Regulating Mechanisms Of Alloying Elements On Microstructures And Properties Of Al-Zn-Mg Alloys

With the development of rail transit,Al-Zn-Mg alloy profiles with excellent strength,toughness,corrosion resistance and fatigue performance have been widely used as important lightweight structural materials.The improvement of rail train lightweight index puts forward the requirements of higher strength and high comprehensive performance for this kind of aluminum alloy.Combined with the Major State Research Program of China,the regulation mechanisms of main alloying elements Zn,Mg and trace Sc and V on microstructure,mechanical properties and corrosion resistance of Al-Zn-Mg alloy profiles are studied in this paper,which provides a basis for composition regulation for new alloy design,composition optimization of existing alloys and preparation of materials with high comprehensive properties.The main innovative conclusions are as follows:(1)In the range of(Zn+Mg)total amount 2.66～9.48 wt.%and Zn/Mg ratio 3.0～15.9,the equivalent precipitation curves of equilibrium η phase is calculated based on the equilibrium phase diagram of Al-Zn-Mg ternary system.Equivalent precipitation curves show that the total amount of(Zn+Mg)is constant,while the amount of Zn/Mg ratio is less than 4.9,the amount of η phase precipitation increases with the increase of Zn/Mg ratio.When Zn/Mg ratio is greater than 4.9,the amount of precipitation decreases with the increase of the Zn/Mg ratio.Meanwhile,a mathematical model of equivalent precipitation is established to reflect the relationship between the amount of precipitated phase-the total amount of(Zn+Mg)-Zn/Mg ratio,and a criterion for the composition optimization of Al-Zn-Mg series alloy is proposed based on precipitation strengthening mechanism.(2)The regularities of Zn and Mg regulating the strength and toughness of Al-Zn-Mg series alloys in the(α+η)phase region are revealed by means of equivalence precipitation mathematical model and experimental validation.When the Zn/Mg ratio is greater than 4.9,decreasing the Zn/Mg ratio can shorten the peak aging time,increase the precipitation density of intragranular η’ phase,and improve the strength of the alloys.The peak aging time is shortened from 48h to 15h,and the precipitation density of η’ phase increases from 66521μm-3 to 118142μm-3.On the uniform precipitation curve of η phase,the reduction of Zn/Mg ratio can inhibit the widening of intergranular precipitation-free precipitation zones(PFZs),which is beneficial to the coordination of strength and toughness.The tensile strength(Rm)and yield strength(Rp0.2)of the test alloy(T6 condition)with Zn/Mg=3.6 are similar to those of the test alloy with Zn/Mg=8.1,and the Rm and Rp0.2 are 407 MPa,353 MPa and 413 MPa,359 MPa respectively,and the impact toughness(αk)are 26J/cm2 and 33J/cm2 respectively,improving 7J/cm2.(3)The basic alloy within the composition range of Zn and Mg of 7B05 alloy is selected,and different contents of Sc are introduced.The regulation effects of trace Sc on the microstructure and strength and toughness of the alloy are studied and revealed.Sc-containing dispersed phase can pin the grain boundary,inhibit recrystallization,increase the proportion fraction of low angle grain boundaries(LAGBs),reduce the thickness of coarse-grained layer of the profiles,and the proportion fraction of LAGBs increases from 74.6%to 88.5%,the thickness of coarsegrained layer reduces from 68.9μm to undetected.Sc can promote the precipitation of η’ phase,reduce the average length of η’ phase,and increase the spacing of η phase at grain boundary.The average length of η’ phase decreases from 7.1nm to 6.3nm.Meanwhile,Sc-containing particles provide additional precipitation strengthening effect.The comprehensive properties of the alloy(T6 condition)with Sc content of 0.16 wt.%are the best:Rm,Rp0.2 and elongation after fracture(A)are 450MPa,422MPa and 13.2%,respectively,and αk is 37J/cm2.(4)The effects of trace V on microstructure and properties of 7B05 alloy are revealed by using a similar method to that of trace Sc.Vcontaining dispersed phase inhibits recrystallization,promotes precipitation of η’ phase,reduces average size of η’ phase and inhibits PFZ broadening,which is conducive to the synergy of strength and toughness of the alloys.The fraction of recrystallization decreases from 25.4%to 14.7%,the average length of T74 alloys decreases from 7.2nm to 6.1nm,and the width of PFZ decreases from 192.1nm to 55.7nm.The comprehensive properties of the alloy(T74 condition)with V content of 0.05 wt.%are the best:Rm,Rp0,2 and A are 370MPa,322MPa and 17.4%respectively,αk is 45J/cm2.(5)The dispersed phases and crystalline phases with Sc,V formed after adding trace Sc and V respectively into the alloys can improve the bonding force of grain boundaries,reduce the electrochemical difference between the grain boundaries and the matrix,decrease the cracking and initiation tendency along the grain boundaries,and present weak cathode compared with the alloy matrix,which are conducive to the synergistic improvement of the strength,toughness and corrosion resistance of the alloy.