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Ammonium nitrate (AN) is of considerable interest to researchers in developing new types of energetic mixtures due to the release of environmentally benign gaseous products during burning and thermal decomposition. However, poor ignition and a low burning rate require special additives to speed up this process. The advantage of this research is the use of high-energy aluminum-based alloys as fuel to compensate for the disadvantages of AN. In addition, the effect of copper oxide (CuO) on the burning kinetics and thermodynamics of the energetic mixture based on ammonium nitrate–magnesium–aluminum alloys (AN/MgAl) is investigated. Alloys based on aluminum were created through a process of high-temperature diffusion welding, conducted in an environment of argon gas. The structure and thermal characteristics of alloys are determined by X-ray diffraction, scanning electron microscopy, and DTA-TG analyses. It has been found that CuO has significant effects on the thermal decomposition of an AN/MgAl-based energetic mixture by shifting the decomposition temperature from 269.33 °C to 261.34 °C and decreasing the activation energy from 91.41 kJ mol−1 to 89.26 kJ mol−1. Adding CuO reduced the pressure deflagration limit from 2 MPa to 1 MPa, and the linear burning rate of the AN/MgAl energetic mixture increased approximately twice (rb = 6.17 mm/s vs. rb = 15.44 mm/s, at a chamber pressure of P0 = 5 MPa). [ABSTRACT FROM AUTHOR]

Magnesium-aluminum (Mg–Al) system alloys have good casting properties, thermal conductivity, and mechanical processing properties; however, hot tearing is one of the most serious defects in the production of Mg alloy castings, which greatly limits its further development and application prospects. Therefore, it is important to study the mechanism of hot crack sprouting and expansion to prevent the occurrence of hot tearing and to expand the application range of Mg alloy materials. This review presents the research progress on the hot tearing behavior of Mg–Al alloys in recent years, summarizes the main hot tearing susceptibility testing and experimental methods, and discusses the influence of alloying elements, microstructure, and casting process parameters on the hot tearing behavior of Mg–Al alloys, in order to provide a basis for the application of the Mg–Al alloys. [ABSTRACT FROM AUTHOR]

In this study, the effects of Al addition on the corrosion behavior of pure Mg with controlled impurity contents were systematically analyzed according to the processing history. The results revealed that the corrosion behavior of high-purity Mg-Al alloys is strongly related to changes in the microstructure, including the β phase and Al-Mn or Al-Fe phases, and the protectiveness of the surface film according to the Al content and processing history. In the as-cast alloys, the corrosion rate increased due to the increase of β phase as the Al content increased, but in the as-extruded alloys, the corrosion rate, which was high due to intermetallic compounds caused by impurities in the low Al alloy, decreased as the Al content increased, and then increased again. This is due to the combined effect of the increase of the β phase and decrease of the impurity effect, and the increase of the dissolved Al content. The results suggest that it is necessary to analyze the effect of alloying elements on the corrosion behavior of pure Mg with information concerning the impurity content and processing history.

• The corrosion behavior according to al content in as-cast and as-extruded mg-al alloy show different trends. • Corrosion factors of Mg-Al alloys such as impurity, second phases, and surface firm properties vary according to not only the al content but also the processing history. • Proposal of the need for alloy design from a new perspective considering the influence of final corrosion factors. In this study, the effects of Al addition on the corrosion behavior of pure Mg with controlled impurity contents were systematically analyzed according to the processing history. The results revealed that the corrosion behavior of high-purity Mg-Al alloys is strongly related to changes in the microstructure, including the β phase and Al-Mn or Al-Fe phases, and the protectiveness of the surface film according to the Al content and processing history. In the as-cast alloys, the corrosion rate increased due to the increase of β phase as the Al content increased, but in the as-extruded alloys, the corrosion rate, which was high due to intermetallic compounds caused by impurities in the low Al alloy, decreased as the Al content increased, and then increased again. This is due to the combined effect of the increase of the β phase and decrease of the impurity effect, and the increase of the dissolved Al content. The results suggest that it is necessary to analyze the effect of alloying elements on the corrosion behavior of pure Mg with information concerning the impurity content and processing history. [ABSTRACT FROM AUTHOR]

The dynamic oxidation of molten Mg–Al alloy was investigated via the oxide /metal/oxide (OMO) sandwich method. The characteristics of sandwiches were explored using optical microscopy, scanning electron microscopy, X-ray energy dispersive spectroscopy, and X-ray diffraction analyses. The results showed the formation of porous oxide films with varying thicknesses from 0.43 to 16.7 mm. Both the measurements and calculations confirmed the literature findings that the oxidation product consists mainly of MgO and MgAl 2 O 4 compounds. The increase in thickness and amount of folds formed on the oxide films implies the significant effect of aluminum in reducing the oxidation resistance of magnesium. [ABSTRACT FROM AUTHOR]

As a potent nucleating substrate for α-Mg grain, MgAl2O4 powder was used to inoculate the Mg-Al melt in this study. The effects of MgAl2O4 amount, holding time and Al content on the grain size and grain refining ratio of the inoculated Mg-Al alloys are systematically investigated. The results show that the minimum grain size of Mg-3Al alloy is achieved by adding 2 wt.% MgAl2O4 powder and this alloy exhibits higher grain refining ratio than Mg-5Al and Mg-8Al alloys. The crystallographic misfit calculation indicates the well-matching and possible orientation relationships (ORs) between α-Mg and MgAl2O4. Among these predicted ORs, [10–10]α−Mg//[110]MgAl2O4 in (0002)α−Mg//(1–13)MgAl2O4 possesses the smallest misfit, i.e., 2.34% (fr). Both results of the experiment and crystallographic calculation demonstrate that the grain refinement of Mg-Al alloys is attributed to the MgAl2O4 particles acting as the heterogeneous nucleation substrates for α-Mg grains.

Master alloys containing Zirconium (Zr) serve as effective grain refiners in most commercial cast magnesium (Mg) alloys. However, their efficacy is diminished in Mg–Al series alloys due to the formation of Al–Zr compounds. In this work, a novel Mg-GNP@MgO master alloy was prepared utilizing the in-situ reaction between CO 2 and Mg. The grain refinement efficiency of this composite inoculation on Mg–9Al alloys was systematically investigated using SEM-EBSD, HRTEM observations, Density Functional Theory (DFT), and Sharp Interface Model (SIM) calculations. The findings suggest that the composite grain refiner (GNP@MgO) exerted a notable refining influence on Mg–9Al alloy. Through the conventional casting process, the average grain size was refined to approximately 13 μm upon the addition of 3 vol% GNP@MgO, achieving a grain refinement efficiency of 90 %. The enhanced refining efficiency arises from the synergistic control over the nucleation and growth processes of α-Mg grains, facilitated by GNP@MgO particles. This process includes improved heterogeneous nucleation, triggered by the newly formed Al 4 C 3 phases through the interaction between GNPs and Al element, followed by the limited expansion of α-Mg grains induced by the nano-sized MgO particles. The MgO particles predominantly gather around grain boundaries to establish stable nano-coating layers, thereby exerting a substantial Zener pinning effect. The pinning role played by the MgO nanoparticles was also verified by a sharp interface model. The findings in this work not only carve out a new avenue for the grain refinement of cast Mg–Al alloys but also inspire fresh perspectives for the development of novel grain refiners. [ABSTRACT FROM AUTHOR]

Grain refinement of magnesium (Mg) alloys has been a major research topic over the past decades as one of the effective approaches to increase their strength and ductility simultaneously. In this study, a brief review of the grain refinement of aluminum-bearing Mg alloys is included to provide an in-depth understanding of the detailed mechanisms of grain refinement of Mg alloys. Additionally, the effect of melt superheating on the grain refining of Mg–Al-based alloys has been investigated. It was confirmed that melt superheating caused a significant grain refining effect in the commercial purity (CP) of AZ91 alloy (0.25% Mn). Undercooling of 1.3 °C was observed before superheating and was noticeably reduced after the superheating process. A vacuum filtering experiment was conducted, which involves filtering the melts using fine metal porous filters to separate the particles in the melts. It was observed that a large amount of Al8Mn5 particles were generated in the commercial purity AZ91 alloy by the superheating process. However, because of the poor crystallographic matching between Al8Mn5 and Mg, Al8Mn5 was not considered the nucleation site for Mg grains. A master alloy containing ε-AlMn particles, which are in good crystallographic matching with Mg, was added, and it was found that the grain size of the commercial-grade AZ91 alloy was reduced. Therefore, it is suggested that Al8Mn5 particles, existing as a solid phase in the molten metal of the commercial AZ91 alloy could be transformed into ε-AlMn particles by the superheating process, and these particles can be effective nucleation sites for Mg grains. The transformation of Al8Mn5 into ε-AlMn is considered the main mechanism of grain refinement of the commercial purity of AZ91 alloy by superheating. Notably, the effect of grain refinement by superheating was not observed in the high-purity (HP) AZ91 alloy (0.006% Mn) because Al–Mn particles were likely not formed due to a very small quantity of manganese. [ABSTRACT FROM AUTHOR]

Mg-3%Al alloy was refined by carbon inoculation combining with 0.2%Ca addition. High grain refining efficiency was obtained and the synergistic refining mechanism was deeply discussed in the present study. Al–C–O particles, actually Al4C3 particles, were formed in the carbon-inoculated Mg-3%Al alloy acting as nuclei for α-Mg grains. Ca addition had no obvious effect on size distribution of the nucleating particles. Ca segregation was proved on Al4C3 particles, which should reduce the interfacial energy of nuclei/Mg. The constitutional undercooling in front of nucleus/liquid was increased from 0.12 °C to 0.15 °C induced by 0.2%Ca addition. The synergistic grain refining efficiency can be attributed to the higher constitutional undercooling and lower the interface energy of nucleus/Mg induced by Ca addition. More nucleating particles with small size could be activated acting as potent nuclei of α-Mg grains. Consequently, Mg-3%Al alloy could be effectively refined due to the synergistic effect induced by carbon inoculation combining with Ca.

As a potent nucleating substrate for α-Mg grain, MgAl 2 O 4 powder was used to inoculate the Mg-Al melt in this study. The effects of MgAl 2 O 4 amount, holding time and Al content on the grain size and grain refining ratio of the inoculated Mg-Al alloys are systematically investigated. The results show that the minimum grain size of Mg-3Al alloy is achieved by adding 2 wt.% MgAl 2 O 4 powder and this alloy exhibits higher grain refining ratio than Mg-5Al and Mg-8Al alloys. The crystallographic misfit calculation indicates the well-matching and possible orientation relationships (ORs) between α-Mg and MgAl 2 O 4. Among these predicted ORs, [10–10] α − Mg //[110] MgAl 2 O 4 in (0002) α − Mg //(1–13) MgAl 2 O 4 possesses the smallest misfit, i.e., 2.34% (f r). Both results of the experiment and crystallographic calculation demonstrate that the grain refinement of Mg-Al alloys is attributed to the MgAl 2 O 4 particles acting as the heterogeneous nucleation substrates for α-Mg grains. [ABSTRACT FROM AUTHOR]

Environmental gains of electric cars can be optimized with the use of lightweight and recyclable magnesium in the vehicle’s structural components. Ductility improvement of low-density Mg-Al alloys will extend their use in automotive body applications. The authors achieved 63% ductility improvement in Mg-6wt%Al with trace Y (1.5 ppm) due to the β-phase refinement and predicted that higher levels would not perform as well. As predicted, 0.3wt% of Y addition investigated in this study led to lower mechanical performance and β-phase refinement than those obtained with trace additions. The tensile ductility and yield strength increased by ~13% and 16%, respectively, and the compression strain to fracture by ~22%. Scanning electron and optical microscopy, X-Rays diffraction, mechanical testing and thermodynamic calculations were used to investigate the effect of 0.3wt% Y on the microstructure of Mg-6wt%Al. The matrix dissolution revealed the close association of the Al2Y and the β-Mg17Al12 phases.

Grain refinement of magnesium (Mg) alloys has been a major research topic over the past decades as one of the effective approaches to increase their strength and ductility simultaneously. In this study, a brief review of the grain refinement of aluminum-bearing Mg alloys is included to provide an in-depth understanding of the detailed mechanisms of grain refinement of Mg alloys. Additionally, the effect of melt superheating on the grain refining of Mg–Al-based alloys has been investigated. It was confirmed that melt superheating caused a significant grain refining effect in the commercial purity (CP) of AZ91 alloy (0.25% Mn). Undercooling of 1.3 °C was observed before superheating and was noticeably reduced after the superheating process. A vacuum filtering experiment was conducted, which involves filtering the melts using fine metal porous filters to separate the particles in the melts. It was observed that a large amount of Al8Mn5 particles were generated in the commercial purity AZ91 alloy by the superheating process. However, because of the poor crystallographic matching between Al8Mn5 and Mg, Al8Mn5 was not considered the nucleation site for Mg grains. A master alloy containing ε-AlMn particles, which are in good crystallographic matching with Mg, was added, and it was found that the grain size of the commercial-grade AZ91 alloy was reduced. Therefore, it is suggested that Al8Mn5 particles, existing as a solid phase in the molten metal of the commercial AZ91 alloy could be transformed into ε-AlMn particles by the superheating process, and these particles can be effective nucleation sites for Mg grains. The transformation of Al8Mn5 into ε-AlMn is considered the main mechanism of grain refinement of the commercial purity of AZ91 alloy by superheating. Notably, the effect of grain refinement by superheating was not observed in the high-purity (HP) AZ91 alloy (0.006% Mn) because Al–Mn particles were likely not formed due to a very small quantity of manganese.

Mg-3%Al alloy was refined by carbon inoculation combining with 0.2%Ca addition. High grain refining efficiency was obtained and the synergistic refining mechanism was deeply discussed in the present study. Al–C–O particles, actually Al 4 C 3 particles, were formed in the carbon-inoculated Mg-3%Al alloy acting as nuclei for α-Mg grains. Ca addition had no obvious effect on size distribution of the nucleating particles. Ca segregation was proved on Al 4 C 3 particles, which should reduce the interfacial energy of nuclei/Mg. The constitutional undercooling in front of nucleus/liquid was increased from 0.12°C to 0.15°C induced by 0.2%Ca addition. The synergistic grain refining efficiency can be attributed to the higher constitutional undercooling and lower the interface energy of nucleus/Mg induced by Ca addition. More nucleating particles with small size could be activated acting as potent nuclei of α-Mg grains. Consequently, Mg-3%Al alloy could be effectively refined due to the synergistic effect induced by carbon inoculation combining with Ca. [ABSTRACT FROM AUTHOR]

The discrete secondary phases usually cause severe galvanic corrosion, thereby resulting in rapid degradation for Mg–Al alloys in orthopaedics application. In this study, CaO was introduced into Mg–Al–Zn (AZ61) alloy via selective laser melting (SLM) to ameliorate the characterisations of the secondary phases, with an aim to improve its corrosion behaviour. Results revealed that CaO reacted with Mg and Al in Mg–Al alloys during SLM, suppressing the formation of coarse Mg17Al12 phase and promoting the formation of (Mg, Al)2Ca phase. Meanwhile, the rapid solidification during SLM promoted the homogeneous precipitation of the second phase. As a result, inert (Mg, Al)2Ca phase homogeneously wrapped the Mg grains, which effectively protected them from the invasion of corrosion solution. Thus, the degradation rate was remarkably reduced from 0.073 to 0.031 mg cm–2 h–1. Furthermore, AZ61-9CaO exhibited good cytocompatibility. This work suggested that AZ61-9CaO was promising candidates for orthopaedics implants.

The corrosion behavior of pure Mg 17 Al 12 and the effect of ball milling in presence of additives (i.e. graphite (G) and magnesium chloride (MgCl 2)) are evaluated in 3.5 wt% NaCl aqueous solution using electrochemical polarization and impedance measurements. Pure Mg 17 Al 12 and milled Mg 17 Al 12 without additives and with MgCl 2 present an open current potential (OCP) of −1.2 V/SCE while Mg 17 Al 12 + G shows a slightly higher OCP (+10% maximum). Mg 17 Al 12 corrodes with low kinetics and an increase of corrosion rate for the milled Mg 17 Al 12 is observed. The corrosion current densities (J corr) derived from the Tafel plots, exhibit their corrosion reactivity as follow: Mg 17 Al 12 < Mg 17 Al 12 5h < Mg 17 Al 12 + G 5h < Mg 17 Al 12 + MgCl 2 5h. Electrochemical impedance spectroscopy (EIS) results are in good agreement with the measured J corr. Randles circuit models are established for all samples to explain their surface behavior in the aqueous NaCl solution. The variation of the fitted parameters is attributed either to the effect of ball milling or to the effect of the additive. Our results are helpful in elucidating the effect of ball milling and the additives. • Mg 17 Al 12 corrodes in a 3.5 wt% aqueous solution of NaCl with a corrosion current density of 5.3 μA/cm2. • Mg 17 Al 12 can act as cathode in Mg–Al alloy with an OCP of −1.2 V/SCE. • Electrochemical impedance spectroscopy can explain the effect of ball milling additives. • Ball milling with MgCl 2 increases the corrosion current density by 22 times. [ABSTRACT FROM AUTHOR]

In most of the Mg–Al alloys, Al forms with Mg the intermetallic compound Mg 17 Al 12. In order to understand the hydrogen production from the Mg–Al alloys waste by the hydrolysis reaction in "model" seawater (i.e. 3.5 wt % NaCl), hydrolysis with Mg 17 Al 12 was investigated. The effect of ball milling time, the nature of the additives (graphite, NaCl, MgCl 2 and AlCl 3) and the synergetic effects of both graphite and AlCl 3 were investigated. It has been established that increasing ball milling time up to 5 h is necessary to activate the intermetallic and to decrease sufficiently its crystallites and particles size. On one hand, the presence of AlCl 3 provides the best hydrolysis performance (14% of the theoretical hydrogen volume in 1 h). On the other hand, the mixture obtained by simultaneous addition of graphite and AlCl 3 shows the best hydrolysis performances with 16% of the theoretical H 2 volume reached in 1 h. • Mg 17 Al 12 can produce hydrogen by the hydrolysis reaction. • Ball milling with graphite and AlCl 3 are the best additives to improve Mg 17 Al 12 hydrolysis. • Hydrolysis of Mg–Al alloys may be enhanced by ball milling with graphite and AlCl 3. [ABSTRACT FROM AUTHOR]

Models for grain size dependence of flow stress typically ignore contributions from geometrically necessary dislocations (GNDs) in the grain boundary regions. Ashby had proposed the following equation for flow stress (τ) as a function of grain size (d) and effective plastic strain (ε ¯): τ = τ 0 + C ′ G b ε ¯ d using an unknown empirical coefficient, C' , In this study, the coefficient, C' , and equation for the grain size dependence of flow stress were investigated in Mg-4Al using nanoindentation. C' ranges from 1.4 to 7.11 that is related to the hardness difference (∆H 0) between the adjacent grains as C ′ = 0.62 Δ H 0 . Ashby equation was used to predict uniaxial stress-strain curves in grain sizes of 55 µm, 187 µm, and 333 µm. The results showed lower flow stress and lower hardening rate compared to experimentally measured tensile stress-strain response. The discrepancy seems to be a result of hardening effect of SSDs. We propose a modified Ashby equation that takes into account the hardening effect of SSDs and GNDs separately, resulting in a better agreement between calculated and experimentally measured values. This analysis demonstrates estimation of uniaxial stress-strain response using nanoindentation measurements that captures effects of grain size, and strain hardening from SSDs and GNDs. The combination of the revised equation and nanoindentation method will enable rapid exploration of stress-strain response of a diverse range of single-phase polycrystalline alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

A high-throughput experimental approach is presented to extract the anisotropic interdiffusion coefficient by combining information over the composition profiles obtained by the electron probe microanalysis (EPMA) and the grain orientation spectrum by the electron backscatter diffraction (EBSD) on polycrystalline diffusion couples. Following the forward-simulation scheme, the interdiffusion coefficients in grains with diverse orientation are obtained and subsequently used to determine the anisotropic interdiffusion coefficients perpendicular ( D ⊥ (x)) and parallel ( D ∥ (x)) to the c axis of the hcp Mg lattice in a Mg–Al alloy as a function of the Al solute content at 673 K and 723 K, respectively. It was found that the interdiffusion coefficients generally increased with the Al content and the rotation angle with respect to the c axis with a valley point around θ ≈ 30 ° at 723 K. And it was noticed that diffusion along the basal plane was always faster than along the c axis. A comprehensive explicit expression of the interdiffusion coefficients was provided as a function of Al content, grain orientation and temperature. The anisotropic impurity diffusion coefficients of Al in hcp Mg derived by the extrapolation of the results in this paper are in good agreement with first principles calculations in the literature. Image 1 • High-throughput experimental approach extracts the anisotropic interdiffusion coefficient. • Coincide composition profiles by EPMA and grain orientation by EBSD in diffusion couple. • Anisotropic interdiffusion coefficients D ˜ were split into D ⊥ (x) and ( D ∥ (x)) for HCP Mg–Al alloys. • Explicit expression presents D ˜ as function of content, grain orientation and temperature. [ABSTRACT FROM AUTHOR]

Magnesium hydride (MgH 2) is a promising candidate as a hydrogen storage material. However, its hydrogenation kinetics and thermodynamic stability still have room for improvement. Alloying Mg with Al has been shown to reduce the heat of hydrogenation and improve air resistance, whereas graphite helps accelerating hydrogenation kinetics in pure Mg. In this study, the effects of simultaneous Al alloying and graphite addition on the kinetics and air-exposure resistance were investigated on the Mg 60 Al 40 system. The alloys were pulverized through high-energy ball milling (hereinafter HEBM). We tested different conditions of milling energy, added graphite contents, and air exposure times. Structural characterization was conducted via X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). H 2 absorption and desorption properties were obtained through volumetry in a Sieverts-type apparatus and Differential Scanning Calorimetry (DSC). The desorption activation energies were calculated using DSC curves through Kissinger analysis. Mg 60 Al 40 with 10 wt% graphite addition showed fast activation kinetics, even after 2 years of air exposure. Graphite addition provided a catalytic effect on ball-milled Mg–Al alloys by improving both absorption and desorption kinetics and lowering the activation energy for desorption from 189 kJ/mol to 134 kJ/mol. The fast kinetics, reduced heat of reaction, and improved air resistance of these materials make them interesting candidates for potential application in hydride-based hydrogen storage tanks. • 2-year air-exposed with added-graphite sample was as active as fresh one. • Air-exposure resistance of Mg–Al Alloys added with graphite is remarkable. • The catalytic effect of graphite was present also in the Mg–Al system. • Adding graphite improved activation kinetics, and eliminated incubation time. • Graphite addition prevented the formation of agglomerates. [ABSTRACT FROM AUTHOR]

Mg–Al alloys were inoculated by MgO powder. The effects of MgO addition amount, holding time after MgO addition and Al content on the grain refining efficiency were systematically investigated in the present study. The refining mechanism was discussed. MgO cannot serve as the effective substrate of primary α-Mg phase. Significant grain refinement of Mg–Al alloys with the Al content over than 2% could be obtained. Al is an indispensable element to ensure the grain refining efficiency of MgO addition on Mg alloys. The optimum MgO powder addition amount and the holding time were about 1.5% and 10 min, respectively. MgO could react with Al in Mg melt, resulting in the formation of MgAl2O4 particles. As the comparative experiments, the grain refinement on the pure Mg and Mg–3%Al alloy inoculated by MgAl2O4 powder was proven. Based on theoretical calculation and experimental results, the grain refinement of Mg–Al alloy inoculated by MgO powder is attributed to the newly formed MgAl2O4 particles, which could act as the effective heterogeneous nuclei of primary α-Mg phase. [ABSTRACT FROM AUTHOR]

The discrete secondary phases usually cause severe galvanic corrosion, thereby resulting in rapid degradation for Mg-Al alloys in orthopaedics application. In this study, CaO was introduced into Mg-Al-Zn (AZ61) alloy via selective laser melting (SLM) to ameliorate the characterisations of the secondary phases, with an aim to improve its corrosion behaviour. Results revealed that CaO reacted with Mg and Al in Mg-Al alloys during SLM, suppressing the formation of coarse Mg17Al12 phase and promoting the formation of (Mg, Al)2Ca phase. Meanwhile, the rapid solidification during SLM promoted the homogeneous precipitation of the second phase. As a result, inert (Mg, Al)2Ca phase homogeneously wrapped the Mg grains, which effectively protected them from the invasion of corrosion solution. Thus, the degradation rate was remarkably reduced from 0.073 to 0.031 mg cm-2 h-1. Furthermore, AZ61-9CaO exhibited good cytocompatibility. This work suggested that AZ61-9CaO was promising candidates for orthopaedics implants. [ABSTRACT FROM AUTHOR]

Compressive creep behavior of a Mg-Al alloy containing a small amount of Nd and Gd (Mg-6Al-1Nd-1.5Gd) was investigated at temperatures from 150 °C to 200 °C under a constant applied stress of 90 MPa, and its microstructure before and after creep testing was compared. Results showed that steady-state creep rate of the alloy was only 1.946 × 10 ^−8 /s at 150 °C, and was increased by four times and almost one order of magnitude at 175 °C and 200 °C, respectively. The microstructure of the alloy mainly consists of α -Mg, β -Mg _17 Al _12 phases, and Al _2 RE phases, which were distributed both in dendrites of α -Mg and at grain boundaries originally. After creep for 120 h, more Al _2 RE phases were aggregated at grain boundaries. The continuous β -Mg _17 Al _12 phase turned into dispersed dot-like or blocky particles. As the test temperature increased, the number of dislocation lines gradually increased due to the increase of creep strain. Meanwhile, dislocation tangle and dislocation pile-ups occurred near grain boundaries. However, obvious slip traces and slip lines appeared inside α -Mg dendrites at 175 °C and 200 °C, respectively, indicating that 〈c + a〉 non-basal slip system was activated, creep resistance decreased dramatically.

Cyclic loading-unloading tests in tension and compression were carried out on pure Mg and alloys with 0.4, 1.5 and 4.2 at% Gd, over a range of grain sizes, to quantify the solute and strain dependence of the materials anelasticity in the form of hysteresis loops. For a given grain size, the anelastic effect was more pronounced, i.e., the loops were wider, for pure Mg, and it decreased rapidly with the Gd concentration. The effect was larger for the finer grains in all materials, and in compression for the pure Mg and the 0.4Gd alloy. No difference between tension and compression was observed for the 1.5Gd alloy, whereas the loops were wider in tension than in compression for the 4.2Gd alloy. In comparison with existing studies in Mg-Al and Mg-Zn alloys, for a given solute concentration, Gd was more effective in reducing the magnitude of the effect than either Zn or Al, in that order. The overall behavior is discussed in terms of the hardening effects of short range order of the alloys on {10 1 ¯ 2} and {10 1 ¯ 1} twinning. [ABSTRACT FROM AUTHOR]

Magnesium based alloys containing γ Mg 17 Al 12 complex phases have been prepared by solidification. Owing to a methodology based on Scheil Gulliver calculation, a series of composites with controlled volume fraction of γ phase has been obtained. According to SEM and TEM observations, the microstructures of the Mg-γ composites consists in magnesium rich dendrites in various fraction embedded in a eutectic mixture formed by γ phase and Mg rich fibers. Preliminary investigations of the mechanical and electrical behaviors of the composites have been carried out. Microstructures and properties of the Mg-γMg 17 Al 12 in situ composite series are discussed with respect to in situ composites containing Bulk Metallic glasses. [ABSTRACT FROM AUTHOR]

The area fraction and number density of twins were determined in pre-polished tensile specimens of cast pure Mg, Mg-Al [0.5, 2 and 9 at.% Al] and Mg-Zn [0.8 and 2.3 at.% Zn] solid solutions, for different applied tensile strains. Profuse twinning developed rapidly in pure Mg, while it occurred very gradually in the Mg-Zn alloys, with Mg-Al showing an intermediate behavior. In the more dilute Mg-Al alloys the area fraction and the number density decreased with the concentration, increasing again for the 9 Al, reaching a level above that of pure Mg at large strains. In Mg-Zn both parameters decreased monotonically with the Zn concentration up to the solubility limit (2.5 at.%). For given alloy and solute content, the number density increased (the twins became smaller) for finer grain sizes in all three materials. It is argued that the availability of nucleating sites created by the microplasticity preceding profuse twinning, which is greater for the alloys due to the delayed onset of twinning, or for fine grained polycrystals due to the increased specific grain boundary area, accounts for the increased number density with the solute concentration or reduced grain size. Solute specific effects are rationalized using concepts of solid solution softening of the prismatic planes in all alloys, and the stronger solid solution hardening due to short range order in the concentrated Mg-Zn alloys. [ABSTRACT FROM AUTHOR]

The present study investigates the role of titanium in grain refinement of Mg-3Al and Mg-9Al alloys. Minor amount of Ti addition (<1 wt% of Al-Ti master alloy) brings about grain refinement in both Mg-3Al and Mg-9Al alloys. Ultrasonic treatment further improves the grain size reduction in Ti added Mg-Al alloys. The average grain size of Mg-Al alloys are reduced by 20-30 % from the non-refined alloy by the addition of 1 wt% of Al-Ti master alloy and ultrasonic treatment further enhances the grain refinement by 10-20 %. In addition, ultrasonic treatment transforms the network type β-MgAl phase into fine discontinuous precipitates in Ti containing Mg-9Al alloys. The mechanism of grain refinement in ultrasonic treated Ti containing Mg-Al alloy is discussed in terms of solute and heterogeneous nucleation perspective. [ABSTRACT FROM AUTHOR]

In this study, new erbium modified Mg–Al alloys were developed by integrating trace erbium (in the form of Al 94.67 Er 5.33 master alloy) into pure Mg using disintegrated melt deposition technique. The developed Er- modified Mg–Al alloys were investigated for their microstructural, mechanical and corrosion characteristics in comparison with their unmodified counterparts. Microstructural investigation revealed (i) improved purity, (ii) (marginal) grain refinement, (iii) more uniform second phase distribution and (iv) Al 3 Er phase formation due to Er modification. Mechanical property measurements revealed an overall enhancement under indentation, tension and compression loads. A remarkable improvement in tensile ductility (without adverse effects on strength) by +19%, +29%, and +58% was obtained in Mg–3Al–0.1Er, Mg–6Al–0.3Er and Mg–9Al–0.5Er when compared to Mg–3Al, Mg–6Al and Mg–9Al respectively. While the Mg–6Al–0.3Er alloy exhibited best ductility, the Mg–9Al–0.5Er has the best strength under both tension and compression loads. Corrosion characteristics evaluated by hydrogen evolution, salt spray and electrochemical impedance experiments revealed improved corrosion resistance of Er modified Mg–Al alloys by the enhanced purity levels and the formation of Al–Er phases. [ABSTRACT FROM AUTHOR]

A new mathematical model was developed to predict the average eutectic spacing under high pressure solidification. The model can be used in the binary alloy system which contains solid solution and intermetallic compound. Following this model, the change of eutectic spacing under high pressure can be predicted if the volumetric conditions of the phases are known. Application of the model to the binary Mg-20.3wt.%Al solidified under different pressure was conducted. It was found that the results predicted by this model show an excellent agreement with the eutectic spacing of Mg-20.3wt.%Al alloy solidified under high pressure. And during experiment, an ultimate pressure was found, the microstructure tends to grow as divorced eutectic when the solidified pressure is higher than this ultimate pressure, the formula is not applicable. [ABSTRACT FROM AUTHOR]

The present study investigates the grain refinement of Mg–3Al, Mg–6Al and Mg–9Al alloys by calcium addition. The maximum reduction in grain size has been observed at 0.2% Ca addition in Mg–Al alloys, in which any further addition (up to 0.4%) has marginal improvement in grain refinement. The mechanism associated with the grain refinement of Mg–Al alloys by Ca addition is discussed in terms of growth restriction factor ( Q ) and constitutional undercooling (Δ T CS ) using analytical model. The influence of growth restriction factor ( Q ) on the final grain size of Ca-added Mg–Al alloys are calculated with the help analytical model by assuming that the number of nucleant particles is not altered through Ca addition. For accurate grain size calculations, the value of Q has been estimated with reliable thermodynamic database using Scheil solidification simulation. The comparison of predicted and experimental grain size results indicate that constitutional undercooling activation of nucleation events plays dominant role in grain refinement in Mg–Al alloys by calcium addition, whereas the increase in growth restriction value has negligible effect. [ABSTRACT FROM AUTHOR]

Magnesium hydride (MgH 2) is one of the most promising materials for solid state hydrogen storage, but overly stable thermodynamics and sluggish kinetics of hydrogenation and dehydrogenation hinders its application. In this study, Mg 90 Al 10 was prepared by hydriding combustion synthesis (HCS), and then various Ti-based compounds (Ti, TiH 2 , TiO 2 and TiF 3) were introduced into Mg 90 Al 10 by ball milling. Mg 90 Al 10 + 10 wt% TiF 3 displays the most excellent dehydrogenation property compared with those doped with other Ti-based compounds. For instance, due to the addition of 10 wt% TiF 3 , the peak dehydrogenation temperature and the apparent dehydrogenation activation energy of the composite are 86 °C and 77.1 kJ/mol lower than those of Mg 90 Al 10 (364 °C and 155.95 kJ/mol). In addition, the catalytic effect of TiF 3 on the hydrogen storage properties of Mg 90 Al 10 has been investigated in detail. Except for the unreacted TiF 3 , Al 3 Ti and MgF 2 can be found during the first dehydrogenation process and remain stable in the subsequent de/hydrogenation cycles, which can act as the active sites to accelerate the hydrogen dissociation and recombination. Therefore, the excellent hydrogen storage properties of Mg 90 Al 10 + 10 wt% TiF 3 can be attributed to the catalytic effect of TiF 3 , in-situ formed Al 3 Ti and MgF 2. Our result is very significant to emphasize the practical application of the Mg-Al hydrogen storage alloys. • The hydrogen storage property of Mg 90 Al 10 doped with TiF 3 is remarkably improved. • The onset dehydrogenation temperature is reduced to 175 °C after TiF 3 addition. • The Mg 90 Al 10 -TiF 3 shows superior hydrogenation/dehydrogenation cycling stability. • TiF 3 , the in-situ formed Al 3 Ti and MgF 2 exhibit excellent catalytic effect. [ABSTRACT FROM AUTHOR]

First-principles calculations have been carried out to investigate the structural stabilities, electronic structures and elastic properties of MgAl, AlCa and AlSr phases. The optimized structural parameters are in good agreement with the experimental and other theoretical values. The calculated formation enthalpies and cohesive energies show that AlCa has the strongest alloying ability, and AlSr has the highest structural stability. The densities of states (DOS), Mulliken electronic populations, and electronic charge density difference are obtained to reveal the underlying mechanism of structural stability. The bulk modulus, shear modulus, Young's modulus, and Poisson's ratio are estimated from the calculated elastic constants. The mechanical properties of these phases are further analyzed and discussed. The Gibbs free energy and Debye temperature are also calculated and discussed. [ABSTRACT FROM AUTHOR]

Structural stabilities, thermodynamics stabilities, elastic properties and electronic structures of MgAl, AlY and AlBa phases were analyzed by first-principles calculations with Castep and Dmol3 program based on the density functional theory. The calculated results of heat of formation indicate that AlY phase has the strongest alloying ability. The calculated thermodynamic properties show that the thermal stability of these compounds gradually increases in the order of MgAl, AlY and AlBa phases. Y or Ba addition to the Mg-Al alloys could improve the heat resistance. The calculated bulk modulus B, shear modulus G, elastic modulus E and Poisson ratio ν show that the adding Y or Ba to Mg-Al alloys could promote the brittleness and stiffness, and reduce tenacity and plasticity by forming AlBa and AlY phases. The calculated cohesive energy and density of state (DOS) show that AlY has the strongest structural stability, then AlBa and finally MgAl. The calculated electronic structures show that AlY has the strongest structure stability because of the strong ionic bonds and covalent bonds combined action. [ABSTRACT FROM AUTHOR]

Abstract: Electronic structures, elastic properties and thermal stabilities of Mg17Al12, Mg2Si and Al2Y have been determined from first-principle calculations. The calculated heats of formation and cohesive energies show that Al2Y has the strongest alloying ability and structural stability. The brittle behavior and structural stability mechanism is also explained through the electronic structures of these intermetallic compounds. The elastic constants are calculated, the bulk moduli, shear moduli, Young''s moduli and Poisson ratio value are derived, the brittleness and plasticity of these phases are discussed. Gibbs free energy, Debye temperature and heat capacity are calculated and discussed. [Copyright &y& Elsevier]

We report initial results on high-temperature deformation of a complex metallic alloy, the γ-Mg17Al12 phase, isostructural with α-Mn. Although, like all complex metallic alloys, the γ-Mg17Al12 phase is brittle at room temperature, a high deformability accompanied by a strong softening was observed above 573K (∼2/3 melting point). Transmission electron microscopy investigation indicates that dislocation climb should be involved in the deformation mechanisms. Similarities with the deformation behaviour of quasicrystals and metallic glasses are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Significant grain refinement in Mg–3Al alloy is achieved with the addition of charcoal due to the formation of Al4C3 particles, which act as effective nuclei for magnesium grains. Addition of 0.5wt% charcoal has lead to reduced grain size of Mg–3Al alloy from 500 to 80μm and no substantial grain refinement is obtained on further addition of charcoal. The results further reveal that the prolonged holding of the melt after the addition of charcoal has not affected the grain refining efficiency of Al4C3. Steady increase in tensile properties observed with increasing amount of charcoal addition has been attributed to the grain refinement and the presence of fine Al4C3 particles. The strengthening mechanisms due to charcoal addition are discussed in terms of Hall–Petch relation and dispersion strengthening. The predicted values are in good agreement with experimental results. [Copyright &y& Elsevier]

Abstract: The evolution of microstructure during the pressure-composition-temperature (PCT) test at 350 °C for Mg-10at%Al and Mg-4at% alloy powders fabricated by electrodeposition as well as pure Mg powder were studied. The results of this study show that the starting microstructure of the Mg-10at%Al alloy powders consist mainly of the hcp-Mg phase. However, due to the rejection of Al by the MgH2 phase, a considerable amount of the Mg17Al12 phase forms upon the initial hydrogenation stage. Consequently, this powder exhibits three transformational steps with the final microstructure consisting of MgH2 and fcc-Al phases. The dehydrogenation takes place at relatively high pressure by formation of the Mg17Al12 phase at the MgH2/fcc-Al interface. The hydrogen released diffuses through the interconnected fcc-Al/Mg17Al12 network to the surface. In contrast, Mg-4at%Al and pure Mg powders release hydrogen at a low pressure by the nucleation of the hcp-Mg phase on the surface. [Copyright &y& Elsevier]

Abstract: A preliminary investigation has been carried to evaluate the influence of Al content on the microstructures and mechanical properties of Mg–Al alloys in as-cast condition. Three types of Mg–Al alloys were studied and characterized by Mg–10Al–Zn, Mg–20Al–Zn and Mg–44Al, respectively. The results indicate that the microstructures of all three alloys mainly consist of α-Mg solid solution, β-Mg17Al12 and the eutectic mixture (α-Mg+β-Mg17Al12). Increasing the Al content leads to the microstructures changes and increase the micro-hardness of the Mg–Al alloys. When the Al content increases to 44wt%, the main phase of the alloy is β-Mg17Al12 and the micro-hardness reaches to about 280 (HV). The reasons for these changes were also discussed in this paper. [Copyright &y& Elsevier]

Abstract: The corrosion of commercial die-cast Mg–Al alloys was elucidated by a study, of the corrosion in 3% NaCl, of (i) high-pressure die-cast (HPDC) model Mg–Al alloys, (ii) low-purity Mg, (iii) high-purity (HP) Mg and (iv) HP Mg heat treated at 550°C. HPDC is the most important route for the production of Mg components. The corrosion of the model alloys was dominated by the Fe impurity element. The present research identified the appearance of the Fe-rich particles in the microstructure. In high magnification (∼1000× to 5000×) secondary electron images, they appear as small white features, typically less than 1μm in diameter. In order to understand the impurity tolerance limits, (i) the appropriate corrosion literature was summarised and reviewed and (ii) Mg phase diagrams were calculated using the Pandat software package. Calculated phase diagrams can explain (i) the tolerance levels for Fe and Cu and (ii) the production of high-purity castings by means of control of melt conditions; this has high significance for the production of quality castings from recycled Mg. A full analysis requires that the Mg database be extended to include Ni, Co and some RE. The Fe tolerance limit is ∼5–10ppm for cast HP Mg heat treated at 550°C. Analysis of the Mg corrosion literature indicates that several studies have been dominated by the Fe impurity content and have not dealt with the stated aims; it means that the full chemical composition should be reported in all studies of the corrosion of Mg alloys. [Copyright &y& Elsevier]

Abstract: The CALPHAD method can be applied as a tool for both alloy development and process guideline determination. In this study, two Mg alloys were designed, their process parameters derived and, using the CALPHAD method, the final results simulated. These results were later confirmed using tangible experimental methods. It was found that - Mg17Al12 precipitates along the grain boundaries (GB), Mg2Sn forms both along the GB and as fine precipitates in the -Mg matrix and the addition of Ce mishmetal (MM) leads to the formation of elongated Al- rare earth (RE) precipitates along the GB. The microstructural stability at 200 ∘C is high, showing no decrease in microhardness for 32 days. It is shown that the CALPHAD method considerably reduces the effort of alloy design and that the reliability of the results is high. [Copyright &y& Elsevier]

In this study, carbon addition affects the formation mechanism and morphology of Al8(Mn,Fe)5, which serves as a nucleation site for α-Mg, conclusively leading to grain refinement of Mn-containing Mg–Al alloys. However, the grain size was decreased on slightly by carbon addition in Mn-free Mg–Al alloys, where Al4C3 particles were observed in the eutectic region. Therefore, it is suggested that Mn is necessary for considerable grain refinement of Mg–Al alloys, and that Al8(Mn,Fe)5 is an effective nucleant in Mn-containing Mg–Al alloys. [Copyright &y& Elsevier]