.However,with longer aging time,they tend to reside in the Al-matrix.The Sc and Zr additions also have influence on the precipitate in the grains and on the grain boundaries.Results suggest that the Sc and Zr additions can promote the nucleation of ?' phase during aging,increasing the number density and volume fraction of ?' phase,and it can also promote the transition of ?' to ?.The alloy containing Sc and Zr has larger precipitates than that in the alloy without Sc and Zr after the same aging treatment.The precipitates on the grain boundaries is more dispersed,the width of PFZ is decreased and the Cu content in the precipitates on the grain boundaries decreased after Sc and Zr additions in Al-Zn-Mg-Cu alloy.Micro-alloying with Sc and Zr can significantly improve the mechanical properties of Al-Zn-Mg-Cu alloy.The yield strength of Al-6.48Zn-2.20Mg-1.64Cu-0.18Sc-0.16Zr?wt.%?alloy after aging at 120 ? for 24 h is around 614 MPa,which is around 106 MPa higher than that of alloy without Sc and Zr.The effects of Sc and Zr additions on the strength of alloys are mainly through the fine-grain strengthening,dispersion strengthening of Al₃?Sc,Zr?precipitates and precipitation strengthening,the effect of dispersion strengthening is the most significant.The improvement in the strength of alloy after Sc and Zr additions when aging at 120 ? is more pronounced than that at 160 ?,and the strength improvement decreases as the increase of Zn content in alloy.Furthermore,the Sc and Zr addition,and the removal of Cu can greatly improve the pitting,EXCO and stress corrosion resistance of Al-Zn-Mg-Cu alloy.This is mainly attributed to the decrease of the potential difference between second-phase and matrix,and difference between grain boundary precipitate and matrix,and the elimination of dealloying effects caused by the Cu containing second phase,as well as the weakening of the micro-galvanic effect.Besides,the refined grains,the larger size and dispersed distribution of grain boundary precipitates are also helpful to reduce the corrosion sensitivity of Al-Zn-Mg alloys.The corrosion resistance of alloys under T6 and T74 treatments has been compared,it was found that the Al-6.54Zn-2.26Mg-0.23Sc-0.13Zr?wt.%?alloy under T74?120 ?/6 h + 160 ?/24 h?treatment shows the best resistance to pitting,EXCO and stress corrosion resistance,and the addition of Sc and Zr can significantly improve the stress corrosion resistance of Al-Zn-Mg-Cu alloy under T6 heat treatment,making the alloys have high strength and good corrosion resistance after T6?120 ?/24 h?at the same time.Sc and Zr micro-alloying can significantly affect the microstructure evolution process of Al-Zn-Mg alloys,including grains,the second phase and the precipitates,and thus affects the mechanical properties and corrosion resistance.After proper hot processing and heat treatments,the novel Al-6.48Zn-2.20Mg-1.64Cu-0.18Sc-0.16Zr?wt.%?alloy shows high strength and good local corrosion resistance,which is better than the 7050 alloy under the same state.Al-Zn-Mg-Cu-Sc-Zr alloy is potential aluminum alloy with high strength and good corrosion resistance.