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The understanding of metastable hardening precipitate evolution in Al-Mg-Si(-Cu) is still quite confused and elusive due to the disordered nature of several precipitates and the complex processes involved. Atomic-resolution scanning transmission electron microscopy is used here to study the nucleation and transformation mechanisms of the B′/Q′ precipitates in the Al-Mg-Si(-Cu) alloys. It is emphasized that understanding the formation and evolution of short-range order (SRO) in the precipitates is a core to revealing their transformation mechanisms. Although the B′ and Q′ phases have the same crystal structure, and both originate from the β″ precipitate, the formation mechanisms of these two precipitates are different. For B′, formed in the Al-Mg-Si alloy, the β″ phase initially transforms to the U2 phase by the transition of the “low density cylinder” (LDC) to Mg hexagons. Subsequently, the U2 phase transforms to the B′ phase by the rotation of Al-Si columns, and the formation and ordering of triangle B′ sub-units. For Q′, formed in the Al-Mg-Si-Cu alloy, the Cu atoms incorporate initially the interior of the β″ phase and form the substructure of Cu sub-unit clusters. In most cases, these Cu sub-unit clusters are randomly distributed, and the QM and QP lattices are formed throughout the precipitates. In some precipitates, the arrangement of Cu sub-unit clusters however constitutes a hexagonal lattice and leads to the formation of the QC phase. During subsequent aging, the formation and ordering of the triangle Q′ sub-units can lead to the formation of the Q′ phase. Clarifying the role of the SRO in these processes provides a new insight in the understanding of transformation mechanisms of precipitates in Al-Mg-Si(-Cu) alloys.

Self-healing strategies aim at avoiding part repair or even replacement, which is time consuming, expen-sive and generates waste. However, strategies for metallic systems are still under-developed and solid-state solutions for room temperature service are limited to nano-scale damage repair. Here we propose a new healing strategy of micron-sized damage requiring only short and low temperature heating. This new strategy is based on damage localization particles, which can be healed by fast diffusing atoms of the matrix activated during heat treatment. The healing concept was successfully validated with a com-mercial aluminum alloy and manufactured by Friction Stir Processing (FSP). Damage was demonstrated to initiate on particles that were added to the matrix during material processing. In situ 2D and 3D nano -imaging confirmed healing of the damaged material and showed that heating this material for 10 min at 400 degrees C is sufficient to heal incipient damage with complete filling of 70% of all damage (and up to 90% when their initial size is below 0.2 mu m). Furthermore, strength is retained and the work of fracture of the alloy is improved by about 40% after healing. The proposed Programmed Damage and Repair healing strategy could be extended to other metal based systems presenting precipitation. (C) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The precipitation mechanism of the Q phase in AlMgSiCu alloys has long been the subject of ambiguity and debate since its metastable phase (Q′) has the same crystal structure and similar lattice parameters as its equilibrium counterparts. In the present work, the evolution of the Q′ (Q) phase during aging is studied by combination of quantitative atomic-resolution scanning transmission electron microscopy and atom probe tomography. It was found that the transformation from the Q′ to the Q phase involves changes of the occupancy of Al atoms in atomic columns of the Q′ (Q) phase. The Al atoms incorporated in the Cu, Si and Mg columns are gradually released into the Al matrix, while mixing between Cu and Si atoms occurs in the Si columns. This transformation process is mainly attributed to the low lattice misfit of the equilibrium Q phase. Besides, the formation of various compositions of the Q phase is due to the different occupancy in the atomic columns of the Q phase. The occupancy changes in the columns of the Q phase are kinetically controlled and are strongly influenced by the alloy composition and aging temperature.

Carbon black (CB) supported Ni, Fe, or Fe-Ni alloy catalysts were synthesized by sol-gel to elucidate the reaction pathways over each catalyst, as well as synergistic effects in glucose to sorbitol hydrogenation. The bimetallic materials presented small and alloyed nanoparticles that were richer in reduced metallic sites at the surface than their monometallic counterparts. Glucose isomerization to fructose was favoured over Fe/CB, while glucose hydrogenation to sorbitol is the dominating pathway over Ni/CB catalyst. By contrast, sorbitol production was promoted and undesired isomerization was suppressed when Fe and Ni formed a nanoalloy. In addition, the alloy catalyst presented better stability than the corresponding monometallic catalyst. A comparison with a mechanical mixture of Fe/CB and Ni/CB monometallic catalysts demonstrated the synergy at the nanoscale in the alloy. By comparing different Fe:Ni ratios, the 1:1 formulation was identified as the best compromise to achieve a high activity while maintaining high sorbitol selectivity.

Extensive use of titanium alloys is partly hindered by a lack of ductility, strain hardening, and fracture toughness. Recently, several β-metastable titanium alloys were designed to simultaneously activate both transformation-induced plasticity and twinning-induced plasticity effects, resulting in significant improvements to their strain hardening capacity and resistance to plastic localization. Here, we report an ultra-large fracture resistance in a Ti-12Mo alloy (wt.%), that results from a high resistance to damage nucleation, with an unexpected fracture phenomenology under quasi-static loading. Necking develops at a large uniform true strain of 0.3 while fracture initiates at a true fracture strain of 1.0 by intense through-thickness shear within a thin localized shear band. Transmission electron microscopy reveals that dynamic recrystallization occurs in this band, while local partial melting is observed on the fracture surface. Shear band temperatures of 1250–2450 °C are estimated by the fusible coating method. The reported high ductility combined to the unconventional fracture process opens alternative avenues toward Ti alloys toughening., Specific titanium alloys combine transformation-induced plasticity and twinning-induced plasticity for improved work hardening. Here, the authors show that these alloys also have an ultra-large fracture resistance and an unexpected fracture mechanism via dynamic recrystallization and local melting in a deformation band.

The microstructural characteristics of two polycrystalline silicon (poly-Si) films with different electrical properties produced by low-pressure chemical vapour deposition on top of high resistivity silicon substrates were investigated by advanced transmission electron microscopy (TEM), including high resolution aberration corrected TEM and automated crystallographic orientation mapping in TEM. The results reveal that the nature of the poly-Si film/Si substrate interface is the main factor controlling the electrical resistivity of the poly-Si films. The high resistivity and high electrical linearity of poly-Si films are strongly promoted by the Sigma 3 twin type character of the poly-Si/Si substrate interface, leading to the generation of a huge amount of extended defects including stacking faults, Sigma 3 twin boundaries as well as Sigma 9 grain boundaries at this interface. Furthermore, a high density of interfacial dislocations has been observed at numerous common and more exotic grain boundaries deviating from their standard crystallographic planes. In contrast, poly-Si film/Si substrate interfaces with random character do not favour the formation of such complex patterns of defects, leading to poor electrical resistivity of the poly-Si film. This finding opens windows for the development of high resistivity silicon substrates for Radio Frequency (RF) integrated circuits (ICs) applications.

The impact of In addition on the precipitation behavior and properties for Al-Mg-Si alloys under different aging treatments was systematically studied by hardness measurements, atom probe tomography (APT), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and density functional theory (DFT) calculations. We found that the In addition effectively suppresses the harmful effect of natural aging (NA), and improves the age hardening for the Al-Mg-Si alloy. Mg-Si-In clusters are formed in the early stage of aging caused by the incorporation of In atoms, which can facilitate the nucleation of the β" phase. During peak aging, In atoms distort the well-ordered structure of the β" and occupy the positions in the interface of the β", facilitating the transition of β" to post-β" (i.e. U2 and β′). During the following aging, In atoms are presented in two different configurations: forming In-rich particles and aggregation at the interfaces of β′. Most of the In-rich particles are formed at the edge of the β′ particles. The segregation of In atoms at the precipitate/matrix interface and the formation of In-rich particles can probably be attributed to the reduced strain energy of the β′ phase it brings about.

The effect of extrusion and heat treatment processes on the microstructure and mechanical properties of the spray formed 7055 alloy were systematically studied by three-dimensional X-ray microscope, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, atom probe tomography and tensile testing. It was found that the main second-phases in the as-deposited 7055 alloy include θ (Al2Cu) and Mg(Zn,Al,Cu)2 phases with similar structure to the ƞ (MgZn2) phase dissolved with Al and Cu. The Al2Cu particles embedded in Mg(Zn,Al,Cu)2 phase are widely distributed on the grain boundaries. Compared with the as-cast 7055 alloy, the S (Al2CuMg) phase and T (Al2Mg3Zn3) phases were not detected in the as-deposited alloy due to the change of solidification paths caused by rapid solidification. The microporosity caused by spray forming is always observed in the as-deposited alloy, these micro-pores can be effectively eliminated by the extrusion processing. Meanwhile, compared with extrusion directly after preheating at 420 °C/24 h, additional high temperature two-stage homogenization treatment before extrusion can reduce the adverse effect of S (Al2CuMg) phase and promote the precipitation of high density of Al3Zr dispersoids, which significantly increases the recrystallization resistance and improve the mechanical properties of the alloy.

Cu segregation at the habit plane of lath-like β′ and QP2 precipitates for the over-aged Al-Mg-Si-Cu alloys were investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). An even number spacing of Cu segregation (dCu) was found in the habit plane of the lath-like β′ and QP2 precipitates, which is caused by the strong interaction between the segregated Cu atoms and the Si atoms within the precipitates. The Cu segregation at habit plane of these precipitates can accommodate the lattice misfit that exists between β´ and α-Al matrix.

Solidification microstructure, precipitation microstructure, and mechanical properties of foundry Al-Si-Cu-Hf alloy were investigated to elucidate the role of Hf during the solidification and precipitation processes. In the as-casting condition, α-Al dendrites, eutectic Si phases, and intermetallic compounds (Al2Cu, the needle-like β-Fe and Chinese-script α-Fe) were observed in the base alloy. However, no β-Fe was detected in the modified alloy, strongly indicating that Hf addition promotes the formation of the α-Fe phase but suppress the formation of the β-Fe phase. Furthermore, Hf addition can result not only in the formation of Hf-containing intermetallic compounds with different morphologies, which were identified as Si2Hf, but also, more importantly, in a significant decrease in the size, number, density, and volume fraction of porosity. After treatment with solution at 520 °C for 20 h, the formation of thermally stable Si2Hf precipitate was observed. After aging treatment at 180 °C up to 72 h, a delayed aging hardening was observed, which may be because Hf addition suppressed the formation of θ’ precipitate. Clearly, Hf addition in Al-Si-Cu alloy can modify the solidification microstructure and the precipitation microstructure, which is believed to be beneficial for mechanical properties. Indeed, a significant increase in elongation by 51.4% without a sacrifice of strength was obtained, compared with the unmodified Al-Si-Cu alloy. This investigation highlights the importance of a well-controlled solidification microstructure and precipitation microstructure for the improvement of mechanical properties, in particular to elongation.

The structural and compositional evolution of precipitates in Al-Mg-Si-Cu alloys were systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography. In under-aged alloys, most of precipitates have a disordered structure, with a substructure of β″ (LDC) and Cu sub-unit cluster or C unit cell. After aging to peak strength, disordered precipitates including β″, QP1 and QP2 phases are formed. The disordered QP1 and QP2 phases, which contain the unit cells of Q′ and C phases, respectively, are the precursor phases of Q′ phase in these alloys. The β″ phase can transform into the disordered QP1 phase by incorporating Cu atoms, forming Cu sub-unit clusters and QP lattice. When the alloy is over-aged, the ordering and transformation of QP1 to Q′ occurs by the formation of Cu sub-unit clusters, the ordering of QP lattice, and the ordering of QC lattice. In contrast, the transformation of the disordered QP2 phase are rather sluggish. After sufficient aging, Q′, C and disordered QP2 transform into the Q phase. During the evolution of the precipitates in these alloys, a continuous incorporation of Mg, Si and Cu atoms and release of Al atoms occur. These findings provide new insights in understanding precipitation in Al-Mg-Si-Cu alloys.

The nature and orientation of anti-phase boundaries (APBs) and interphase boundaries in Al3Zr precipitates of the Al-Cu-Zr alloy were investigated by aberration-corrected scanning transmission electron microscopy (STEM). Two groups of APBs ({100} and {110}) are identified. The {100} APBs are in the majority, while the {110} APBs are for the first time observed in the Al3Zr precipitate. Both kinds of APBs are associated with a displacement vector of a/2 in corresponding planes. Besides, two different L12/D023 interfaces (“in-phase” and “anti-phase” relationships) are formed in the Al3Zr precipitate, which is mainly caused by the activation of multiple APBs in different planes. It is clarified that the APBs play an important role in the L12-D023 transformation.

The addition of Sn in Al-Mg-Si alloys has drawn widely attention due to its strong interaction with the vacancies. In the present work, the effect of different amount of Sn addition on natural aging and precipitation hardening in Al-Mg-Si alloys are investigated. During natural aging (NA), the Sn addition can effectively suppress the clustering by capturing vacancies, and the NA hardness is gradually decreased with increasing Sn content. However, during artificial aging (AA) at 170 °C after NA, it is found that the positive effect of Sn addition is highly dependent on the Sn content. A Sn content ≤0.1 wt% can facilitate the precipitation of β″ phase due to the release of vacancies at high aging temperature. While a Sn content >0.1 wt% can result in a decrease of AA hardness. This is mainly due to the coarser and sparser distribution of β″ phases in the α-Al matrix. Besides, the excess Sn atoms preferentially form coarse primary Sn particles and Mg2Sn particles instead of being solute atoms in the α-Al matrix, which can deteriorate the mechanical properties of this alloy. Some Sn particles are formed at the interface of Fe-rich intermetallic particle, which probably refine the Fe-intermetallic phases in the high-Sn alloys. These results indicate that the Sn content should be accurately controlled for the Al-Mg-Si-Sn alloys with increased properties.

6000-series aluminium alloys with high Cu or excess Si addition were susceptible to intergranular corrosion (IGC). In order to obtain good IGC resistance, four alloys with low/high Cu and various Mg/Si ratios were designed. The corrosion behaviour of four alloys was investigated by accelerated corrosion test, electrochemical test and electron microscopies. It was revealed that IGC susceptibility of alloys was the result of microgalvanic coupling between the noble grain boundary precipitates and the adjacent precipitates free zone (PFZ), which was closely related to a combination of Cu content and the Mg/Si ratio. Excess Mg could improve the IGC resistance of alloys by forming discontinuous precipitates on the grain boundaries. The designed alloy with high Cu and excess Mg has the same corrosion level as the commercial alloy with low Cu and excess Si, which provides possibility for developing new alloy.

The solution heat treatment of AA6014 sheet is important to meet the required dent resistance in assembled automotive structures. In this study, the effect of solution heat treatment conditions on the Mg2Si particle size distribution and volume fraction were quantitatively investigated using SEM observations, electrical conductivity, and hardness measurements. Precipitation during artificial aging was studied by DSC analysis. After soaking at 570 °C for 1min, the volume fraction of soluble Mg2Si was reduced dramatically from 1.58% to nearly zero. Hardness measurements, EC, and SEM characterization of T4 samples were compared as measures of Mg2Si dissolution.

The effects of Ag and/or Cu additions on the natural aging and precipitation hardening behavior of Al-Mg-Si alloys were investigated by using hardness test, differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). Both Ag and Cu additions enhanced the hardening kinetics of Al-Mg-Si alloys during natural aging (NA) and artificial aging (AA) treatments. The strong interaction of Cu, Ag and Mg atoms is responsible for the improved precipitation kinetics of these alloys, resulting in a refinement of clusters and precipitates. The stronger interaction between Ag and Mg results in lower T4 hardness and higher AA hardness, compared to Cu. This is potentially beneficial for automobile applications where rapid hardening during paint baking is required. Both Ag and Cu additions reduce the detrimental effect of NA on subsequent AA, due to the formation of Ag or Cu-containing clusters. The clear advantage of Ag addition, compared to Cu, is the improvement of precipitation hardening response of Al-Mg-Si alloys.

The effect of pre-straining on the natural aging and bake hardening of Al-Mg-Si alloys with different additions of Cu were investigated by Vickers microhardness measurements, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM) characterization. It is revealed that introducing pre-straining can suppress the natural aging and increase the bake hardening response (BHR) for both low and high Cu addition alloys. A pre-strain of 2% is found to be the optimum process. Addition of Cu can increase the effect of pre-straining in suppressing the natural aging and improving the BHR after paint baking. The slow diffusion rate of Cu atoms in the Al matrix is most probably the main reason for the significant suppression of natural aging in the high content Cu alloy after pre-straining. During paint baking, the dislocations introduced promote the preferential precipitation of the Q′ phase and produce a disordered structure within the precipitates. Thus designing new pre-straining processes should fully consider the influences of Cu addition to the alloys.

The design of high-strength martensitic stainless steels requires an accurate control over the stability of undesired phases, like carbides and ferrite, which can hamper strength and ductility. Here, the ferrite to austenite transformation in Fe-11Cr-0.06C has been studied with a combined experimental-modelling approach. Experimental observations of the austenization process indicate that austenite growth proceeds in multiple steps, each one characterized by a different transformation rate. DICTRA based modelling reveals that the dissolution of the M23C6 Cr-rich carbides leads to Cr partitioning between austenite and parent phases, which controls the rate of transformation through (i) a soft-impingement effect and (ii) consequent stabilization of the ferrite, which remains untransformed inside chromium-enriched-zones even after prolonged austenization stage. Slow heating rate and smaller initial particle sizes allow the design of ferrite-free microstructure. (C) 2018 Published by Elsevier Ltd.

Selective laser melting (SLM) manufactured AlSi10Mg alloys present a fine silicon-rich network and precipitates which grant high mechanical strength but low ductility. Post-treatments, aiming at eliminating inherent defects related to SLM such as residual stresses, porosity or inhomogeneity, result in significant changes in the microstructure and impact both the hardening and the damage mechanisms of the post-treated material. The present work is dedicated to the investigation of the fracture of SLM AlSi10Mg under as built and three post-treatment conditions, namely two stress relieve heat treatments and friction stir processing (FSP). It is found that the interconnected Si network fosters damage at low strain due to the brittleness of the Si phase. The onset of damage transfers load to the enclosed Al phase which then fractures quickly under high stress, thus leading to low material ductility. In contrast, when the Si network is globularized into Si particles, the ductility is highly increased even in the case where the porosity and inhomogeneity of the microstructure remain after the post-treatment. The ductility enhancement results from the delay in void nucleation on the Si particles as well as from the tolerance for void growth in the Al matrix.

The effect of Ag addition on the precipitation evolution and interfacial segregation for Al–Mg–Si alloys was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atom probe tomography (APT) and density functional theory (DFT) calculation. At the early aging stage, Ag atoms could enter clusters and refine the distribution of these clusters. Then, Ag atoms preferentially segregate at the GP zone/α-Al and β"/α-Al interfaces at the peak aging stage by the replacement of Al atoms in FCC matrix. With prolonging aging time, Ag atoms generally incorporate into the interior of β" precipitate, facilitating the formation of QP lattice (a hexagonal network of Si atomic columns) and the local symmetry substructures, Ag sub-unit (1) and Ag sub-unit (2). At the over-aged stage, the Ag sub-unit (1) and Ag sub-unit (2) could transform to the β′Ag (i.e. β′Ag1 and β′Ag2.) and Q′Ag unit cells, respectively. All the precipitates at the over-aging stage have a composite and disordered structure due to the coexistence of different unit cells (β′Ag1, β′Ag2, Q′Ag and β′) and the non-periodic arrangement of Ag atoms within the precipitate. In the equilibrium stage, the incorporated Ag atoms in the precipitates release into the α-Al matrix as solute atoms or form Ag particles. In general, Ag atoms undergo a process of “segregate at the precipitate/matrix interface → incorporate into the interior of precipitate → release into the α-Al matrix” during the precipitation for Al–Mg–Si–Ag alloys. Besides, Ag segregation is found at the interfaces of almost all metastable phases (including GP zone, β″, β′/β′Ag phase) in Al–Mg–Si–Ag alloys. The Ag segregation at the β′/α-Al interface could increase the length/diameter ratio of β′ phase and thus promote the additional strengthening potential of these alloys. These findings provide a new route for precipitation hardening by promoting the nucleation and morphology evolution of precipitates. © 2019. This is the authors’ accepted and refereed manuscript to the article. Locked until 30 November 2021 due to copyright restrictions. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

In the framework of metal matrix composites, a research gap exists regarding tailoring damage mechanisms. The present work aims at developing an Al/NiTi composite incorporating internal stresses in the vicinity of reinforcements. The composite is manufactured by friction stir processing which allows a homogenous NiTi distribution and a good Al/NiTi interface bonding. The internal stresses are introduced via shape memory effect of the embedded NiTi particles. The induced internal strain field is confirmed by digital image correlation and the corresponding stress field is evaluated by finite element simulation. It is found that the damage mechanism is modified in the presence of internal stresses. The consequent enhancement of fracture toughness arises by the fact that the internal stresses foster discrete damages shifted from the fracture ligament line. These damages release the stress concentration at the main crack tip and lead to a deviated crack path when coalescing to accommodate fracture propagation.

The effect of combined addition of Ag and Cu on the precipitation behavior for an Al-Mg-Si alloy was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and three-dimensional atom probe (3DAP) analysis. At the early aging stage, Ag and Cu atoms participate in the clustering process and accelerate the transformation of clusters to GP zones and β″ phases. In the peak aging stage, Ag atoms preferentially segregate at the β″/α-Al interfaces, while Cu atoms enter into the interior of the β″ phase and form a symmetry substructure, Cu sub-unit clusters. With prolonged aging, Ag atoms gradually incorporate in the β″ phase and form Ag sub-unit clusters. The incorporated Ag and Cu atoms can promote the formation of β′Ag, disordered QP1 and QP2 phases during the over aging stage. Almost all of these precipitates have a composite structure with various unit cells (β′, β′Ag, Q′ and C) or substructures (LDC, Cu sub-unit clusters, Ag sub-unit clusters), and Ag segregation is found in the interfaces of these precipitates. For the lath-like QP2 phase, Cu and Ag atoms segregate at the coherent and semi-coherent QP2/α-Al interfaces, respectively, which can reduce their strain energy and interfacial energy. These findings provide new insights in understanding the precipitate evolution of the Al-Mg-Si-Ag-Cu alloy, which is a potential alloy for automobile applications.

The heterogeneous precipitation of the η (MgZn2) phase on the E (Al18Mg3Cr2) dispersoids of the 7075 aluminum alloy was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectrometry (EDX). It is found that coarse η particles are heterogeneously precipitated at the E particle interface after water quenching and isothermal aging at 120 °C. The incoherent E/Al interface is responsible for the high tendency of heterogeneous precipitation of the η phase. Two different orientation relationships (ORs) between the η, E and Al matrix are identified: OR1 [2 1 ¯ 1 ¯ 0]η//[011]E//[ 1 ¯ 12]Al, (01 1 ¯ 0 )η//(13 3 ¯ )E//(2 0 1)Al, OR2 [ 1 ¯ 12]E//[0001]η//[011]Al, (01 1 ¯ 0 )η//(220)E//(3 4 4 ¯ )Al. The η phase is preferential to nucleate along the {111}E or the {220}E planes, depending on its OR. The heterogeneous nucleation of η phase on the E particle could stabilize the E/Al interface by introducing a coherent E/η interface, which increases the drive force of heterogeneous precipitation. The reorientation of η phase and mutual diffusion of solute atoms could assist the coherency of the E/η interface. The present results suggest that increasing the coherency of the E/Al interface is a promising method to suppress the heterogeneous precipitation of the η phase.

The morphologies and orientation relationships (OR) of the Q′ phase in an over-aged Al––Mg–Si–Cu alloy were characterized by atomic resolution high-angle annular dark-field scanning transmission electron microscopy. Statistical results revealed that the OR of the rod-like Q′ phase with the Al matrix has a variable angle ranging from ~ 3.4° to 15.5° between ( 11 2 - 0 ) Q′ and (510) α , while the lath Q′ phase has a conventional OR with a constant angle of 11°. Such variations likely to be attributable to the ordering of Si-network in the Q′/β″ phase formed in the peak aging condition.

The disordered structures of the Q′ and C phases in the over-aged Al–Mg–Si–Cu alloy were studied by the atomic resolution high-angle annular dark-field scanning transmission electron microscopy and density functional theory calculations. A Cu cluster-like zone is found in both the well-ordered and disordered regions of the disordered precipitates, which acts as a stable skeleton of these phases. The statistic result shows that the C phase is more preferential to produce disordered structure than the Q′ phase. The mechanism for forming the highly disordered C phase is attributed to the relatively weak metallic bonding between the Al–Si1 and Mg2–Si1 atoms.