Mg17Al12 Phase Research Articles

Rheological Solidification Behavior and Mechanical Properties of AZ91-Sn Alloys

The solidification and tensile deformation behaviors of rheo-cast AZ91-Sn alloys were revealed to study the effects of Sn alloying on improvement of AZ91 alloy’s mechanical properties. Two kinds of Mg17Al12 phases precipitated from the supersaturated magnesium matrix during rheo-solidification were observed: coarse discontinuous precipitates (DP) at grain boundaries and small-sized continuous precipitates (CP) inside grains. With increasing Sn content, the amount of Mg17Al12 phases was increased whilst the amount of Al atoms in the matrix was decreased. Due to the higher melting point of Mg2Sn than Mg17Al12, Mg2Sn precipitated earlier from the melt, and therefore provided heterogeneous nuclei for Mg17Al12 during the eutectic reaction. Due to grain refinement and solid solution strengthening, AZ91-2.4Sn (mass%) gained 52% increase in tensile strength and 93% increase in elongation compared with pure AZ91 alloy. The higher-density twins and microcracks induced by Sn alloying relaxed stress concentration during plastic deformation, so the fracture mode was transformed from cleavage fracture of pure AZ91 alloy to ductile fracture of AZ91-Sn alloys.

The microstructure, mechanical and wear properties of AZ91-x%B4C metal matrix composites in as-cast and extruded conditions

In the present work, the AZ91 Mg alloy reinforced with different weight percentages of B4C (1 wt%, 3 wt%, 5 wt%, 10 wt% and 15 wt%) was investigated with the aim of improving its mechanical and wear resistance in the as-cast and extruded conditions. A simple and cost-effective method, stir casting process, was employed to fabricate light-weight AZ91-x%B4C composites. Microstructural characterization of the as-cast specimens revealed the reasonably uniform distribution of B4C particles, well-bonded B4C particles to the matrix and the presence of minimal porosity. It was revealed that B4C addition has a slight influence on the microstructure in as-cast state, whereas extrusion process had intense grain refinement and fracturing and dispersion of the Mg17Al12 phase resulted in the significant enhancement of compressive strength, hardness and wear properties. After the incorporation of B4C, a significant reduction in weight loss was observed, although the friction coefficient was increased. Scanning electron microscopic observation of the worn surfaces was conducted and the dominant wear mechanism was recognized as oxidation assisted abrasive for AZ91-xB4C composites. These results designate that the AZ91-xB4C composite can be considered as an outstanding material where high compressive strength and wear-resistant components are of great importance, primarily in the aerospace and automotive engineering parts.

The influence of Al content from filler metals on tungsten inert gas welding-brazing of Mg-Ti

Three types of Mg-based fillers with various Al-containing were applied for the tungsten inert gas (TIG) welding-brazing of MB3 alloy and Ti-6Al-4V alloy. Results reveal that coarse α-Mg grains accompanied with black precipitates of Mg17Al12 phase occurred in the fusion zone, whereas thicker TiAl3 phase was formed in the brazing zone with the increasing Al-containing from filler wire. The evident decline of micro hardness was observed in the joint fusion area, owing to the comprehensive effect of grain growth and Orowan hardening mechanism. With optimum welding current of 65A and AZ91 filler, the sound Mg/Ti joint with joining strength of 202 N mm−1 was obtained. Eventually, the formation of interfacial layer was analyzed to further reveal the joining mechanism of Mg/Ti joint.

Solidification process and hot tearing behaviors of AZ series magnesium alloys

In the present work, the solidification process and hot tearing behaviors of AZ series magnesium alloys were investigated. The solidification process was analyzed by differential thermal analysis (DTA), and the hot tearing behaviors were studied by a home-made T-shaped mold. The Clyne and Davis’ model was modified to suit for the prediction of the hot tearing susceptibility (HTS) of ternary alloys. The results showed that the predicted HTS using the modified CSC’ are consistent with the experimental tested hot crack volume for AZ series alloys. The HTS of the alloys are, CSC’ (AZ31) > CSC’ (AZ61) > CSC’ (AZ91). With the increasing of Al, the content of Mg17Al12 phase increased and the temperature of coherent point decreased. The Mg17Al12 phase significantly influenced the later stage refilling during the solidification process.

Effect of Ca addition on microstructure and impression creep behavior of cast AZ61 magnesium alloy

Abstract Microstructure and creep properties of AZ61 alloy containing 1 and 3 wt.% Ca were investigated. The creep properties were examined using impression method under different stresses between 200 and 500 MPa at the temperature ranging from 423 to 491 K. The microstructure of AZ61 alloy contains α(Mg) matrix and Mg17Al12 intermetallic phases. It is shown that adding Ca to AZ61 alloy reduces the amount of Mg17Al12 phase via forming (Mg, Al)2Ca phase; furthermore, increasing the Ca content to 3 wt.% leads to the formation of (Mg, Al)2Ca phase, as well as the elimination of the Mg17Al12 phase. Creep properties of AZ61 alloy are improved with the Ca addition. The improvement in creep properties is attributed to the reduction in the amount of Mg17Al12 phase and the formation of (Mg, Al)2Ca phase with high thermal stability. According to the obtained creep data, it is concluded that the pipe diffusion−climb controlled dislocation creep is the dominant creep mechanism and Ca addition has no influence on this mechanism. The effect of pre-deformation on the creep properties of AZ61+3%Ca alloy reveals that the creep resistance of the alloy depends on the continuity of (Mg, Al)2Ca phase. It is decreased by reducing the phase continuity.

Microstructure and Mechanical Properties of Recycled Al-7%Si-1%Fe Cast Alloy with Part Addition of Beverage Cans

The effect of un-modified addition of beverage cans material on the microstructure and mechanical properties of Al-7%Si-1%Fe alloy were investigated. Beverage can material included lids and bodies which were added into the liquid metal at 5 and 10 wt.% respectively. The microstructure of the beverage can material was found to consist of Al(Mn,Fe)3 and Mg2Si phase, with a larger phase fraction of Al(Mn,Fe)3 in the bodies compared to lids. The X-ray diffraction revealed that the lids and bodies contained Al, Si, Al(Mn,Fe)3 and Mg2Si phases. An Al3Mg2 phase unique to the lids was also found. The microstructure of un-modified Al-7%Si- 1%Fe reference alloy consisted of acicular silicon and Al5FeSi (β phase) in α-Al matrix. Addition of can lids at 5 and 10 wt.% decreased the β phase. Addition of can bodies at 5 and 10 wt.% also decreased the β phase, formed an Al15(MnFe)3Si2 (α phase) and also formed a Mg2Si phase especially with 10 wt.% addition. The addition of both lids and bodies modified the β phase similarly to the bodies only samples. A fibrous silicon morphology was found for all conditions. Finally, the ultimate tensile strength increased by 20-30% while the elongation increased with beverage can addition compared to the un-modified Al-7%Si-1%Fe reference cast alloy.

Microstructure and mechanical properties of multi-pass TIG welded joint of thick Al–Zn–Mg alloy plate

The microstructure and tensile mechanical properties of multi-layer multi-channel TIG welded joints of Al–Zn–Mg alloy plates were studied in this paper. The mechanical properties of the welded joint including microhardness and tensile performance were tested, respectively, while the microstructure was observed by scanning electron microscope, energy dispersive spectrometer, x-ray diffractor and transmission electron microscope. A short rod-shaped MgZn2 phase and a strip-shaped Al3Mg2 phase were found in the intergranular column of the weld seam, and an ellipsoidal MgZn2 phase was found in the intragranular column. The EDS test results of the micro-fracture and the fracture surface of the tensile specimen indicated that these particles could act as a strengthening phase or a crack source during the tensile fracture process. In addition, the distribution of microhardness in the weld zone was investigated.

Age hardening responses of as-extruded Mg-2.5Sn-1.5Ca alloys with a wide range of Al concentration

This article aims to explore the age hardening responses of both as-extruded and as-aged Mg-2.5Sn-1.5Ca-xAl alloys (x = 2.0, 4.0 and 9.0 wt%, termed TXA322, TXA324 and TXA329, respectively) through microstructural and mechanical characterization. Results indicate that grain size of as-extruded TXA322, TXA324 and TXA329 alloys were ∼ 16 μm, ∼ 10 μm and ∼ 12 μm, respectively. A number of <a> and <c+a> dislocations were observed in all the as-extruded samples. Guinier – Preston (GP) zones were evidently identified in TXA322 alloy, while only a small number of Mg17Al12 phases existed in both TXA324 and TXA329 alloys. An aging treatment facilitated the precipitation of a high number density of GP zones within the matrix of TXA322 alloy. In contrast, no obvious nano-precipitates were in as-aged TXA324 alloy. Numerous nano-Mg17Al12 phases were formed through a following aging treatment in TXA329 alloy. In terms of mechanical properties, it is apparent that an increment in ultimate tensile strength of ∼ 46 MPa and ∼ 40 MPa was yielded in peak-aged TXA322 and TXA329 alloys, while no obvious variations in UTS were present in peak-aged TXA324 alloy, in comparison with the as-extruded counterparts.

Microstructure evolution and creep behavior of high-pressure die-cast Mg–9Al–1Zn–1Sr alloy

The creep behavior of high-pressure die-cast Mg–9Al–1Zn–1Sr alloy was investigated in detail under temperatures ranging from 130 to 170 °C and stresses of 30–80 MPa. The studied alloy exhibited a high stress exponent of 8.4 and low activation energy of 102 kJ/mol. The obtained high stress exponent in studied alloy indicated the breakdown of the conventional power law. For rationalization, a threshold stress was introduced in the analysis, and then the high stress exponent was modified to be 5.2. Simultaneously, taking into account the significant influence of tested temperature on the minimum creep rate, a normalized stress exponent of 5 was achieved via the normalization method. Both methods suggested that the creep deformation of the studied alloy was mainly controlled by the dislocation climb. Moreover, transmission electron microscopy observations revealed that the origin of the threshold stress could be associated with the Orowan strengthening of γ-Mg17Al12 platelets that dynamically precipitated in Mg matrix during creep. Microstructure analysis results clarified that dislocation climb is the dominant creep mechanism of AZJ911 alloy, while the reticular eutectic Mg17Al12 phase disintegration, dynamical precipitation of γ-Mg17Al12, and twinning also influence the creep deformation of AJZ911 alloy.

Microstructure Evolution and Improved Creep Behavior of Mg-Al-Si Alloy Matrix Composite Reinforced with SiC Nanoparticles

The Mg-9Al-1Si matrix composite reinforced with SiC nanoparticles is fabricated by semisolid stirring-assisted ultrasonic vibration, which contains finer grains and a more uniform microstructure compared to that of the Mg-9Al-1Si alloys. The sizes of the Mg17Al12 and Mg2Si phases decreased continuously as the volume fraction of n-SiCp increased from 0 to 1% and then increased apparently when the volume fraction of n-SiCp increased to 1.5%. The steady-creep rates of the 1 vol.% n-SiCp/Mg-9Al-1Si are approximately two times lower than those of the Mg-9Al-1Si matrix alloy at 200 °C under applied stress of 70 MPa due to the strengthening effect of the nanosized SiCp. The reason for the improved creep properties is mainly attributed to the obvious grain boundary strengthening, the Orowan strengthening mechanism, the load transfer effect, and the pinning effect of the n-SiCp and the refined Mg17Al12 and Mg2Si phases.

The Effect of Variable-Frequency Ultrasonic Melting Treatment Technology on Microstructure and Properties of AZ80 Magnesium Alloy

The traditional fixed-frequency ultrasonic technology (FUT) and the variable-frequency ultrasonic technology (VUT) were used in direct-chill (DC) casting of an AZ80 magnesium alloy with the diameter of ∅255mm. The microstructure evolution, mechanical properties and macro-segment of Al element were investigated. The results show that both FUT and VUT can refine the α-Mg grain while the VUT has better efficiency which can reduce the grain size from 679-1454 μm (DC) to 150-241 μm. The homogeneity of grain size was also improved. These two technologies can also change the morphology and distribution of Mg17Al12 phase and tiny and refined morphology of Mg17Al12 phase and a homogeneous distribution can be obtained after VUT treatment. The YS and UTS were increased following the microstructure evolution, displaying the YS of 88.9-95.1 MPa and UTS of 164.3-173.1 MPa after VUT treatment. The segregation rate of Al element were improved with VUT treatment. Keywords: magnesium alloy; variable frequency ultrasonic technology; microstructure; mechanical properties;

Formation Investigation of Intermetallic Compounds of Thick Plate Al/Mg Alloys Joint by Friction Stir Welding.

5A06 Aluminum (Al) alloy and AZ31B magnesium (Mg) alloy with 20 mm thickness were successfully butt joined by friction stir welding. In order to control the composition of Al and Mg alloys along thickness direction, an inclined butt joint was designed in this study. The microstructure and phase identification at the interface of Al/Mg joints were examined using scanning electron microscopy with an energy-dispersive spectroscopy and Micro X-ray diffraction. The results indicated that there were two different formation mechanisms of intermetallic compounds at the interface of thick plate Al/Mg joint. The first was constitutional liquation, and eutectic structure consisting of Al12Mg17 and Mg solid solution existed at the top and upper-middle of the Mg side interface. The second was diffusion reaction, and the two sub-layers of Al12Mg17 and Al3Mg2 formed at the lower middle and bottom of the Mg side interface. In addition, the diffusion thickness values of Al12Mg17 and Al3Mg2 layers decreased gradually from the lower middle to bottom of the Mg side interface. As the position changes from the middle to the bottom near the Mg side interface, the diffusion coefficient of Al3Mg2 phase rapidly decreases from 3.14 × 10−12 m2/s to 6.9 × 10−13 m2/s and the diffusion coefficient of Al12Mg17 phase decreases from 6.8 × 10−13 m2/s to 1.5 × 10−13 m2/s.

An in-situ study on the diffusion growth of intermetallic compounds in the Al–Mg diffusion couple

Study on the diffusion growth of intermetallic compounds in the Al–Mg based light-weight metal materials is important due to its close interrelation with property. The current work aims at solving the existing problems of estimating interdiffusivities of intermetallic compounds, such as the early-stage effect on the accurate determination of parabolic growth constant and the simultaneous calculation of composition- and temperature-dependent interdiffusion coefficient. We present the first usage of High-Temperature Laser-Scanning Confocal Microscopy for effective in-situ observation of the time-dependent intermetallic compound growth in the Al–Mg diffusion couples. The method is verified by successfully recording the growth of Al3Mg2 and Al12Mg17 phases in diffusion couples annealed at 380 °C, 400 °C and 420 °C, and the accurate parabolic growth constant of intermetallic compound are estimated without early-stage effect. The experimental data are then subject to a numerical inverse methods to generate a set of self-consistent interdiffusivities, which can reproduce the observed diffusion growth behavior of intermetallic compounds in the Al–Mg diffusion couples. The applicability of current method to other alloy systems is also discussed.

Microstructures and mechanical properties of a high pressure die-cast Mg–4Al−4Gd−0.3Mn alloy

A new high pressure die-cast (HPDC) magnesium (Mg) alloy with high performance, namely Mg–4Al–4Gd–0.3Mn (AGd44), was developed by using the heavy rare earth element Gd. The microstructures under as-cast and peak-aged states were investigated in detail, especially by transmission electron microscopy (TEM). The studied alloy has finer cells although clearly less intermetallic phases than the conventional Mg–Al–RE based alloys. However, the structures of intermetallic phases in AGd44 alloys are fairly complicated. Most of the blocky phases are Al2Gd while minor of them are Al10Gd2Mn7 and Al8GdMn4, and the petaloid and the lamellae phases are Al2Gd and Mg17Al12, respectively. Peak-aging treatment can further enhance the yield strength of the studied alloy, which is mainly attributed to more coarse lamellae Mg17Al12 phase and fine lath-shaped Mg17Al12 precipitate appearing at cell boundaries and in α-Mg cells, respectively. Furthermore, the relatively smaller cells as well as the novel intermetallic phase component contribute to the more excellent strength-ductility balance of the studied alloy than the commercial/experimental HPDC Mg–Al-based alloys. The results provide new insight into developing HPDC Mg alloys with high mechanical properties.

Optimization of biocompatibility in a laser surface treated Mg-AZ31B alloy.

Biodegradable bone implants can remove the need for subsequent bone-implant surgeries by controlled biomineralization and degradation. Although Mg-alloys generally possess biocompatible properties, they corrode rapidly, thereby preventing sufficient hydroxyapatite formation and biomineral growth. In an attempt to address these limitations, laser surface treatments were performed via the employment of a continuous wave Nd:YAG laser on the Mg-AZ31B alloy using laser fluences in the range of 1.06-4.24 J/mm2 (250-1000 W). The laser-treated samples were investigated for their wettability in simulated body fluid. In vitro analyses were performed in simulated body fluid to examine corrosion and biomineralization behavior on the laser-treated samples. Statistical optimization algorithms based on wettability data predicted an optimal laser fluence of 3.286J/mm2 (775 W) within the range of laser fluences used in the present study for achieving a balance between biodegradation and biomineralization. Confirmatory tests on optimized samples indicated an up to 84% grain size reduction in laser-treated surface regions, a several-fold increase in Mg17Al12 (β) phase volume fraction, a reasonably abundant formation of hydroxyapatite, and increased rates of biomineralization that exceeded degradation. These findings indicate the potential of laser surface engineering to realize Mg-AZ31B alloy as a viable biodegradable bone implant material.

Effect of deep surface rolling on microstructure and properties of AZ91 magnesium alloy

Abstract A solution-treated AZ91 bulk material was deep-surface-rolled at room temperature to investigate the effect of deep surface rolling on the microstructure and mechanical properties of the alloy. Microhardness and microstructure along the depth of the treated surface layer were characterized. The results show that the affected layer was up to 2.0 mm thick and consisted of three sublayers: a severe deformation layer with thickness of about 400 μm from the topmost surface, a medium deformation layer with thickness of around 600 μm and a small deformation layer up to 1000 μm thick. In addition to grain refinement in the deformation layer, strain-induced precipitation of β phase (Mg17Al12) was observed, particularly in the severe and medium deformation layers. It is believed that the cooperative effects of grain refinement, strain hardening and precipitation strengthening led to the significant increase in hardness of the AZ91 alloy after the deep surface rolling.

Study on corrosion behaviour of AZ91 alloys at different aging time

The current work presents the corrosion behavior of AZ91 magnesium alloys in 0.01M NaCl. 2 different aging time samples were used in this study. It is found that sample with 2 hrs aging time shows a better corrosion resistance as compared to the 4 hrs aging time. It is also showing a significant small amount of hydrogen evolution reaction (HER) during the hydrogen gas collection examination. XRD indicates a few β- Mg17Al12 phase formed at 4 hrs aged samples. This is suggested that the formation of β phase in the alloy were accelerating the corrosion rate due to the galvanic corrosion mechanism.

Microscopic Analysis of Layers Containing Mg2Si and Mg17Al12 Phases Fabricated on AZ91 Through Thermochemical Treatment

Microscopic Analysis of Layers Containing Mg2Si and Mg17Al12 Phases Fabricated on AZ91 Through Thermochemical Treatment

Influence of Torsion on Precipitation and Hardening Effects during Aging of an Extruded AZ91 Alloy

The effect of torsional deformation by various routes prior to artificial aging treatment on the microstructure of extruded AZ91 rods was investigated. The tension and compression properties along the extruded direction were tested to evaluate the hardening effect. The results showed that torsional deformation can largely enhance the subsequent age-hardening effect. A detailed microstructural characterization revealed that torsional deformation produced a high density of dislocations and {10-12} twins, which can promote continuous precipitation and increase the number density of the Mg17Al12 phase. In addition, part of dislocations and the {10-12} twins were retained after aging treatment. The combination of the increased precipitation hardening by Mg17Al12, dislocation strengthening and refinement strengthening contributed to the high hardening effect. Moreover, the hardening effect can be further increased by tailoring the torsion route. The torsion route exhibited little influence on the age-hardening effect, while it remarkably affected the texture of the twisted AZ91 rods. Reciprocating torsion had little influence on the texture. In contrast, unidirectional torsion caused the c-axis of the texture to rotate toward the extrusion direction, resulting in a texture softening effect. Therefore, combining reciprocating torsion and aging caused a higher hardening effect than combining unidirectional torsion and aging. The detailed strengthening mechanism was analyzed and is discussed.

Effect of Cooling Rate on Microstructure and Mechanical Properties of Sand-Casted Al–5.0Mg–0.6Mn–0.25Ce Alloy

This study examines the relationship among cooling rate, microstructure and mechanical properties of a sand-casted Al–5.0Mg–0.6Mn–0.25Ce (wt%) alloy subjected to T4 heat treatment (430 °C × 12 h + natural aging for 5 days), and the tested alloys with wall thickness varying from 5 to 50 mm were prepared. The results show that as the cooling rate increases from 0.22 to 7.65 K/s, the average secondary dendritic arm spacing (SDAS, λ2) decreases from 94.8 to 27.3 μm. The relation between SDAS and cooling rate can be expressed by an equation: $$\lambda_{2} = 53.0R_{\text{c}}^{ - 0.345}$$. Additionally, an increase in cooling rate was shown not only to reduce the amount of the secondary phases, but also to promote the transition from Al10Mn2Ce to α-Al24(Mn,Fe)6Si2 phase. Tensile tests show that as the cooling rate increases from 0.22 to 7.65 K/s, the ultimate tensile strength (UTS) increases from 146.3 to 241.0 MPa and the elongation (EL) increases sharply from 4.4 to 12.2% for the as-cast alloys. Relations of UTS and EL with SDAS were determined, and both the UTS and EL increase linearly with (1/λ2)0.5 and that these changes can be explained by strengthening mechanisms. Most eutectic Al3Mg2 phases were dissolved during T4 treatment, which in turn further improve the YS, UTS and EL. However, the increment percent of YS, UTS and EL is affected by the cooling rate.