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Ultra-high strength Al-Zn-Mg-Cu alloy extrusion bars were processed by equal channel angular pressing (ECAP) at room temperature (RT). Post aging treatment was used to determine the optimum time of aging. ECAPed sample was divided into two types: near the upper channel (P1) and lower channel (P2). Micro-hardness and tensile tests showed that ECAPed sample reached peak aging state at 8 h, which greatly shortened the time to T6 state. The mechanical properties of ECAPed samples were also better than traditional T6 samples. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) were used to study the phase transformation from extrusion bars to ECAPed samples. AlZnMgCu phase and Al 2 CuMg phase containing Zn can be found in ECAPed sample. Meanwhile, the coarsening phase Al 7 Cu 2 Fe decreased significantly. Optical microscopy (OM) and transmission electron microscopy (TEM) were used to analyze the microstructure evolution such as shear bands and grain size during ECAP process. In P1, the shear bands with width ranging from 90 to 300 µm can be found. These shear bands were divided into many thinner shear bands and became intragranular deformation bands gradually in P2. Compared with P2, the average grain size in P1 decreased greatly and the precipitate η′ phase became finer and denser. Grain boundary strengthening and precipitation strengthening in P1 were much better than P2. The diameter of the channel should not exceed 20 mm when it used to press ultra-high strength Al-Zn-Mg-Cu alloy and the 3/4 parts near the top channel of product should be used to avoid unrefined grains. [ABSTRACT FROM AUTHOR]

The superplastic behavior of Al–Mg–Sc–Zr samples with standard gauge size (18 mm by 6 mm) were prepared using simple rolling and were tested in the temperature range from 450 °C to 525 °C at strain rates ranging from 1.67×10 –3 s −1 to 1×10 –1 s −1 . With proper deformation parameters, the Al–Mg–Sc–Zr alloy has an elongation to failure much higher than 300% and the maximum elongation is 740%. The Microstructure and dislocation substructure investigation using optical microscopy (OM) and transmission electron microscopy (TEM) revealed a dynamic recrystallization in it. The grain size and activation energy on the deformation mechanisms of superplastic is discussed. Results also show that these nano-scale Al 3 (Sc 1−x Zr x ) particles play an important role in the superplastic process. Al 6 FeMn particles were found to induce the formation and growth of cavities, which can lead to the fracture of specimens. [ABSTRACT FROM AUTHOR]

2.5 mm thick cold-rolled Al–Zn–Mg–Zr alloy plates with trace amount of Sc were subjected to metal–inert gas (MIG) welding with Al–Mg–Sc as the filler metal. The influence of Sc additions on microstructure and mechanical properties of the welded joints was investigated by means of optical microscopy (OM), scanning electron microscope (SEM), energy dispersive X-ray (EDX), transmission electron microscopy (TEM), Vickers micro-hardness test and room temperature tensile test. Experimental results indicated that trace amount of Sc had significant effect on microstructure and mechanical properties of the joints. In both cases, base metal consisted of a typical fiber structure, different width of the equiaxed zones (EQZs) with fine grains were observed in the weld metal at the fusion boundary, fusion zones exhibited dendritic microstructure and average size of Al–Zn–Mg–0.10%Sc–Zr was 120 μm, while in Al–Zn–Mg–0.25%Sc–Zr, grain size decreased drastically to 50 μm. The grain refinement is mainly caused by the extremely fine, coherent Al 3 (Sc, Zr) particles with L1 2 structure, which act as heterogeneous nucleation. The observed grain refinement resulted in an appreciable increase in fusion zone strength and micro-hardness. Tensile strength of Al–Zn–Mg–0.25Sc–Zr alloy welded joint was 460 MPa, approximately 11% higher than Al–Zn–Mg–0.10Sc–Zr alloy welded joint. Micro-hardness in the center of fusion zone rose from 95 HV to 110 HV. The main strengthening mechanisms of Al–Zn–Mg–Sc–Zr MIG welded joints are precipitation strengthening derived from η′ precipitates, dispersion strengthening and fine grain strengthening caused by coherent secondary Al 3 (Sc, Zr) particles. [ABSTRACT FROM AUTHOR]

Stress corrosion cracking (SCC) behavior of Al–Zn–Mg–Cu alloys with different Sc, Zr contents and heat treatments was studied using slow strain rate test. Grain boundary microstructures were identified by transmission electron microscopy (TEM) and statistical analysis. It was found that the SCC resistance of alloys is improved by increasing Sc, Zr contents and aging degree. Grain boundary precipitates (GBPs) area fraction was found to be an important parameter to evaluate the SCC susceptibility. The results reveal that for Al–Zn–Mg–Cu–0.25Sc–0.10Zr (wt%) alloy with different aging degrees, hydrogen induced cracking dominates the SCC when the area fraction of GBPs is relatively low. For peak-aged Al–Zn–Mg–Cu alloy and Al–Zn–Mg–Cu–0.10Sc–0.10Zr (wt%) alloy, anodic dissolution dominates the SCC when the area fraction of GBPs is sufficiently high. [ABSTRACT FROM AUTHOR]

Microstructures and properties of Al–Zn–Mg–Mn alloy with trace amounts of Sc and Zr have been investigated. The results show that the addition of minor Sc and Zr to Al–Zn–Mg–Mn alloy can effectively refine grain size caused by the formation of Al 3 (Sc, Zr) particles with cubic L1 2 structure, which is coherent with α(Al) matrix. After solution treatment at 470 °C for 1 h, more homogenous recrystallization grains are observed in Al–Zn–Mg–Mn alloy; partial recrystallization occurs in Al–Zn–Mg–Mn–0.12Sc–0.12Zr alloy. However, Al–Zn–Mg–Mn–0.24Sc–0.12Zr alloy still remains an unrecrystallized fiber-like structure, which reveals that minor Sc and Zr can remarkably inhibit the occurrence of recrystallization. After aging treatment at 24 h for 120 °C, the tensile strength of Al–Zn–Mg–Mn–0.12Sc–0.12Zr alloy increased by 40 MPa as compared to Al–Zn–Mg–Mn alloy. When the Sc content increased to 0.24 wt%, the tensile strength of Al–Zn–Mg–Mn–0.24Sc–0.12Zr alloy reached a maximum. The main strength mechanisms are grain refinement strengthening with combined Sc and Zr additions and precipitation strengthening of Al 3 (Sc, Zr) particles. Compared with Al–Zn–Mg–Mn alloy, Al–Zn–Mg–Mn alloy with trace amounts of Sc and Zr exhibits higher corrosion resistance due to the discontinuity distribution of η precipitates along the grain boundary and the resistance effect on recrystallization behavior by Al 3 (Sc,Zr) particles. [ABSTRACT FROM AUTHOR]

Highlights: [•] Investigate effect of aging temperature on corrosion behavior of an Al–Zn–Mg–Sc–Zr alloy. [•] Find the relationship between corrosion and microstructures of the studied alloy. [•] Corrosion behavior is determined using both electrochemical and immersion tests. [•] Improvement is due to absence of coarse precipitates and precipitate-free zones. [ABSTRACT FROM AUTHOR]

Highlights: [•] Hot compression test were conducted under different deformation conditions. [•] The dynamic recovery and dynamic recrystallization occurred during hot deformation. [•] ANN model and constitutive equations were established to predict the flow stress. [•] Material constants and Q value in the constitutive equations were calculated. [•] ANN model has a better prediction power than constitutive equations. [ABSTRACT FROM AUTHOR]

New aerospace high strength Al–Zn–Mg and Al–Zn–Mg–0.25Sc–0.10Zr (wt%) alloys were welded by tungsten inert gas (TIG) process using a new Al–6.0Mg–0.25Sc–0.10Zr (wt%) filler material, and friction stir welding (FSW) process, respectively. Mechanical property and microstructure of the welded joints were investigated comparatively by tensile tests and microscopy methods. The results show that Sc and Zr can improve the yield strength and ultimate tensile strength of Al–Zn–Mg alloy by 59 MPa (23.3%) and 16 MPa (4.0%) in TIG welded joints, and by 77 MPa (23.8%) and 54 MPa (11.9%) in FSW welded joints, respectively. The ultimate tensile strength and elongation of new Al–Zn–Mg–Sc–Zr alloy FSW welded joint are 506±4 MPa and 6.34±0.2%, respectively, showing superior post welded performance. Mechanical property of welded joint is mainly controlled by its “weakest microstructural zone”. TIG welded Al–Zn–Mg and Al–Zn–Mg–Sc–Zr alloys reinforced with weld bead both failed at fusion boundaries. Secondary Al 3 Sc x Zr 1− x particles originally present in parent alloy coarsen during TIG welding process, but they can restrain the grain growth and recrystallization here, thus improving welding performance. For two FSW welded joints, fracture occurred in weld nugget zone. Secondary Al 3 Sc x Zr 1− x nano-particles almost can keep unchangeable size (20–40 nm) across the entire FSW welded joint, and thus provide effective Orowan strengthening, grain boundary strengthening and substructure strengthening to strengthen FSW joints. The positive effect from Sc and Zr additions into base metals can be better preserved by FSW process than by TIG welding process. [ABSTRACT FROM AUTHOR]