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In recent years, wire-arc directed energy deposition (waDED), which is also commonly known as wire-arc additive manufacturing (WAAM), has emerged as a promising new fabrication technique for magnesium alloys. The major reason for this is the possibility of producing parts with a complex geometry as well as a fine-grained microstructure. While the process has been shown to be applicable for Mg-Al-Zn alloys, there is still a lack of knowledge in terms of the influence of the WAAM process on the age-hardening response. Consequently, this study deals with the aging response of a WAAM AZ91 alloy. In order to fully understand the mechanisms during aging, first, the as-built condition was analyzed by means of high-energy X-ray diffraction (HEXRD) and scanning electron microscopy. These investigations revealed a fine-grained, equiaxed microstructure with adjacent areas of alternating Al content. Subsequently, the difference between single- and double-step aging as well as conventional and direct aging was studied on the as-built WAAM AZ91 alloy for the first time. The aging response during the various heat treatments was monitored via in situ HEXRD experiments. Corroborating electron microscopy and hardness studies were conducted. The results showed that the application of a double-step aging heat treatment at 325 °C with pre-aging at 250 °C slightly improves the mechanical properties when compared to the single-step heat treatment at 325 °C. However, the hardness decreases considerably after the pre-aging step. Thus, aging at lower temperatures is preferable within the investigated temperature range of 250-325 °C. Moreover, no significant difference between the conventionally aged and directly aged samples was found. Lastly, the specimens showed enhanced precipitation kinetics during aging as compared to cast samples. This could be attributed to a higher amount of nucleation sites and the particular temperature profile of the solution heat treatment. [ABSTRACT FROM AUTHOR]

The effect of Ca addition on the age hardening response and precipitation behavior of a Mg-7Y-0.5Zr (wt%) alloy during isothermal aging at 350 ℃ has been studied by hardness test and high-angle annular dark-field scanning transmission electron microscopy. The addition of 0.6 wt% Ca can effectively enhance the age hardening response of Mg-7Y-0.5Zr alloy at 350 ℃, and the enhanced hardness is mostly attributed to the uniform precipitation of a novel strengthening precipitate phase, designated β p. The β p phase forms as plates on { 10 1 ̅ 0 } α planes. It has a Cd 45 Sm 11 -type structure (space group F 4 ̅ 3m , lattice parameter a = 2.24 nm) and a composition of Mg 5 Y 0.8 Ca 0.2. Its orientation relationship with the α-Mg matrix phase is such that [ 1 ̅ 11 ] β p // [2 1 ̅ 1 ̅ 0] α and (110) β p // (0001) α. After prolonged aging at 350 ℃, the metastable phase β p can transform in-situ to the equilibrium phase β-Mg 24 Y 5 with a cube-to-cube orientation relationship. The β p → β phase transformation is associated with the transition of constituent atomic clusters and the re-distribution of selected atoms within clusters. • The addition of 0.6 wt% Ca to Mg-7Y-0.5Zr alloy can enhance the age hardening response at 350 ℃. • A novel strengthening phase β p is observed in the peak-aged Mg-7Y-0.6Ca-0.5Zr alloy. • The β p phase transforms in-situ to the equilibrium phase β-Mg 24 Y 5 after prolonged aging. [ABSTRACT FROM AUTHOR]

Effects of ageing treatment on the microstructures, mechanical properties and corrosion behavior of the Mg-4.2Zn-1.7RE-0.8Zr-xCa-ySr [x=0, 0.2 (wt.%), y=0, 0.1, 0.2, 0.4 (wt.%)] alloys were investigated. Results showed that Ca or/and Sr additions promoted the precipitation hardening behavior of Mg-4.2Zn-1.7RE-0.8Zr alloy and shortened the time to reaching peak hardness from 13 h to 12 h. The maximum hardness of 77.1±0.6 HV for the peak-aged Mg-4.2Zn 1.7RE-0.8Zr-0.2Ca-0.2Sr alloy was obtained. The microstructures of peak-aged alloys mainly consist of α-Mg phase, Mg51Zn20 phase and ternary T-phase. The Zn-Zr phase is formed within the α-Mg matrix, and the Mg2Ca phase is formed near T-phase due to the enrichment of Ca in front of the solid-liquid interface. Furthermore, fine short rod-shaped β′1 phase is precipitated within the α-Mg matrix in the peak-aged condition. The peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca 0.2Sr alloy exhibits optimal mechanical properties with an ultimate tensile strength of 208 MPa, yield strength of 150 MPa and elongation of 3.5%, which is mainly attributed to precipitation strengthening. In addition, corrosion properties of experimental alloys in the 3.5wt.% NaCl solution were studied by the electrochemical tests, weight loss, hydrogen evolution measurement and corrosion morphology observation. The results suggest that peak-aged alloys show reduced corrosion rates compared with the as-cast alloys, and minor additions of Ca and/or Sr improve the corrosion resistance of the Mg-4.2Zn-1.7RE-0.8Zr alloy. The peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy possesses the best corrosion resistance, which is mainly due to the continuous and compact barrier wall constructed by the homogeneous and continuous second phases.

The microstructure evolution and age-hardening response for different Sm/Sn ratios (0–2.55, in wt.%) of Mg-Sn-Sm alloys were investigated. The second phase formation in as-cast alloys and the Mg3Sm precipitates formed in aged alloys were characterized using XRD, FESEM and HAADF-STEM with EDS techniques. Results indicate that the Sm/Sn ratio has a great influence on the phase constitution, α-Mg grain size and age-hardening response. With the increment of Sm/Sn ratio, Mg41Sm5 and thermally stable MgSnSm phases precipitate. When the Sm/Sn ratio is about 1.19, the secondary dendrite arm spacing of α-Mg grains significantly decreases. Furthermore, the alloy with Sm/Sn ratio up to 2.55 exhibits the highest age-hardening response, the hardness value increases from 52 HB at solution-treated condition to 74 HB at peak-aged condition (ageing at 220 °C for a short time of 4 h). This is attributed to the large volume fraction of needle-like Mg3Sm precipitates formed in the α-Mg matrix during ageing treatment, which results in a significant precipitation strengthening effect. [ABSTRACT FROM AUTHOR]

Effects of ageing treatment on the microstructures, mechanical properties and corrosion behavior of the Mg-4.2Zn-1.7RE-0.8Zr-xCa-ySr [x=0, 0.2 (wt.%), y=0, 0.1, 0.2, 0.4 (wt.%)] alloys were investigated. Results showed that Ca or/and Sr additions promoted the precipitation hardening behavior of Mg-4.2Zn-1.7RE-0.8Zr alloy and shortened the time to reaching peak hardness from 13 h to 12 h. The maximum hardness of 77.1±0.6 HV for the peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy was obtained. The microstructures of peak-aged alloys mainly consist of α-Mg phase, Mg51Zn20 phase and ternary T-phase. The Zn-Zr phase is formed within the α-Mg matrix, and the Mg2Ca phase is formed near T-phase due to the enrichment of Ca in front of the solid-liquid interface. Furthermore, fine short rod-shaped β′1 phase is precipitated within the α-Mg matrix in the peak-aged condition. The peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy exhibits optimal mechanical properties with an ultimate tensile strength of 208 MPa, yield strength of 150 MPa and elongation of 3.5%, which is mainly attributed to precipitation strengthening. In addition, corrosion properties of experimental alloys in the 3.5wt.% NaCl solution were studied by the electrochemical tests, weight loss, hydrogen evolution measurement and corrosion morphology observation. The results suggest that peak-aged alloys show reduced corrosion rates compared with the as-cast alloys, and minor additions of Ca and/or Sr improve the corrosion resistance of the Mg-4.2Zn-1.7RE-0.8Zr alloy. The peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy possesses the best corrosion resistance, which is mainly due to the continuous and compact barrier wall constructed by the homogeneous and continuous second phases. [ABSTRACT FROM AUTHOR]

The strain-amplitude-dependent and strain-amplitude-independent damping capacities of a cast AZ91 magnesium alloy as functions of its age-hardening response induced by continuous precipitation have been investigated. The damping capacities of the AZ91 alloy exhibited a decreasing tendency in the order of solutionised, underaged, overaged, and peak aged states in the strain-amplitude-dependent region, whereas they increased continuously with aging time in the strain-amplitude-independent region. In view of the microstructural evolution during aging, the lower Al concentration in the α-(Mg) matrix and the lower number density of continuous precipitates are thought to be responsible for the better damping capacities of the AZ91 alloy in the strain-amplitude-independent and strain-amplitude-dependent regions, respectively. [ABSTRACT FROM AUTHOR]

In this research, precipitation evolution and strengthening behavior in Mg-3Nd-0.2Zn alloy during isothermal age treatment at 200 °C are quantitatively studied by a combination of experimental investigation and computational simulation. Microstructure evolution, including volume fraction, number density, and size of precipitates are first measured by TEM and DSC techniques and then modeled using an integrated computational simulation method combining the CALPHAD (CALculation of PHAse Diagram) approach and classical nucleation model. The focus of the analysis is set on the evolution of key strengthening precipitate β’. Critical kinetic parameters such as particle interfacial energy and nucleation site number are obtained computationally by fitting the model to the experimental data. Mechanical properties of a series of Mg-Nd-Zn alloys are predicted using the simulated microstructure evolution as inputs. Good agreement is found between simulated and measured results. Strategies for enhancing the precipitation hardening effect are presented and discussed based on the simulation results. The merit of this study is to use the obtained parameters to extrapolate microstructure evolution and mechanical properties when adjusting alloy composition and/or heat treatment conditions within a certain range, which will be very useful for further development and optimization of multi-component Mg alloys. [ABSTRACT FROM AUTHOR]

The microstructure and age-hardening behaviors of Mg-7Sn, Mg-7Sn-1Ca and Mg-7Sn-1Ca-1Ag alloys have been investigated in this study. The results show that the combined addition of Ca and Ag in Mg-Sn alloy has a positive effect on grains refinement from 113.5 µm of Mg-7Sn alloy to 54.3 µm of Mg-7Sn-1Ca-1Ag alloy. Meanwhile, the size of CaMgSn phase is refined due to suppressing the formation and growth of CaMgSn phase by Ag element. The average length of CaMgSn phase decreases from 27.2 to 14.9 µm. Compared with Mg-7Sn and Mg-7Sn-1Ca alloys, Mg-7Sn-1Ca-1Ag alloy exhibits the highest age-hardening response. The peak-aged hardness value increases from 61.1 HV to 80.4 HV by ageing treatment at 200 ℃ and the incubation period of age-hardening response also shortens from 80 h to 20 h with Ag addition. It is attributed to the effective promotion of dispersive precipitation and the volume fraction increasement of Mg 2 Sn precipitates in Mg-7Sn-1Ca-1Ag alloy, which can lead to a strong precipitation strengthening effect in α-Mg matrix. [ABSTRACT FROM AUTHOR]

Effects of Ag content (0-2 wt%) on the solid-solution and ageing behavior of a Mg-5Sn alloy were investigated. In the solution-treated state, addition of 0.5 wt% Ag can clearly improve the hardness of the Mg-5Sn alloy, while further additions of Ag increase the hardness only a little. On contrary, the age-hardening response of the Mg-5Sn alloy is enhanced proportionally with the amount of Ag added. The microstructure of the peak-aged Mg-5Sn-xAg (x = 0, 0.5, 1, 2) alloys are mainly composed of α-Mg matrix and Mg 2 Sn precipitates. TEM results show that as the Ag additions increase, the number density of the Mg 2 Sn precipitates increase and the aspect ratio is also reduced, which mainly account for the improved precipitation-hardening response. Retained Ag atoms in α-Mg solid solutions also contribute to the strengthening effect in the peak-aged state. Minor blocky Mg 54 Ag 17 particles were observed in the Mg-5Sn-2Ag alloy. The particles adopt an orientation relationship of [1 2 0] p //[1 0 -1 0] α , (0 0 2) p //(0 0 0 2) α with respect to α-Mg matrix. The role of Ag during the precipitation process has also been discussed. [ABSTRACT FROM AUTHOR]

High quality WE43 magnesium alloy ingots with diameter of 500 mm are successfully fabricated by direct-chill (DC) casting. The alloying elements distributed almost homogeneously across the large ingot. With the average grain size decreasing from the center to the edge of the ingot, the mechanical properties are improved gradually. After solution treatment at 525 °C for 8 h, the Mg 14 Nd 2 Y phases distributing along the grain boundaries are well dissolved into matrix. The direct-chill (DC) casting ingot exhibits remarkable age-hardening response from 85 Hv to 117 Hv after T6 treatment at 250 °C. The T6 peak-aged sample exhibits ultimate tensile strength of 274 MPa, yield strength of 215 MPa and elongation to failure of 3.4%, which are higher than those of sand-cast and permanent mold cast WE43 alloys. The large amount of nano prismatic β ′ and β 1 phases precipitated during T6 treatment contribute to improved strength of the WE43 alloy. [ABSTRACT FROM AUTHOR]

Precipitation evolution and its correspondence to mechanical properties in Mg 4Sm xZn 0.4Zr (x = 0, 0.3, 0.6, 1.3) (wt%) alloys are systematically investigated in this work. Precipitation sequences at 200 °C are identified using transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). A new precipitate β′ Z is found to present with the increase of Zn addition. With higher Zn content (>∼1 wt%), basal γ-series precipitates dominate. The transition in precipitation sequence agrees well with the change of mechanical properties. The microstructure and property evolution in the optimum Mg 4Sm 0.3Zn 0.4Zr alloy is modeled based on obtained experimental data through a combination of classical nucleation and growth model and the CALPHAD (CALculation of PhAse Diagram) simulation. The obtained parameters are then used to predict the properties of other Mg Sm Zn Zr alloys under other heat treatment conditions. Good agreements are found between the calculated and measured data, which validate the accuracy of the current database and suggest a powerful tool for future optimization of this promising alloy system. [ABSTRACT FROM AUTHOR]

The effects of extrusion and macro-alloying of Ag and Zn on the age-hardening behavior and microstructure of a Mg-7Sn alloy were investigated. The results show that the extrusion process and extrusion combined with 2 wt%Ag or 2 wt%Ag+1 wt%Zn addition can significantly improve the hardness of the unextruded Mg-7Sn alloy from 46 HV to 52 HV, 72.5 HV and 77.5 HV, respectively at the unaged state. The strengthening effect is mainly attributed to the dynamic precipitation and much refined grain size obtained by the extrusion process. Due to the depletion of the alloying elements by dynamic precipitation, the hardness increments resulted from artificial ageing at 160 °C are almost equal (around 10 HV) for the base and macro-alloyed Mg-7Sn extruded alloys. The extruded alloys exhibit a bimodal grain size distribution containing major dynamically recrystallized fine grains and minor severely deformed coarse grains. There are two types of Mg 2 Sn precipitates. One is the spherical particles distributed randomly and the other is the laths formed within the matrix. Ag or Ag+Zn addition to the extruded Mg-7Sn alloy can significantly refine the size of Mg 2 Sn laths. Blocky ε’-Mg 54 Ag 17 phase is also formed along grain boundaries in the extruded Mg-7Sn-2Ag-1Zn alloy. [ABSTRACT FROM AUTHOR]

Effects of thermal history on precipitation behavior and precipitation kinetics in Mg–3Nd–0.2Zn (wt.%) alloy were studied. Microstructure analysis of the aged alloys revealed the presence of new phases in the microstructure in addition to the old description: Super saturated solid solution (SSSS) → G. P. zones → β ″ → β ′ /β 1 → β , reported for Mg–Nd system. In this regard, the sequence of precipitation for the studied alloy was identified as: Super saturated solid solution (SSSS) → Clusters of atoms → G. P. zones (I, II, III) → β ′ → β 2 → β 1 /γ ′ → β. The formation of clusters of solute atoms in this system was confirmed by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and atom probe tomography analysis. In addition, new metastable phases designated as G. P. zones (I, II, III) and β 2 were identified in the microstructure of the aged alloys by means of high resolution HAADF-STEM. The β 2 phase was found to be a transition phase in transformation of β ′ to β 1 . Furthermore, the microstructure analysis revealed that the structure of β 1 phase is indeed a body centered tetragonal structure in spite of face centered cubic structure that has been reported for this phase in the literature. [ABSTRACT FROM AUTHOR]

Mg–Sn-based alloys are considered as a promising precipitation-hardening system for applications at elevated temperatures, but the hardening effect is not satisfactory owing to sluggish nucleation and rapid coarsening of the major Mg2Sn lath precipitates. In this study, Cu and Al are added to a Mg–6Sn–1Mn base alloy. The age-hardening response and the microstructures of these modified alloys have been investigated and are compared to that of the base alloy. The additional elements are found to bring several beneficial effects to the alloys for applications at elevated temperatures. Firstly, a eutectic structure consisting of strong intermetallic phases, i.e. Mg2Cu in the Mg–6Sn–1Mn–2Cu alloy and Al0.93Cu1.07Mg in the Mg–6Sn–1Mn–2Cu–2Al alloy, remains stable along the grain boundaries after solution and ageing heat treatments. Secondly, the precipitate density has been increased significantly and the precipitate size has been refined remarkably during ageing at 200?°C. Moreover, the growth of the precipitates is inhibited remarkably during the over-ageing period. Therefore, the age-hardening response and over-ageing resistance are notably improved. [ABSTRACT FROM AUTHOR]

Highlights: [•] The strongest age-hardening response was found in 0.81Mg alloy. [•] Quantitative APT study showed strong dependence of Mg–Ag co-clustering on Mg content. [•] A critical Mg content related to the greatest Mg–Ag co-clustering was revealed. [•] The evolution from Mg–Ag co-clusters to Ω phase was accelerated in 1.18Mg alloy. [Copyright &y& Elsevier]

Abstract: The effect of 0.5at% Ag and its combination with 0.4at% Zn on the precipitation hardening behavior of Mg–1.5Sn–0.5Mn (at%) alloy have been investigated using hardness measurements and transmission electron microscopy (TEM). The results show that sole addition of Ag can remarkably improve the peak hardness from 58HV to 77HV after ageing at 200°C, which is attributed to the increased number density, the size and so the volume fraction of the Mg2Sn lath precipitates. A combination addition of Ag and Zn further improves the peak-hardness to 83HV and the time to peak hardness has been reduced from 120h to 100h. The joint addition of Ag and Zn both increases the number density and refines the Mg2Sn precipitates. Zn also promotes the Mg2Sn precipitates lying on the non-basal plane of matrix, as well as the blocky Mg54Ag17 phase which has an orientation relationship with α-Mg matrix as (001)ɛ′//(0001)α, [200]ɛ′//[−1−120]α, [0−20]ɛ′//[1−100]α. In the peak-aged Ag modified alloys, Ag atoms are segregated in the Mg2Sn precipitates. Nucleation, growth and coarsening of the Mg2Sn precipitates simultaneously occur as the ageing proceeds. The growth of Mg2Sn precipitates exhibits a significant anisotropic behavior. Addition of Ag can promote the precipitation process during the underage period and retarding the growth of Mg2Sn during the overage period, leading to a more thermal stable microstructure of the Ag and Ag+Zn modified alloys, from which only a small hardness decrease after 700h ageing is detected. [Copyright &y& Elsevier]

Abstract: The microstructures and mechanical properties of the new quaternary Mg–6Zn–xCu–0.6Zr alloys (x =0, 0.5, 1.0 and 2.0wt.%) have been investigated. The results show that the Cu content has a significant effect on the age-hardening response, tensile performance and fracture behavior of the alloys. The addition of 0.5wt.% Cu resulted in a remarkable age-hardening response and a striking improvement of the room temperature tensile properties after an isothermal ageing at 180°C. However, an excessive Cu addition of 2.0wt.% caused the formation of continuous brittle grain-boundary intermetallic particles, thus degrading the age-hardening response and the mechanical properties of the alloy. The microstructural factors associated with the improvements in the mechanical properties are discussed in detail. [Copyright &y& Elsevier]

Abstract: The microstructures and age-hardening response of a Mg–5wt.%Gd alloy and a Mg–5wt.%Gd–1.1wt.%Mn–0.8wt.%Sc alloy have been examined under different solution treatment conditions. Both alloys exhibit little or no age-hardening response during isothermal ageing at temperatures above 200°C when the alloys are solution treated at 520°C for 24h. Solution treatment at 610°C for 24h does not improve the age-hardening response of the binary alloy. However, it leads to a significant increase in hardness during ageing for the quaternary alloy. This improved age-hardening response in the quaternary alloy is ascribed to the dense precipitation of two fine-scale precipitate phases during ageing. [Copyright &y& Elsevier]

In this work, a bulk nanocrystalline WE43 alloy prepared through rotary swaging was subjected to the aging treatment at different temperatures. The mechanical property and microstructure variations during aging were systematically studied. Aging at 175 °C enhanced the alloy to be maximumly with 516 MPa in ultimate tensile strength (UTS), 477 MPa in yield strength (YS) and 8.5% in elongation to failure. The significant strengthening effect resulted from rare-earth elements segregation at grain boundary. During aging at 225 °C, tremendous enhancement was obtained in the initial stage of aging, while the formation of massive precipitates along grain boundary deteriorated the strength as aging time was prolonged. • A nanocrystalline magnesium alloy is obtained by a combination of cold rotary swaging and aging treatment. • Low temperature aging treatment promotes the segregation of rare earth elements along nanocrystalline grain boundaries. • The tensile properties of the nanocrystalline WE43 alloy are comprehensively enhanced after aging. [ABSTRACT FROM AUTHOR]

The accelerated age-hardening response and the enhanced peak-hardness in Al-Mg-Si with Zn addition were investigated by three-dimensional atom probe and transmission electron microscopy. The results showed that Zn addition significantly increased the number density of solute aggregates by stimulating the formation of more Mg Si precipitates and resulted in a higher Mg/Si ratio because of the formation of Mg Zn precipitates. Furthermore, a synergistic strengthening from Mg Si and Mg Zn solute aggregates was quantitatively estimated in accelerating the age-hardening response at the early aging stage, and a coexistence of β″ phases for Mg 2 Si system and GPI zones/η′ phases for MgZn 2 system was confirmed to enhance the peak-hardness of Al-Mg-Si alloy with Zn addition. Unlabelled Image • The age-hardening response was investigated in Al-Mg-Si alloy with Zn addition. • Zn addition stimulated the formation of more Mg Si precipitates. • The formation of Mg Zn precipitates contributed a higher Mg/Si ratio. • A synergistic strengthening at the early aging stage was quantitatively estimated. • The coexistence of β″ phases and GPI zones/η′ phases enhanced the peak-hardness. [ABSTRACT FROM AUTHOR]

Effects of Bi additions on age-hardening response and precipitation of Mg–2.31Zn–0.31Co (at.%) alloy have been investigated. The addition of Bi can substantially promote the age-hardening response at 200 °C. The precipitates microstructure is characterized using high-angle annular dark-field scanning transmission electron microscope, and the results show that the remarkable improvement is attributed to a much higher number density of β 1 ' rods and the formation of prismatic Mg 3 Bi 2 plates. At the early stage of ageing, Mg 3 Bi 2 precipitates out first and then Zn segregates to the Mg 3 Bi 2 /Mg interface, which could provide chemical environment and heterogeneous nucleation sites for the subsequent precipitation of the β 1 ' phase. Image 1 • The addition of Bi can substantially enhance the age-hardening response of Mg–2.31Zn–0.31Co alloy. • Bi additions result in a higher number density of β 1 ' rods and the formation of prismatic Mg 3 Bi 2 plates. • Zn atoms segregate at Mg 3 Bi 2 /Mg interface. • Rod-like β 1 ' phase heterogeneously nucleates at Mg 3 Bi 2 /Mg interfaces. [ABSTRACT FROM AUTHOR]

Two types of nanoclusters, Cluster (1) and Cluster (2), formed at around room temperature and 100 °C, respectively, affect the age-hardening behavior in Al-Mg-Si alloys. Formation of Cluster (1) during natural aging (NA) is more accelerated in the high-Mg (9M10S) alloy than in the low-Mg (3M10S) alloy. Hardness at the early stage of two-step aging at 170 °C is not increased for the natural aging samples. On the other hand, hardness is directly increased for the pre-aged (PA) specimens. Furthermore, the formation of Cluster (1) during natural aging is suppressed by the formation of Cluster (2) during pre-aging at 100 °C. To understand the effects of heat treatment histories and Mg contents on the microstructure, transmission electron microscopy (TEM) was utilized. All the images were obtained at (001) plane, and peak aged samples with different heat treatments were used. Lower number density of precipitates is confirmed for the natural aging samples compared with the single-aged and pre-aged specimens. A higher number density of precipitates is confirmed for 9M10S in comparison to 3M10S. Hardness results correspond well to the TEM images. [ABSTRACT FROM AUTHOR]