Effect Of Composition And Grain Size On Precipitation Behaviour Of Al-Zn-Mg-(Cu) Alloys

Al-Zn-Mg-(Cu)series alloys are widely used in aerospace,rail transit and automobile fieldsdue to their heat-treatable and strengthenable properties and good overall performance.The improvement of the strength of Al-Zn-Mg-(Cu)alloys is still limited(600MPa～800MPa)only by adjusting the alloying elements and heat treatment process.However,combining the two methods of precipitation strengthening and fine-grain strengthening can improve the strength of Al-Zn-Mg-(Cu)series alloys.The strength of the series alloy is increased to nearly1GPa,so this method is used as an important means of strengthening Al-Zn-Mg-(Cu)alloy and put into application.Therefore,this paper mainly studies the influence of grain size and Zn/Mg atomic ratio on the precipitation behavior of Al-Zn-Mg-(Cu)alloys.In this paper,7075 alloy was prepared by solid solution,equal channel angular extrusion(ECAP)and high-pressure torsion(HPT)processes to prepare four samples with different grain sizes,and two samples with different Zn/Mg atomic ratios based on 7N01.Homemade alloy materials.By means of differential scanning calorimeter(DSC),electron backscatter diffraction(EBSD),transmission electron microscope(TEM),transmission electron microscope combined with high-angle annular dark field imaging(HAADF-STEM)and three-dimensional atom probe(APT),The microscopic characterization of the sample’s grain morphology and the precipitated phases in the grains was carried out,and the main conclusions are as follows:(1)The effect of grain size on the precipitation behavior of Al-Zn-Mg-(Cu)alloy.The DSC test of 7075-CG,UFG1,UFG2,and NC samples with different grain sizes,whichare coarse,ultra-fine and nanocrystalline,showed that the Al-Zn-Mg-(The decomposition path of the supersaturated solid solution of Cu)alloy is different,that is,the number and type of precipitated phases are obviously different at the same temperature.The CG sample has the same precipitation path as the supersaturated solid solution aluminum alloy with a Zn/Mg ratio<2 recognized by the researchers,which is supersaturated:solid solution→GP zones→η’→η-Mg Zn2.In the UFG1,UFG2 and NC samples with smaller grain sizes,the GPηp phase andη_pprecipitated phases that originally appeared in the aluminum alloy with Zn/Mg atomic ratio>2appeared.In the small grain size samples that obtained by processing methods of large plastic deformation,large precipitated phases rich in Zn and Cr/Mn are formed at grain boundaries.(2)The influence of Zn/Mg atomic ratio on the precipitation behavior of Al-Zn-Mg alloys.The Al-5.75Zn-1.20Mg alloy with a Zn/Mg atomic ratio of about 1.88 and the Al-9.38Zn-0.95Mg alloy with a Zn/Mg atomic ratio of about 3.67 were aged at 120℃and150℃for 1h,24h,and 120h,respectively.The results showed that the same sample was Aging at 120°C and 150°C,there is no difference in the types of precipitated phases;150°C samples with the same aging time have more precipitated phases and larger diameters than samples at 120°C.Under the treatment of 150℃/120h,some of theη_pprecipitates in the Al-9.38Zn-0.95Mg sample changed from the initial state to the mature state.Through DSC analysis of the two alloys,it was verified that the decomposition path of the supersaturated solid solution of the two alloys conformed to the decomposition path corresponding to the Zn/Mg ratio.APT analysis was carried out on the two alloys at 120℃/120h.The data results proved from the composition and morphology that the precipitated phase in the Al-9.38Zn-0.95Mg sample is different from the Al-5.75Zn-1.20Mg precipitated phase,and the precipitation in the Al-9.38Zn-0.95Mg sample with a higher Zn/Mg atomic ratio There are moreηp phases and GPηp precipitated phases.