Effect Of Minor Sc,Zr Addition On Microstructure And Properties Of Al-Zn-Mg-cu Alloy

Three kinds of Al-Zn-Mg-Cu alloys with different content of Sc, Zr, produced bylow-frequent electromagnetic casting technology, were subjected to homogenizationtreatment, hot extrusion, solution and aging treatment. The tensile mechanical propertiesand microstructure of alloys containing different Sc, Zr were studied by using opticalmicroscopy (OM), differential thermal analysis (DSC), scanning electron microscope(SEM), transmission electron microscope (TEM), micro Vickers hardness tester,universal testing tensile tester and conductivity tester, as well as the existing forms andacting mechanisms of minor Sc, Zr in Al-Zn-Mg-Cu alloy, optimization of the heattreatment processes. Several conclusions can be summarized as follows:①Minor Sc, Zr addition can fine grains of alloy3containing0.15%Sc and Zr,respectively, by precipitating of primary Al3(Sc,Zr) phase from the melt duringsolidification as heterogeneous nuclei.②Minor Sc, Zr addition can strongly pin dislocation and subgrain boundaries,which can effectively stop recrystallization and remarkably improve the temperature ofrecrystallization of Al-Zn-Mg-Cu alloy during solution treatment by precipitatinguniform, fine and diffuse secondary Al3(Sc,Zr) particles, coherented with the α(Al)matrix, during homogenization.③Secondary Al3(Sc,Zr) particles can remarkably improve tensile properties ofalloy, due to strong interaction between Secondary Al3(Sc,Zr) particles and dislocation,which can consume extra energy when dislocation cut or bypassing the particles.Compared with alloy1without Sc and Zr, the tensile mechanical properties of alloy3containing0.15%Sc and Zr, respectively, is better in any state.④Compare alloy2with alloy3, the grain size of as-cast alloy decreases with theincrease of Sc content, but having the same effect on effectively inhibitingrecrystallization. Minor Sc addition can remarkably improve mechanical properties ofAl-Zn-Mg-Cu-Zr alloy and strengthening mechanisms were come from the precipitation,substructure and grain refining strengthening.⑤Heat treatment process of Al-Zn-Mg-Cu-Zr alloy containing Sc was studied.Suitable heat treatment process is420℃/3h+475℃/2h+120℃/24h+170℃/30min+120℃/24h. At this condition, alloy not only has high strength and toughnessbut also has better resistance to stress corrosion.