The multilayered 7XXX series aluminum alloy was impacted by 7.62 mm ogival projectiles at velocities ranging from 787 to 851m·s^（-1）. The deformed microstructure under various impacting velocities and fracture surfaces of different sections were investigated at different physical scales to determine the process of failure.Optical microscopy（OM）,electron back-scattered diffraction（EBSD） and scanning electron microscopy（SEM） were used in the investigation. The results show that crater is constrained in the 7B52 front layer and two types of adiabatic shear bands which are transformed bands and deformed bands and different types of cracks are observed.Spall fracture is the significant failure mode of 7B52 front layer, and the resulting delamination leads to the presence of bending tensile fracture instead of the shear plugging.The ductile 7A01 layer blunts and deflects the spall crack tips, preventing the targets from full spall, and induces a constraint of 7A52 rear layer. The level of the constraint determines different fracture modes of 7A52 layer,accounting for the asymmetry of damage.
In this study, the changing trend of crystal-lattice constant and the influential factors of the stability of supersaturated solid solutions with various alloying additions in the Al–X（Zn, Mg, Cu） binary alloys were investigated. The samples were analyzed using X-ray diffraction（XRD）,X-ray absorption fine structure（XAFS）, and scanning electron microscope（SEM）. It is found that the addition of Cu causes the largest change of crystal-lattice constant of the Al–xCu supersaturated solid solution binary alloy. The most dramatic change occurs in the initial stage of Cu addition.The change is stabilized thereafter. Also, at the same alloying element addition to the Al–X（X = Zn, Mg, Cu）binary alloys, the Al–xCu is the most unstable system.Influential factors of the stability include the lattice constant change and the type of alloying element. The larger the lattice constant changes, the more unstable the supersaturated solid solution is. The alloying element, easy to aggregate, often leads to the solid solution less stable.
With the aid of scanning electron microscopy （SEM）, energy-dispersive spectroscopy （EDS）, X-ray diffraction （XRD）, differential scanning calorimetry （DSC） analysis and electron backscatter diffraction （EBSD）, the microstructure of the alloy in as-extruded state and various solution-treated states was investigated. The results indi- cate that second phase of the as-extruded 7136 aluminum alloy mainly consists of Mg（Zn, Cu, Al）2 and Fe-rich phases. The Mg（Zn, Cu, Al）2 phase directly dissolves into the matrix during solution treatment with various solution temperatures. After solution treated at 475℃ for 1 h, Mg（Zn, Cu, Al）2 phases are dissolved into the matrix, while Fe-rich phases still exist. Fe-rich phases could not dissolve into the matrix by prolonging solution time. The mechanical property test and EBSD observation show that two-stage solution treatment makes no significant improvement in mechanical properties and recrystallization of the alloy. The optimized solution treatment parameter is chosen as 475 ℃/1 h.
In the present work,the precipitate compositions and precipitate amounts of these elements（including the size distribution,volume fraction,and inter-precipitate distance） on the Cu-containing 7000 series aluminum alloys（7150 and 7085 Al alloys）,are investigated by anomalous small-angle x-ray scattering（ASAXS） at various energies.The scattering intensity of 7150 alloy with T6 aging treatment decreases as the incident x-ray energy approaches the Zn absorption edge from the lower energy side,while scattering intensity does not show a noticeable energy dependence near the Cu absorption edge.Similar results are observed in the 7085 alloy in an aging process（120℃） by employing in-situ ASAXS measurements,indicating that the precipitate compositions should include Zn element and should not be strongly related to Cu element at the early stage after 10 min.In the aging process,the precipitate particles with an initial average size of ~ 8 ？A increase with aging time at an energy of 9.60 ke V,while the increase with a slower rate is observed at an energy of 9.65 ke V as near the Zn absorption edge.
The effect of copper content on quench sensitivity in novel Al-Zn-Mg-Cu alloys containing high zinc content was investigated by Jominy end quench test.Electrical conductivity and hardness test,temperature collecting,and transmission electron microscopy(TEM)technique were adopted for the properties and microstructure characterization of three alloys with different copper contents.The results indicate that the electrical conductivity of all three alloys increases with the increase of distance from the quenched end,while the hardness shows an opposite trend.If the dropping of 10%hardness is defined as the critical evaluation standard of quenching,the depth of quenched layer of AlloysⅠ,Ⅱ,andⅢare 70,55,and 40 mm,respectively.The precipitation behavior on grain boundaries of three alloys is similar except for a little difference in size,while the size of precipitates in grains of AlloyⅢwith higher copper content is larger than those of the other two alloys at the same location.Considering all results,the stability of the supersaturated solid solution of AlloyⅢis lower than those of the other two alloys,meaning that AlloyⅢshows the highest quench sensitivity.Higher copper content leads to higher quench sensitivity in novel Al-Zn-Mg-Cu alloys with the same content of magnesium,zinc,and other trace elements.
The quench sensitivity and their influential factors of 7,021, 7,085, and 7,050 alloys were investigated by the end quenching test method and the measurement of electrical conductivity, hardness, and microstructure after aging. The results indicate that 7,050 alloy has the largest changes with hardness decreasing from HV 199 to HV 167,and electrical conductivity increases from 16.6 to18.2 MS m-1when the distance from quenched end increases from 2 to 100 mm. Alloys 7,085 and 7,021 have relatively smaller changes. According to the relationship between the hardness and electrical conductivity of a supersaturated solid solution, 7,050 alloy has higher quench sensitivity than 7,085 and 7,021 alloys. The microstructure of 7,050 alloy with higher major alloy element（Zn ？ Mg ？ Cu） addition and Cu element addition is mostly affected by the changes of distance from quenched end. In 7,050 alloy, the size of intragranular precipitates is from about 10-200 nm, and the（sub） grain boundary precipitates are about 20-300 nm. Alloy 7,085 with lower Cu content is moderately affected, while 7,021 is least affected. It is found that with the increase of distance from quenched end, quenched-induced precipitate preferentially nucleates and grows in the（sub） grain boundary and then on the pre-existing Al3 Zr particles.
