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Nanocrystalline (NC) structure can lead to the considerable strengthening of metals and alloys. Obtaining appropriate comprehensive mechanical properties is always the goal of metallic materials. Here, a nanostructured Al-Zn-Mg-Cu-Zr-Sc alloy was successfully processed by high-pressure torsion (HPT) followed by natural aging. The microstructures and mechanical properties of the naturally aged HPT alloy were analyzed. The results show that the naturally aged HPT alloy primarily consists of nanoscale grains (~98.8 nm), nano-sized precipitates (20–28 nm in size), and dislocations (1.16 × 1015 m−2), and exhibits a high tensile strength of 851 ± 6 MPa and appropriate elongation of 6.8 ± 0.2%. In addition, the multiple strengthening modes that were activated and contributed to the yield strength of the alloy were evaluated according to grain refinement strengthening, precipitation strengthening, and dislocation strengthening, and it is shown that grain refinement strengthening and precipitation strengthening are the main strengthening mechanisms. The results of this study provide an effective pathway for achieving the optimal strength–ductility match of materials and guiding the subsequent annealing treatment.

The synergistic effects of 0-0.3 wt.% Sc addition and retrogression and re-aging (RRA) on microstructure and mechanical properties were evaluated in Al-8.0Zn-2Mg-2Cu-0.15Zr alloy. The minor Sc addition can effectively refine the grains of the tested alloy. When the Sc content increased from 0 to 0.2 wt.%, the grain size decreased from 200-250 to 30 μm. As the Sc content increased from 0.2 to 0.3 wt.%, the grain size was still 30 μm, almost without change. The grain refining is due to the formation of Al3(Sc, Zr), which has a better coherent relationship with the matrix than Al3Zr. The RRA treatment results in a slight decrease in the tensile strength (Rm) and significant improvement in the total elongation (A) of the tested alloy. The sample's Rm before and after the RRA decreased slightly from 517 to 510 MPa when the Sc is 0.3 wt.%; the A increased significantly from 6.0 to 12.0%. The tested alloy prepared by adding 0.1-0.3 wt.% Sc has a higher tensile strength and toughness than those alloys without Sc due to the formation of Al3(Sc, Zr). After the RRA treatment, both the formation of η (MgZn2) and the release of internal stress decrease the strength and increase the total elongation of the tested alloy relative to values before the RRA treatment.

Tensile experiments were conducted for Al-7.0Zn-2.5Mg-2.0Cu-0.1Zr-0.2Sc alloy in different aging states (18 h, 24 h, 36 h) with temperature environments including room temperature, −10 °C and −30 °C. Comparative studies were made on the evolution of the precipitate phase in alloys at three kinds of aging times and the evolution of tensile properties in alloys under different ambient temperatures. The findings showed that the precipitates in Al-7.0Zn-2.5Mg-2.0Cu-0.1Zr-0.2Sc alloy were mainly in the GP zone after the solution + aging treatment η’ phase, the secondary Al3 (Sc, Zr) phase and the θ’ (Al2Cu) phase. As the aging time was prolonged, the η’ phase gradually grew and the PFZ gradually widened. At the three test temperatures, the tensile strength (TS) and yield strength (YS) of the alloys both showed a trend of first increasing before decreasing with the extension of aging time, while the elongation (A) and section shrinkage (Z) showed a decreasing trend. As the test temperature decreased, the TS and YS of the alloys increased and the A and Z of the alloys decreased. At room temperature, alloys showed a ductile fracture mode, which changed to mixed ductile and brittle fracture with decreasing test temperature.

Microstructural evolution, precipitation behavior, mechanical properties and strengthening mechanism of a novel Al–Zn–Mg–Cu–Li–Sc–Zr alloy (7075-1.2Li–0.1Sc–0.1Zr alloy, wt%) were investigated. The strong tendency for preferential precipitation of Al 3 Li was observed. After single-stage aging (120 °C/48 h), the main precipitation phase was core–shell Al 3 (Sc, Zr, Li), consisting of Al 3 (Sc, Zr) core formed at homogenization (460 °C/32 h) and Al 3 Li-shell formed at subsequent aging. However, under two-stage aging (120 °C/16 h + 150 °C/48 h), co-precipitation of core–shell Al 3 (Sc, Zr, Li) and rod-shape η ′ (MgZn 2 ) happened, triggering the further improvement in microhardness of Al–Zn–Mg–Cu–Li–Sc–Zr alloy. The grain boundary precipitates (GBPs) and precipitate free zones (PFZs) existed in the alloys after both single-stage aging and two-stage aging. The strength of Al–Zn–Mg–Cu–Li–Sc–Zr alloy was gradually improved after two aging treatments, however, the ductility was continuously decreased. The strengthening mechanisms of Al 3 (Sc, Zr, Li) and η ′ (MgZn 2 ) at single-step and two-step aging treatments were both bypass strengthening.

Post-heat treatment was conducted on the Al-Zn-Mg-Cu-Sc-Zr alloy treated by friction stir processing (FSP) to optimize its strength and corrosion resistance. Different post-heat routes had significant bearing on the phase composition of the FSP sample, while its fine equiaxed grain structure could be retained during post heating. Enhanced strength of the FSP sample was obtained by post-heat treatments. The FSP sample after T6 treatment (artificial aging to peak hardness value after solid solution treatment) exhibited the highest strength in virtue of its high precipitation strengthening effect, and the high ductility of the FSP sample could be retained after solid solution treatment. Furthermore, the FSP sample presented better corrosion resistance after artificial aging and T6 treatment by reducing its solution atom numbers.

The mechanical properties and stress corrosion cracking (SCC) resistance of an Al-Zn-Cu-Mg-Sc-Zr alloy under different aging conditions were investigated. The dependence of microstructure and mechanical properties on aging parameters was evaluated by tensile test, hardness test and conductivity measurement. The results show that for the alloys with retrogression and re-aging treatment (RRA), the conductivity increases with the retrogression time and temperature, while the tensile strength decreases. The transmission electron microscopy (TEM) results show that the precipitates η(MgZn2) at grain boundary aggregate apparently with retrogression time and the precipitates inside the matrix exhibit the similar distribution to T6 temper, which comprises fine GP zones, large η′(MgZn2) and η(MgZn2) phases. According to the mechanical properties and microstructure observations, the optimal RRA regime is recommended to be 120 °C, 24 h + 180 °C, 30 min + 120 °C, 24 h. The strength level of the alloy after the optimum RRA treatment is similar to that in T6 condition and the SCC resistance is improved obviously in contrast to T6 condition.

The deformation behavior of a new Al-Zn-Cu-Mg-Sc-Zr alloy was investigated with compression tests in temperature range of 380–470 °C and strain rate range of 0.001–10 s−1 using Gleeble 1500 system, and the associated microstructural evolutions were studied by metallographic microscopy and transmission electron microscopy. The results show that true stress—strain curves exhibit a peak stress, followed by a dynamic flow softening at low strains (ɛ<0.05). The stress decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener-Hollomon exponential equation with the activation energy for deformation of 157.9 kJ/mol. The substructure in the deformed specimens consists of few fine precipitates with equaixed polygonized subgrains in the elongated grains and developed serrations at the grain boundaries. The dynamic flow softening is attributed mainly to dynamic recovery and dynamic recrystallization.