Mg17Al12 Phase Research Articles

An elastoplastic phase-field study of the precipitation behaviors of Mg17Al12 phase in Mg-Al-based alloys: Part II. Precipitation under various loadings

An elastoplastic phase-field model, developed in Part I, was employed to study stress-assisted diffusional phase transformation, taking plastic deformation into consideration. Different loading conditions were applied to a single crystal of Mg-Al-based alloys during precipitation, aiming at investigating the effects of elastoplastic deformation associated with both phase transformation and external loadings. The simulation results show that even when external loadings are below the materials’ yield limits, plastic deformation may occur due to stress accumulation triggered by the volume expansion of β-Mg17Al12 precipitates and the lattice mismatch between the precipitate and the matrix. Under both axial and shear loading conditions, plasticity plays a significant role in the morphological evolution of β-Mg17Al12 precipitate with Burgers orientation. In contrast to the lath morphology observed in the absence of external loadings, a rhombic-like precipitate is formed when applying an external tensile loading of 30 MPa along [112̅0]α direction, while a mirror-symmetrical-parallelogram-like morphology emerges under a shear loading of + 30 MPa. Additionally, plasticity plays a similar dual role during precipitation under different loading conditions, which would promote the growth of the precipitate at the initial stage but tend to retard it afterward. These findings illustrate that the plastic strain generated under external loadings has a great impact on the precipitate microstructure, which provides guidelines for designing the precipitate microstructure to obtain better mechanical properties.

Hybrid calphad DFT modelling of the Mg–Al–Ca system

The Mg–Al–Ca system is an important system for Mg alloys. Al is the most common alloying element for Mg alloys, and additions of Ca can be used to produce ductile alloys and alloys with good creep properties. The Mg–Al–Ca system has been investigated extensively in the past, in particular the Mg-rich corner. Also, calculations using density functional theory (DFT) have been extensively applied in the past. DFT calculations generally show high accuracy and consistency in this system. The Mg–Al–Ca system contains all three Laves phases (C14, C15 and C36), but existing thermodynamic models of this system only contain rather simplified and limited models for the Laves phases. In this work, DFT calculations are used to provide energies of formation for all possible end-members of the Laves phases and the Mg17Al12 (A12) phase. This is combined with a thermodynamic modelling of the system to provide a thermodynamic description of the system where the Laves phases and the Mg17Al12 phase have reasonable Gibbs energies for the complete composition space. The new model description also provides a good representation of most existing experimental data, in particular in the important Mg-rich corner.

Synthesis and characterization of silicon dioxide reinforced AZ91D magnesium composites

In the present investigation silicon dioxide of varying weight proportion was added in to AZ91D magnesium alloy matrix and its microstructure and mechanical properties was characterized for functional applications. Optical microscope is used to analyzed its microstructure studies and its performance measures was characterized using ASTM standards. From the experimentation it was observed that it has good wettability between the matrix and reinforcement and also it was found that the ceramic strengthening particulates are uniformly distributed through the cross section of matrix alloy. The increase in hardness and tensile strength can be attributed to the presence of hard reinforcement in a AZ91D alloy matrix, which absorbs the load and restricts dislocation movement due to the difference in thermal expansion coefficients between matrix and reinforcing particulates. On the other hand, the increase in percentage of reinforcement in matrix minimizes the ductility and reducing the percentage of elongation between the intermixture The compressive strength increased due to uniform dispersion of silicon dioxide on magnesium alloy and the presence of interface reaction of Mg2Si between the immixture. Owing to the inclusion of silicon dioxide that the formation of Mg17Al12 phase reduced and hence improvement in corrosion resistance of synthesized composites was observed.

Improvement of Hot Tearing Resistance of AZ91 Alloy with the Addition of Trace Ca.

Hot tearing is the most common and serious casting defect that restricts the light weight and integration of magnesium alloy components. In the present study, trace Ca (0-1.0 wt.%) was added to improve the resistance of AZ91 alloy to hot tearing. The hot tearing susceptivity (HTS) of alloys was experimentally measured by a constraint rod casting method. The results indicate that the HTS presents a ν-shaped tendency with the increase in Ca content, and reaches its minimum value in AZ91-0.1Ca alloy. Ca is well dissolved into α-Mg matrix and Mg17Al12 phase at an addition not exceeding 0.1 wt.%. The solid-solution behavior of Ca increases eutectic content and its corresponding liquid film thickness, improves the strength of dendrites at high temperature, and thereby promotes the hot tearing resistance of the alloy. Al2Ca phases appear and aggregate at dendrite boundaries with further increases in Ca above 0.1 wt.%. The coarsened Al2Ca phase hinders the feeding channel and causes stress concentration during the solidification shrinkage, thereby deteriorating the hot tearing resistance of the alloy. These findings were further verified by fracture morphology observations and microscopic strain analysis near the fracture surface based on kernel average misorientation (KAM).

Enriching the microstructure of AZ91D alloy by nano MoO3 composites

In the present study, a vacuum-assisted stir-casting process was made to synthesize nano-molybdenum trioxide-strengthened magnesium alloy composites of varying weight proportions. Morphological analysis was used to study the uniform distribution of ceramic reinforcing particles in the synthesized magnesium nanocomposites. ASTM standards were used to examine the mechanical properties of the intermixture. It was observed that the nano-MoO3 ceramics were uniformly distributed within the matrix alloy, no residual porosity was observed, and the grain size of the intermixture was observed as 59.82 + /0.001 nm and due to enhanced grain refinement and interfacial reaction between the intermixtures, the mechanical properties of nanocomposites, such as tensile, compression, flexural and impact strengths, significantly increased over the as-cast magnesium alloy. The corrosion resistance was improved due to the reduced size of the Mg17Al12 phase and the more uniform dissemination of ceramic strengthening particulates. Further, the improved load-bearing capacity and transfer layer enriched the wear resistance of magnesium alloy nanocomposites for all weight proportions.

Influence of titanium diboride reinforced magnesium composites using squeeze casting method

In the current investigation, squeeze casting was used to create titanium diboride-reinforced magnesium composites in a varying weight proportion (2 wt%,4 wt% and 6 wt%). SEM was used for the morphological analysis, and ASTM standards were followed in creating the performance metrics. The distribution of the ceramic reinforcing particles inside the magnesium matrix was observed to be uniform. Due to a powerful interfacial response between the intermixtures, the hardness (24.52%), tensile strength (18.52%), and wear resistance (28.52%) of synthetic magnesium composites dramatically enhanced. The corrosion resistance of magnesium composites was assessed using a salt spray test, and it was found that by reducing the size of the -phase Mg17Al12, the corrosion resistance greatly increased (36.52%) when compared to that of the cast alloy.

Relationship between microstructure and mechanical properties of as-extruded and peak-aged Mg-6Al-3Sn-2Zn alloys

The microstructure and mechanical properties of as-extruded and as-aged Mg–6Al–3Sn–2Zn (ATZ632) alloys were investigated. The as-extruded alloy shows excellent mechanical properties, achieving the yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) of 216.4 MPa, 344.3 MPa, and 18.4%, respectively. After aging treatment, the volume fraction of the precipitated Mg17Al12 phase increases and the Mg4Zn7 phase appears. The YS and UTS of the as-aged ATZ632 alloy increased to 252.5 and 416.2 MPa, respectively; however, the EL decreased to 10.1%. The high strength of the as-aged alloys is attributed to the precipitated Mg4Zn7 phase with an orientation perpendicular to the (0001) plane of α-Mg and the Mg17Al12 phase with an orientation parallel to the basal plane. Furthermore, the precipitation after aging decreases the EL. Static recrystallization grains appear during low-temperature aging at 150 °C, and their orientation shows a high degree of consistency with the c-axis perpendicular to the extrusion direction.

Damping performance of SiC nanoparticles reinforced magnesium matrix composites processed by cyclic extrusion and compression

This work dealt with the damping performance and its underlying mechanism in SiC nanoparticles reinforced AZ91D composite (SiCnp/AZ91D) processed by cyclic extrusion and compression (CEC). It was found that the CEC process significantly affects the damping performance of the composite due to alterations in the density of dislocations and grain boundaries in the matrix alloy. Although there would be dynamic precipitation of the Mg17Al12 phase during processing which increases the phase interface and limits the mobility of dislocations and grain boundaries. The results also showed that the damping capacity of 1%SiCnp/AZ91D composite continuously decreases with adding CEC pass number and it consistently increases with rising the applied temperature. Considering the first derivative of the tanδ–T curve, the dominant damping mechanism based on test temperature can be divided into three regions. These three regions are as follows (i) dislocation vibration of the weak pinning points (≤Tcr), (ii) dislocation vibration of the strong pinning points (Tcr∼TV), and (iii) grain boundary/interface sliding (≥TV).

Effects of RE (RE = Sc, Y and Nd) concentration on galvanic corrosion of Mg-Al alloy: a theoretical insight from work function and surface energy

Alloying rare earth (RE) elements is an effective method to improve corrosion resistance of Mg-Al alloy but excessive RE concentration may aggravate galvanic corrosion. In this study, density functional theory calculations are performed to unravel effects of RE (RE = Sc, Y, Nd) concentration on the galvanic corrosion of Mg-Al alloy from the perspectives of work function and surface energy. The results showed that, when the RE concentration was low, the galvanic corrosion between Mg matrix and Mg17Al12 phase could be alleviated with the increasing of RE concentration. When the RE concentration was high, the precipitated RE phases became cathodes and strongly aggravated galvanic corrosion of Mg matrix. Furthermore, regardless of the change of RE concentration, the effectiveness of alleviating galvanic corrosion in Mg-Al alloy was in the order: Sc > Y > Nd. Possible explanations to experiments were provided and potential strategy to improve corrosion resistance of Mg-Al alloy was also proposed.

An elastoplastic phase-field study of the precipitation behaviors of Mg17Al12 phase in Mg-Al-Based alloys: Part I. Modeling and the role of plasticity

An elastoplastic phase-field model for typical diffusional transformations was developed, where plastic deformation was considered through a modified time-dependent Ginzburg-Landau dynamic equation. The model was applied to investigate the precipitation in Mg-Al-based alloys, with a focus on studying the effects of elastoplastic deformation associated with phase transformation on the morphology and growth kinetics of β-Mg17Al12 precipitates (with Burgers orientation). The elastoplastic modeling results show that the precipitates adopt a lath-shaped morphology with lozenge ends, consistent with experimental observations. Although the plastic strain accumulated in the transformation front has a moderate impact on the morphology of the precipitate, it exhibits a clear dual effect on the growth of the precipitate along [112̅0]α. Specifically, plasticity promotes the growth of the precipitate during the initial stage, owing to its stress relaxation effect, and induces a solute poor zone of atomic Al with a greater concentration gradient at the transformation front. However, the plastic strain tends to retard the growth of the precipitate afterward due to its limited ability to release the increasing compressive strain between the two phases. These results shed light on understanding the role of plasticity in general diffusional transformations and provide guidelines for tailoring the microstructure of alloys.

Corrosion behavior and discharge properties of Al-doped Mg–Sn alloys for high performance Mg-air batteries

In this study, we report a novel, non-rare earth Mg alloy-based anode for Mg air battery applications. The Mg-xAl-1Sn (x = 0, 3, 6, 9) alloy series were synthesized and subjected to extruding. The microstructural analyses indicated that the as-extruded Mg–0Al–1Sn(AT01) alloy showed coarse dynamically recrystallized (DRXed) grains. The Al-doped alloys showed uniform DRXed grain distribution, and their grain size decreased with increasing Al content. Meanwhile, the solid solubility of Sn changed with the addition of Al; as a result, the amount of Sn dissolved in the matrix was reduced. This promoted the precipitation of the Mg2Sn phase, and the precipitate content increased significantly in the Al-doped alloys. Furthermore, different corrosion behaviors were observed in the Al-doped alloys. Numerous discontinuous Mg17Al12 phases and strong cathode Mg2Sn phases induced rapid galvanic corrosion with the Mg matrix, which caused the latter one to be rapidly dissolved. However, the refined grains and discontinuous precipitates that existed in as-extruded Al-doped alloys were beneficial to the initiation of cracks within the alloy surface, making the self-peeling of discharge products easier. The AT91 alloy-based anode exhibited good discharge stability during long-term cycling at all applied current densities.

Work Function, Sputtering Yield and Microhardness of an Al-Mg Metal-Matrix Nanostructured Composite Obtained with High-Pressure Torsion

Severe plastic deformation has proven to be a promising method for the in situ manufacturing of metal-matrix composites with improved properties. Recent investigations have revealed a severe mixing of elements, as well as the formation of non-equilibrium intermetallic phases, which are known to affect physical and mechanical properties. In this work, a multilayered aluminum–magnesium (Al-Mg) nanostructured composite was fabricated using constrained high-pressure torsion (HPT) in a Bridgeman-anvil-type unit. A microstructure investigation and X-ray diffraction analysis allowed us to identify the presence of intermetallic Al3Mg2 and Al12Mg17 phases in the deformed nanostructured composite. The sputtering yield of the Al3Mg2 and Al12Mg17 phases was found to be 2.2 atom/ion and 1.9 at/ion, respectively, which is lower than that of Mg (2.6 at/ion). According to density functional theory (DFT)-based calculations, this is due to the higher surface-binding energy of the intermetallic phases (3.90–4.02 eV with the Al atom removed and 1.53–1.71 eV with the Mg atom removed) compared with pure Al (3.40–3.84 eV) and Mg (1.56–1.57 eV). In addition, DFT calculations were utilized to calculate the work functions (WFs) of pure Al and Mg and the intermetallic Al3Mg2 and Al12Mg17 phases. The WF of the obtained Al-Mg nanostructured composite was found to be 4 eV, which is between the WF value of Al (4.3 eV) and Mg (3.6 eV). The WF of the Al12Mg17 phase was found to be in a range of 3.63–3.75 eV. These results are in close agreement with the experimentally measured WF of the metal matrix composite (MMC). Therefore, an intermetallic alloy based on Al12Mg17 is proposed as a promising cathode material for various gas-discharge devices, while an intermetallic alloy based on Al3Mg2 is suggested as a promising optical- and acoustic-absorbing material.

Microstructure and properties of an AZ61 alloy after extrusion with a forward-backward oscillating die without preheating of the initial billet

We have investigated the microstructure, mechanical and corrosion properties of an AZ61 alloy that was extruded using a newly developed technique with an oscillating die (KoBo). The KoBo method allows the extrusion of AZ61 without preheating of the initial billet at high deformation ratios. The combined SEM, EBSD and TEM investigations of the microstructure revealed significant microstructure refinement as well as changes to the intensity of the texture and the distribution of the Mg17Al12 phase. The size of grains was reduced from coarse (davg 20.4 µm for the initial billet) to fine (davg 6.6 µm for the extrusion ratio of R1 7:1 and davg 4.5 µm for R2 10:1). However, in this study, it does not improve the strength and the corrosion properties of the AZ61 alloys. The continuously precipitated Mg17Al12 phase along the grain boundaries overwhelms the strengthening due to grain refinement. Intense corrosion occurs in the case of the KoBo-extruded samples, and the main mechanism of the corrosion is microgalvanic, taking place between the matrix and the Mg17Al12 formed at grain boundaries.

Effect of P-MIG and CMT on microstructure and properties of 6082-T6 joints

This paper focuses on pulse metal inert gas welding (P-MIG) and cold metal transfer (CMT) of 6082-T6 aluminium alloy. The microstructure and properties of joints were analysed and the phase transformation of the welding zones (WZ) was calculated using JMatPro. The results showed that CMT joints are better than P-MIG joints in tensile strength and hardness. The main reason is that the average grain size and the second phase size of CMT joints are smaller than those of P-MIG joints. Moreover, the larger number of large-angle grain boundaries (LAGB), diffusely distributed Al3Mg2 and Al6Mn phases in CMT WZ, as well as a small amount of distributed Al3Zr, are beneficial to the joints properties too.

The corrosion resistance and discharge performance of as-extruded AZ91 alloy synergistically improved by the addition of submicron SiCp

In order to reveal the influence of submicron SiCp on the corrosion and discharge behaviors of extruded AZ91 alloy, the extruded AZ91 alloy and 0.5 μm SiCp (0.5 vol.%)/AZ91 composite were subjected to immersion, electrochemical, and Mg-air battery tests. The results show that the introduction of submicron SiCp not only reduces the weightless corrosion rate of the extruded AZ91 alloy from 3.09 mm/y to 2.18 mm/y, but also increases its discharge voltage and stability at current densities ranging from 2.5 mA cm−2 to 40 mA cm−2. The improvement of corrosion resistance and discharge activity are attributed to the refined grains and precipitates, as well as the weakened basal texture owing to the addition of the minor submicron SiCp. Extruded AZ91 alloy has a coarse and unevenly distributed Mg17Al12 phase and strong basal texture. The coarse phase and more grains with basal texture as cathode lead to the local corrosion of magnesium in the corrosion process and the local dissolution of magnesium in the discharge process. After adding 0.5 vol.% submicron SiCp, the Mg17Al12 phase is fined and evenly distributed, and the basal texture is weakened, which not only weakens the galvanic corrosion but also promotes the uniform corrosion of magnesium. As a result, the corrosion resistance is improved. In addition, the discharge activity benefits from refined grains and the easily dissolving discharge product resulting from uniform dissolution of magnesium. This work provides new ideas for achieving the synergistic improvement of corrosion resistance and discharge performance of magnesium alloy.

Effect of Y2O3 Addition on Microstructure and Properties of Laser Cladded Al-Si Coatings on AZ91D Magnesium Alloy

The effect of Y2O3 addition on the microstructure and properties of the laser cladded Al-Si alloy coating on the surface of AZ91D magnesium alloy was investigated in this study. The experimental results showed that the Al-Si + Y2O3 cladding layers contained α-Mg, Mg2Si, Al4MgY and a small amount of Al12Mg17 phases. The coarse dendrites, reticulated eutectic structures and massive phases in the coatings tended to be refined and gradually uniformly distributed with the increased amount of Y2O3. The introduction of Y2O3 into the cladding layer favored the improvement of microhardness and wear resistance due to the grain refinement strengthening and dispersion strengthening. The addition of Y2O3 also promoted the reduction of localized corrosion sites and made the corrosion surface smoother, implying that the corrosion resistance of the Y2O3-modified coatings was better than that of the unmodified cladding layer.

Influence of ultrasonic shot peening on the microstructure and impression creep performance of squeeze-cast AZ91 alloy reinforced with graphene nanoplatelets

The present investigation explores the influence of graphene nanoplatelets (GNPs) (0.5, 1.0, and 2.0 wt %) addition as well as ultrasonic shot peening (USP) for 10, 20, and 30 s on the microstructure, hardness, and impression creep response of AZ91 alloy fabricated by squeeze-casting for transmission housing applications. The as-cast AZ91-xGNPs nanocomposites displays lower grain size, lesser Mg17Al12 phase fraction, and higher dislocation density. The USP technique induces compressive residual stress (CRS) and high strain hardening at the surface followed by high dislocation density that increases with time. Both surface roughness as well as the dimple size and thickness after USP treatment are lower for the nanocomposites than for the AZ91. The hardness and creep resistance of the as-cast AZ91-xGNPs nanocomposites are superior to the monolithic alloy, which further improved after USP treatment. In contrast to the base alloy, the GNC3 nanocomposite displays the best hardness and impression creep resistance both in the as-cast and USP-treated condition. Dislocation climb regulated by pipe diffusion is the dominating creep mechanism in the as-cast and USP-treated materials. The improvement of impression creep performance for as-cast nanocomposites is attributed to the reduced Mg17Al12 phase content, high dislocation density, and strengthening from the GNPs. In contrast, the superior creep performance of USP-treated nanocomposites is attributed to the presence of CRS along with high dislocation density and work hardening on the surface.

Optimizing the microstructures and enhancing the mechanical properties of AZ81 alloy by adding TC4 particles

In this work, AZ81 alloys reinforced with 3 wt%, 6 wt% and 9 wt% TC4 (Ti–6Al–4V) particles were fabricated using two-step stirring. The tensile results revealed that the TC4 particles could significantly improve the yield stress (YS), ultimate tensile stress (UTS), and elongation (El.) of AZ81 alloys. By adding TC4, the grains showed refined/coarse bimodal morphologies in the composites. The formation of β (Mg17Al12) phase was notably suppressed due to the interfacial reaction between TC4 and Al. The dynamic recrystallization (DRX) behavior was also restricted by TC4 particles and β phase, ascribing to the refined grains and Orowan strengthening. The good interfacial bonding was achieved at the interface of TC4/α-Mg in 6TC4/AZ81 composite. Compared to the AZ81, 6TC4/AZ81 showed the highest YS, UTS and El. of 241 MPa, 357 MPa, and 17.0%, respectively. Its increased strength was primarily attributed to the mismatch coefficient of thermal expansion (CTE) and grain boundary strengthening. The improved ductility was ascribed to the TC4 particles, which are more deformable to withstand external load.

Processing, microstructure, and mechanical behavior of AZ31 magnesium alloy fabricated by electron beam additive manufacturing

In this work, AZ31 magnesium alloy was successfully manufactured by the electron beam additive manufacturing (EBAM) technique. The microstructure and mechanical properties of the EBAM samples under different energy densities and deposition passes were analyzed. The results show that the energy density has a decisive effect on the manufacturability of the sample, wherein the sample prepared at an energy density of 1.432 × 1010 J·m-3 possesses the best manufacturability. The thermal cycling of the EBAM technique resulted in apparent grain size variation and layered arrangement along the deposition height. The fine grains with an average size of approximately 20 µm account for 30% of the sample while the remaining coarse grains possess an average size of approximately 40 µm. Moreover, a large number of fine Al8Mn5 and Mg17Al12 phases precipitated due to the high cooling rate of the molten pool. The microhardness of the sample first decreased and then increased slightly with increasing deposition height. Under the combined effects of dispersion strengthening and grain refinement, the EBAM specimens demonstrated excellent tensile strength and elongation of 230 MPa and 13.5%, respectively, which are superior to those of the die-cast AZ31 alloy.

Microstructure, mechanical properties and corrosion resistance of friction stir welded joint of Al–Mg–Mn–Zr–Er alloy

Abstract The microstructure, mechanical properties and corrosion resistance of friction stir welded joint of Al–Mg–Mn–Zr–Er alloy were investigated under three FSW parameter sets: 800 rpm × 100 mm min−1, 800 rpm × 80 mm min−1 and 600 rpm × 80 mm min−1. The microstructural transformation sequence of welded joint is discontinuous dynamic recrystallization, dislocation introduction, dynamic recovery and continuous dynamic recrystallization. The mechanical properties of welded joint are highly correlated with the welding process parameters. The welded joint can obtain good tensile strength and elongation under the parameters of 800 rpm × 100 mm min−1. Corrosion resistance of welded joints has little correlation with welding parameters, but is mainly related to different positions of welded joints. The main influencing factor is grain structure state. The intergranular corrosion susceptibility is related to β(Al3Mg2) phase.