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The effects of solution treatment on microstructural and mechanical properties of a novel Al–Zn–Mg alloy by microalloying with Sc and Zr were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), transition electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), differential scanning calorimeter (DSC), X-ray diffraction and tensile test, hardness measurement, respectively. The results show that solution treatment had a significantly effect on microstructural and mechanical properties. There exist lots of residual phases along rolling direction before solution treatment. With increasing solution temperature/time, the residual phases are dissolved into α-Al matrix gradually, and the number density of residual phases decreased. After solution treatment at 460°C/60 min and aging treatment at 120°C/24 h, the precipitated phase inside the grains are homogenously distributed η ′ phase; the distribution of η precipitates is discontinuous and coarsened on the grain boundary. With the increase of solution temperature(≤470 °C) or solution time(≤60 min), the precipitated phases inside the grains began to become more diffuse, and the grain boundary precipitated phases also gradually become smaller. When solution temperature further increased to 490 °C or solution time increased to 120 min, the precipitated phases within the grains and on the grain boundary began to coarsen. In addition, there also exists lots of fine Al 3 (Sc, Zr) particles during aging treatment. Both tensile strength and hardness increase first and then decrease with increasing solution temperature or solution time. The optimum solution treatment is 470°C/60 min. The ultimate tensile strength, yield strength and elongation of the aged studied alloys reached a maximum value, 581 MPa, 561 MPa, and 10.1%, respectively. [ABSTRACT FROM AUTHOR]

Microstructural and property evolution of the Al–Zn–Mg–0.10%Sc–0.10%Zr alloy sheet during its preparation were investigated in detail 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. Stress corrosion cracking (SCC) behavior of the Al–Zn–Mg–0.10%Sc–0.10%Zr alloy under different heat treatments was studied using slow strain rate test. The results showed that serious dendritic segregation existed in as-cast condition. The suitable homogenization treatment for Al–Zn–Mg–0.10%Sc–0.10%Zr alloy was 470 °C/24 h. After homogenization treatment, dissoluble Zn and Mg enriched non-equilibrium phases dissolved into α -Al matrix completely. The suitable solid solution-aging treatment for Al–Zn–Mg–0.10%Sc–0.10%Zr alloy was solution treated at 470 °C for 60 min, followed by water quenching and then aged at 120 °C for 24 h. Under this aging temper, the grain structures were composed of sub-grains, η ʹ phases and nanometer-sized, spherical Al 3 (Sc, Zr) particles. Grain boundary precipitates (GBPs) area fraction was found to be an important parameter to evaluate the SCC susceptibility. The improved corrosion resistance from increasing aging temperature or prolonging aging time was due to the discontinuous η precipitates along the grain boundary and the high area fraction of GBPs. The main strengthening mechanisms of Al–Zn–Mg–0.10%Sc–0.10%Zr alloy are precipitation strengthening derived from η ʹ precipitates, dispersion strengthening, sub-grain strengthening and grain refinement caused by coherent Al 3 (Sc, Zr) particles. [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]

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]

The precipitation behaviour, mechanical properties and corrosion performance of an Al-Zn-Mg-Sc-Zr alloy under one-stage aging (T6), two-stage aging (T74) and retrogression and re-aging (RRA) treatment were mainly investigated. For the RRA treatment, with increasing temperature and duration of retrogression, the grain interior precipitates (GIPs) gradually grew and changed from GP zones + η′ phases to η′ phases + η phases. In addition, the discontinuity of coarsened grain boundary precipitates (GBPs) significantly increased, and the width of precipitate free zones (PFZs) became broader. Consequently, progressive decrease in strength and hardness, and improved corrosion resistance of the alloy can be observed. Compared to conventional T6 and T74 treatments, a good balance of mechanical properties (168.5 HV for hardness, 547 MPa for ultimate tensile strength) and corrosion resistance can be obtained with retrogression treatment at 160 °C for 30 min. Moreover, research on the localized corrosion of the Al-Zn-Mg-Sc-Zr alloy showed that, at the initial stage of pitting corrosion, the Al matrix around the cathodic α-Al(Fe,Mn)Si particles was uniformly dissolved, leading to the detachment of the α-Al(Fe,Mn)Si particles from the matrix and the formation of pitting pits. • The strengthening and corrosion mechanisms of Al-Zn-Mg-Sc-Zr alloy under different aging conditions were studied. • A model for the process of pitting corrosion development is established. • The optimized RRA treatment for Al-Zn-Mg-Sc-Zr is obtained. [ABSTRACT FROM AUTHOR]

• The effects of Sc, Y and Zr on the performances and microstructure evolution of Al-0.7Mg-0.6Si-0.2Ce alloys were studied. • An excellent combination of strength, elongation and electrical conductivity was achieved with the addition of Sc and Y. • The strengthening and electrical resistance mechanism of the studied alloys were revealed by statistical calculation. [Display omitted] In order to achieve the compatibility between excellent mechanical properties and good electrical conductivity of Al-Mg-Si alloys, detailed investigation was carried out to understand the microstructure evolution and their influences on performances of different Al-0.7Mg-0.6Si-0.2Ce-X (X Sc, Y and Zr) alloys prepared by casting, homogenization, hot extrusion, solution and aging treatments. Under aging state, Ce can form a large number of micron-sized AlCeSi and a few AlCeFeSi phase. The strength increased to 334 MPa as a result of decreased grain size and improved precipitation effect and the electrical conductivity increased up to 53.53%IACS because of lower solute Si atoms. The β" precipitates were coarsened and consequently the volume fracture was largely reduced due to the presence of Sc element, leading to the decreased strength of 311 MPa. The more solute Mg, Si and Sc atoms increased the degree of free electron scattering and the electrical conductivity decreased to 51.86%IACS. It is also found that Sc significantly enhanced the elongation of the alloy from 11.8% to 18.0%, which might be attributed to the formation of Al 3 Sc and (Al, Si) 3 Sc phases. The addition of Y raised the electrical conductivity of Al-Mg-Si-Ce alloy to 55.79%IACS by ameliorating the volume fraction and distribution of β" precipitates and decreasing the solute Si atoms. The co-addition of Sc and Y inherited the advantages of such two alloys. The ultimate tensile strength, elongation, electrical conductivity of Al-0.7Mg-0.6Si-0.2Ce-0.2Sc-0.1Y alloy processed by aging treatments is 336 MPa, 19.4% and 53.11%IACS, respectively. [ABSTRACT FROM AUTHOR]

Abstract: A ZK60 alloy was processed by two-step equal channel angular pressing (ECAP) in order to refine the grain size and enhance the mechanical properties. The results show that fine grain size less than 1μm was obtained after the two-step ECAP process, 4 passes at 513K, followed by another 4 passes at 453K. The strength of ZK60 alloy decreased after the single-step ECAP process but recovered after two-step ECAP process. The strength decrease was related to the texture softening. Compared with the single-step ECAP at 513K, the two-step ECAP has significantly improved the yield strength and ultimate tensile strength of the alloy. The mechanism of the notable grain refinement of the ECAP process was mainly attributed to continuous dynamic recovery and recrystallization. [Copyright &y& Elsevier]

In ultra-high strength Al-Zn-Mg-Cu alloys, traditional heat treatments such as peak aging, two-stage aging and retrogression and re-aging cannot perfectly balance the relationship between mechanical properties and corrosion resistance and usually consume more time. Based on previous studies, the aim of this paper is to establish a suitable non-isothermal aging method for ultra-high strength Al-Zn-Mg-Cu alloys. In this study, after enhanced solution treatment and aging process with the heating rate of 20 °C/h, the alloy achieves excellent hardness and ultimate tensile strength of 195.7 HV and 680 MPA, respectively. When heating rate increases to 40 °C/h, non-isothermal aging can also ensure the alloy achieve the excellent mechanical properties (195.6 HV, 675 MPA). Similar to mechanical properties, the samples have outstanding resistance to intergranular and exfoliation corrosion, which is in accordance with the results of potentiodynamic polarization and electrochemical impedance spectroscopy tests. Evidences from high resolution transmission electron microscopy show that non-isothermal aging can promote the formation of a large number of smaller GP zones, η' precipitates and few η 2 precipitates inside grains, which greatly strengthens the mechanical properties. Furthermore, the grain boundaries precipitates transform from continuous to discontinuous and precipitate free zones form gradually, which greatly enhances the corrosion resistance. Because of these precipitates inside grains and grain boundaries, the samples with enhanced solution treatment +20 °C/h aging process and both solution treatments +40 °C/h aging process can achieve good mechanical properties closing to T6 state and corrosion resistance similar to T74 state simultaneously with much less aging time. Image 1 • The non-isothermal aging was designed as a novel route for heat treatments of ultra-high strength Al-Zn-Mg-Cu alloy. • An excellent combination of strength-ductility and corrosion resistance was achieved in the studied alloy. • The strengthening and corrosion mechanisms of non-isothermal aging on the studied alloy were discussed systematically. • Precipitation behavior of MgZn 2 strengthening phase was well studied. [ABSTRACT FROM AUTHOR]

The existing form of Sc in Si-containing 6xxx-series Al alloys and its role in strength has always been controversial. In this study, isothermal section phase diagram of the Al-0.60 wt%Mg-Si-Sc system at 350 °C was calculated by CALPHAD method and Pandat software, and the results show that Sc exists in Al 3 Sc or V (AlSc 2 Si 2) phases, depending on the contents of Si and Sc. Subsequently, we designed a Al-0.60 Mg-0.75Si-0.05Sc (wt%) Melt Inert Gas (MIG) welded alloy, and investigated its microstructure and mechanical property by tensile tests and electron microscopy. HRTEM and STEM-EDS results show that, Sc exists in the form of core(Sc)-shell (Si) structured (Al, Si) 3 Sc transition phases with the size range of 60–80 nm in the fractured zone (heat affected zone, HAZ) of the welded joints, deriving from equilibrium phase diagram, V (AlSc 2 Si 2) theoretically. The HAZ of the Al-Mg-Si-Sc joint consists of partly recrystallized grains and strong rolling texture, different from the completely recrystallized and coarsened grains in the Al-Mg-Si joint. In addition, the (Al, Si) 3 Sc phases increase the density of β'′ aging strengthening phases and prevent them from coarsening under welding heat cycle. The above positive effects of (Al, Si) 3 Sc particles increase the yield strength, ultimate tensile strength and elongation of Al-Mg-Si welded joints by 31 MPa (20.6%), 41 MPa (21.2%) and 0.8% (12.9%), respectively. [Display omitted] • Clarify the existing form of Sc in metal-inert gas welded Al-Mg-Si alloys. • Investigate the thermal stability of (Al, Si) 3 Sc phases and their effects on microstructure under welding thermal cycle. • Quantify the role of Sc in strength of metal-inert gas welded Al-Mg-Si alloys. [ABSTRACT FROM AUTHOR]