β-Mg17Al12 Phase Research Articles

Design of novel Al-Mg-(Zn-Sc) alloys with enhanced mechanical properties and corrosion resistance

The novel Al-Mg-(Zn-Sc) alloys were fabricated using vacuum induction melting and argon-protected casting method. The effects of Zn and Sc on the microstructure evolution, mechanical properties and corrosion properties of the as-cast Al-Mg-(Zn-Sc) alloys were comparatively studied. The results show that the addition of 1.5 wt% Zn facilitates the evolution of coarsen β-Al3Mg2 phases into fine T-Mg32(Al, Zn)49 phases, enhancing the resistance to intergranular corrosion (IGC) of Al-5.5Mg-1.5Zn-0.4Sc (alloy 4) with a maximum corrosion depth of 133.4 µm. The addition of Zn also promotes Mg solid solution, resulting in a 13.7 % increase in Mg solid solution concentration in Al-5.5Mg-1.5Zn (alloy 3) compared to Al-5.5Mg (alloy 2). The addition of 0.4 wt% Sc leads to the formation of smaller (∼2 µm) Al3Sc particles with a larger volume fraction of 0.96 %, providing grain refinement and improved strength in the Al-Mg alloy. Furthermore, the joint addition of Zn and Sc elements reduces the grain size by 64.0 % compared to Al-5.0Mg (alloy 1) and increases the tensile strength by 33.5 %. Alloy 4 exhibits the greatest measured tensile strength of 343 MPa. The solid solution strengthening, grain boundary strengthening and precipitation strengthening contributions to alloy 4 are calculated at 76.8 MPa, 27.9 MPa and 66.6 MPa, respectively.

Improved corrosion resistance of magnesium alloy prepared by selective laser melting through T4 heat treatment for biomedical applications

Selective laser melting (SLM) technology has broad prospects for developing magnesium alloys in structural and biomedical applications. The effects of T4 heat treatment on microstructure, mechanical properties and corrosion resistance of as-built AZ91D alloy were investigated. The results showed that after T4 treatment, β-Mg17Al12 phases distributed around the matrix changed from networks to diffuse dots and then disappeared at 420 °C for 1 h. The tensile strength and hardness of AZ91D alloy decreased after T4 treatment. In addition, electrochemical and immersion experiments showed that the corrosion resistance was greatly improved after solution. Compared with the as-built AZ91D alloy, the solution-treat sample exhibited higher corrosion resistance in Hanks' solution, and its corrosion current density was reduced by 70%, hydrogen evolution rate by 57%, and corrosion rate by about 78% in the weightless experiment. In this study, after solution-treat, the corrosion resistance of as-built AZ91D can be significantly improved while meeting the mechanical requirement of degradable alloys, and has great potential for biomedical applications.

Enhanced corrosion resistance of Mg17Al12 compounds by Ce modification

Mg17Al12-xCe (x = 0, 0.1, 0.5, 1 wt%) intermetallic compounds have been prepared by vacuum induction melting, in order to enhance the corrosion resistance of Mg17Al12 compound by Ce modification. The results have showed that the Mg17Al12 compound compose of β-Mg17Al12 substrate and eutectic structure (α-Mg and β-Mg17Al12 phases) distributed continuously along grain boundaries under non-equilibrium solidification conditions. A high cathode to anode area ratio of the eutectic structure results in poor corrosion resistance of the Mg17Al12 compound. The eutectic structure distributed continuously along the grain boundaries can be interrupted and corrosion could not propagate along the grain boundaries in Mg17Al12 compound alloyed by 0.1 wt% Ce. Hence, the corrosion performance of Mg17Al12 compound has been improved by 0.1 wt% Ce. With the Ce content further increasing to 0.5 wt%, the corrosion resistance of the compounds could be effectively improved due to the formation of dense corrosion products film. Additionally, the complete suppression of eutectic structure effectively reduces electrochemical corrosion of compounds containing more than 0.5% Ce. On the contrary, due to the precipitation of more Al11Ce3 phase, the corrosion resistance of Mg17Al12–1Ce alloy is slightly deteriorated based on the galvanic corrosion mechanism.

Enhancing the creep resistance of a cast AZ91 alloy via minor Sc addition

The influence of 0.2 wt% Sc addition on the microstructure and creep characteristics of a cast AZ91 magnesium alloy was studied by impression creep tests under the constant punch stresses ranging from 175 to 700 MPa in the temperature range 425–525 K. The findings indicated that creep rates decreased at all stress levels and temperatures after Sc addition. This was ascribed to the formation of the thermally stable Al3Sc and Mg5Al4Sc particles, reduction in the quantity of the unstable β-Mg17Al12 phase, and solid solution strengthening of Al in the Mg matrix. The creep stress exponent and activation energy values obtained from the studied alloys ranged from 5–6 and 95–117 kJ/mol, respectively. The detected drop in creep activation energy with increasing stress implies that two concurrent mechanisms of dislocation climb controlled by lattice and pipe-diffusion are competing with each other. The former one was the governing mechanism at low stress levels, while the latter one was the prevailing mechanism in the high stress regimes.

Effect of aging on corrosion and discharge performance of AZ63-1.5RE alloy as anode in magnesium-dissolved oxygen seawater batteries

This study aims to understand how aging treatment affects corrosion resistance and discharge performance of AZ63-1.5 rare earth mischmetals (AZ63-1.5RE) alloy in low-power dissolved oxygen seawater batteries. Solutionizing and aging treatment at 180 °C, 210 °C, and 240 °C for 16 h, 20 h, and 24 h is used to investigate the effects of heat treatment on corrosion and discharge performance. An XRD pattern of alloys after solutionizing and aging indicates a change in crystalline orientation and an increase in Mg phases proportion. A reduction in self-corrosion of alloys is achieved through solutionizing and aging. With solutionizing and aging of the alloys, their discharge potentials is shifted to more negative values by about 0.01–0.02 V. The anodic efficiency also is improved significantly from 53.89% for the as-cast alloy to 67.55% for the aged alloy at 210 °C for 20 h. Following 24 h of discharge, the aged alloy at 210 °C demonstrated the highest polarization resistance. The favorable corrosion performance can be attributed to grain refinement of the β-Mg17Al12 phase, the increased contribution of the Al-RE corrosion-resistant phase, the low contribution of the layered phase in the matrix, and the formation of dense discharge products on the surface.

Influence of minor RE addition on microstructures, tensile properties, and creep resistance in a die-cast Mg–Al–Ca–Mn alloy

In this work, we have investigated the influence of RE content on the tensile property, creep resistance, and microstructures of a die-cast Mg–9Al-0.3Ca-0.2Mn (wt. %). Minor content of RE elements simultaneously increases the strength and ductility, which is mostly since RE addition refined primary dendritic α-Mg and eutectic α-Mg matrix and eutectic β-Mg17Al12 phases and formed thermally stable Al11RE3 phases. The 0.6% RE adding alloy obtains the best strength-ductility synergy along with the UTS of 245 MPa, TYS of 170 MPa, and EL of 5.7%. The 0.6−1.0% RE addition gives rise to a reduction of the minimum creep rate by over one order of magnitude of the alloy, indicating a significant enhancement in creep resistance, which can be mainly attributed to RE addition introducing Al11RE3 phases to prevent grain boundary sliding, and suppressing dynamic precipitation of γ-Mg17Al12 phases during high-temperature creep. These results can provide important references for the design of creep-resistant die-cast Mg alloys and promote the application of commercial Mg–Al based alloys at high temperatures.

Effect of friction stir back extrusion on direction-dependent properties of gradient AZ91-bioactive glass composite rod

This research investigated the microstructure, mechanical properties, and corrosion resistance of friction stir back extruded AZ91-bioactive glass composites in parallel and perpendicular directions to the extrusion direction. The results showed that small grains are observed on the lateral surface, especially where bioactive glass is more prevalent. The grain size increases toward the center of the composite rod. The grain size and relative content of the β-Mg17Al12 phase shift from 30.2 ± 2.3 μm and 0.45 on the lateral surface of the composite rod to 37.4 ± 3.2 μm and 0.12 at the center of the composite rod, respectively. The corrosion resistance and hardness of the lateral surface area of the composite rod are 23 and 26%, respectively, higher than the corrosion resistance and hardness of the transverse surface. The lateral surface of the composite rod shows 74% higher corrosion resistance than the lateral surface of friction stir back extrusion processed wire without bioactive glass particles.

Evaluation of AZ91-bioactive glass composites produced by the friction stir back extrusion technique

In the present study, the effects of bioactive glass (64SiO2-31CaO-5P2O5) reinforcement on the microstructure, mechanical properties, and in-vitro corrosion characteristics of AZ91-bioactive glass composite wire, produced via friction stir back extrusion (FSBE), were investigated. The findings reveal that the bioactive glass reinforcement phase, exhibiting a gradient distribution in the AZ91 matrix and the α-Mg and β-Mg17Al12 phases, predominantly appear in the extruded samples. Additionally, a higher rotational speed during FSBE reduces bioactive glass powder agglomeration within the AZ91 matrix. The optimal yield strength, ultimate tensile strength, and corrosion resistance were observed in the FSBE-processed composite containing 5 vol% bioactive glass. It was revealed that employing a rotational speed of 1200 rpm and an extrusion speed of 30 mm/min during FSBE on as-cast AZ91 alloy with 5 vol% bioactive glass led to an 86 % enhancement in corrosion resistance in simulated body fluid (SBF) compared to the as-cast alloy. Incorporating 5 vol% bioactive glass into the homogenized AZ91 alloy increased corrosion resistance by 98 %. Moreover, raising the rotational speed from 800 to 1200 rpm resulted in a 24 % increase in corrosion resistance, reaching a 0.0427 mm/year rate.

The microstructure and mechanical and corrosion properties of Mg matrix composites reinforced with bioactive glass: The combined effect of bioactive glass content and extrusion speed

This paper studies the influence of 64SiO2–31CaO–5P2O5 bioactive glass particles on the microstructure, and mechanical and corrosion characteristics of AZ91-bioactive glass composite wires fabricated by friction stir back extrusion (FSBE). According to the findings, the extruded composite mostly includes the α-Mg and β-Mg17Al12 phase, as well as the bioactive glass reinforcing phase with a gradient distribution in the AZ91 matrix. The hardness of the FSBE-processed composites increased to 81.32 HV with increasing bioactive glass phase content from 1 to 5 vol % and extrusion speed from 20 to 40 mm/min. In the FSBE-processed composite with 5 vol % bioactive glass particles, the maximum yield strength, ultimate tensile strength, and corrosion resistance of 253.34 ± 9.31 MPa, 298.73 ± 8.95 MPa, and 0.0457 mm/year were obtained, respectively. Compared to AZ91 alloy the polarization resistance in the simulated body fluid solution of AZ91-bioactive glass composite increased and reached 298.21 Ω.cm2. The use of composite samples in circumstances requiring biodegradable implants seems to be promising.

Enhanced corrosion resistance of the as-cast AZ91 magnesium alloy by micro-addition of the strontium and rare earth elements

The synergetic effect of strontium (Sr) and rare earth elements (RE) on the corrosion properties of as-cast Mg–9Al–1Zn alloy were studied. Via the addition of 0.5 wt-% RE and 0.5 wt-% Sr (RE/Sr ratio of 1), a remarkable reduction in the corrosion rate of AZ91 was observed. The reason was found to be related to the decrement of the micro-galvanic corrosion by the modification of the β-Mg17Al12 phase (changing its morphology from semi-continuous to a more spheroid shape) and formation of proper amounts of Al11RE3 and Al4Sr intermetallics. However, the excess RE/Sr ratio impaired the corrosion resistance, which can be related to the micro-galvanic effect by excess amounts of Al11RE3 and Al4Sr intermetallics.

Effect of homogenization heat treatment on the microstructure of AZ91 magnesium alloy at different temperatures and ageing times

Magnesium (Mg) alloys are promising biodegradable implant materials. If successful, they do not require a second surgiabscal operation for their removal. These alloys possessed microstructure, which contained alpha (α-Mg) phase, eutectic (α + β) phase and intermetallic or beta (β-Mg17Al12) phase. The bio-corrosion rate of any intermetallic alloy is related to its β phase volume fraction (VF). However, this β phase contributes galvanic corrosion which hinders the clinical application of Mg alloys. Thus, this study is focused to reduce the VF of the β phase and select the appropriate temperature for Mg homogenization heat treatment (HHT). HHT was carried out on Mg AZ91 alloy samples by using a high-temperature rotary furnace at 350 °C for 10 h, 370 °C for 10 h, 390 °C for 5 h and10h, 410 °C for 5 h and10h and 430 °C for 5 and 10 h. The influence of varying temperature with time was investigated on the microstructure and microhardness of Mg AZ91 alloy by using a field emission scanning electron microscope equipped with an energy-dispersive spectroscope and image Digimizer. The results revealed that the higher the HHT or the longer the homogenization heat dwelling time, the more the homogenous distribution of the large β phase over the microstructure, hence containing little or without a β phase. The microhardness increased with an increase in temperature and ageing time with the peak hardness at 410 °C and 430 °C for 10 h.

Effects of annealing on the interfacial microstructure and mechanical properties of explosively welded AZ80 magnesium alloy and A6005C aluminium alloy

Effects of annealing on the interfacial microstructure, mechanical properties, and residual stress of the explosively welded AZ80 magnesium alloy and A6005C aluminium alloy cladding plate were investigated. By explosive welding, a thin interlayer composed by an intermetallic compound, i.e. γ-Mg17Al12 phase, was formed at the interface of the cladding plate. After annealing at both 373 K and 473 K, the thickness of the interlayer increased. After annealing at 473 K, the interlayer changed from a single layer of γ-Mg17Al12 phase to a double layer of γ-Mg17Al12 phase and β-Al3Mg2 phase, resulting in a decrease in shear strength. As a result of nanoindentation measurement at the interface, the hardness was remarkably high in the β-Al3Mg2 phase. It was suggested that this phase became the crack initiation site for brittle fracture and the shear strength is decreased. Measurements of the residual stress using synchrotron radiation X-rays at the interface of cladding plate revealed the tendency of the generation of tensile residual stress on AZ80 magnesium alloy side and compressive residual stress on A6005C aluminium alloy side. After annealing at 473 K, residual stress in AZ80 magnesium alloy side and A6005C aluminium alloy side changed to compressive and tensile stresses, respectively, and these stress values became smaller in both cases. On the other hand, after annealing at 373 K, compressive residual stress was observed in both AZ80 magnesium alloy side and A6005C aluminium alloy side.

Effect of friction stir back extrusion rotational speed on microstructure, mechanical properties, and corrosion behaviour of AZ91-Ca alloy

In this study, the effect of severe plastic deformation via friction stir back extrusion on microstructure, mechanical properties, and corrosion behavior of AZ91-Ca alloy were investigated. The results show that due to the high temperature during the process at a rotation speed of 1200 rpm, all the β-Mg17Al12, and Al2Ca phases are dissolved, and next to the manganese-rich compounds, re-precipitation of the β-Mg17Al12 and Al2Ca phases is observed. Also, increasing the rotation speed from 800 to 1200 rpm increases the accumulated strain from 0.186 to 0.351. With decreasing rotational speed from 1200 to 800 rpm, the texture parameter of the basal and prismatic planes is reduced, and on the contrary, the texture parameter of the pyramidal planes is increased. The ultimate tensile strength value increases from 332.14 ± 8.71 MPa to 375.13 ± 7.45 MPa by decreasing the rotational speed from 1200 to 800 rpm. The ultimate tensile strength, yield strength, elongation percent, and corrosion resistance in the sample processed at 800 rpm increased by 78, 49, and 32%, respectively, when compared to the as-cast base metal.

Fabrication of a gradient AZ91-bioactive glass composite with good biodegradability

This study used friction stir-back extrusion to fabricate the AZ91 + 3 wt% bioactive glass gradient composite wire. The microstructure, mechanical properties, and corrosion resistance of a material in a simulated body fluid were investigated. Three 2-mm diameter holes with varying hole patterns were drilled in the cross-section of the AZ91 rod to apply 3 wt % bioactive glass to the AZ91 matrix. The results demonstrated that the hole pattern strongly influenced the material's flow in the extruded wire's cross-section. By increasing the distance between the center of the initial rod and the center of the holes, a higher temperature and more uniform distribution of plastic strain are formed during friction stir back extrusion, resulting in uniform distribution of bioactive glass particles and α + β eutectic structure near the surface of composite wires. Introducing bioactive glass particles into the zone near the surface of the AZ91 rod results in the formation of a uniform distribution of bioactive glass particles near the surface and their absence in the central zone of the composite wire. A higher amount of discontinuous β-Mg17Al12 phase and α + β eutectic formed at the grain boundaries by increasing the temperature and plastic strain during friction stir-back extrusion. The crystallographic texture of the AZ91 rod changed from prismatic to basal and pyramidal due to the friction stir-back extrusion method. A gradient AZ91-bioactive glass composite wire with ultimate tensile strength, yield strength, elongation, and corrosion resistance 58, 64, 62, and 34%, respectively, greater than AZ91 as-cat rod can be produced by inserting bioactive glass powder using a hole drilling method and applying a friction stir back extrusion process.

Augmenting the performance of friction stir back extruded AZ91 alloy using the gradient composite design technology

This study investigates the effect of extrusion speed during friction stir back extrusion on the microstructure, mechanical characteristics, and in vitro corrosion behavior of AZ91-3 vol % bioactive glass composite wire. To that end, the bioactive glass powder was inserted into the AZ91 matrix by drilling holes in the cross-section of the AZ91 rod. The results show that the extrusion speed strongly influenced the material's flow in the extruded wire's cross-section. By adding the bioactive glass powder to the processed wire, the ultimate tensile strength and corrosion resistance increased 5 and 23% compared to the additive-free wire. By decreasing the extrusion speed to 20 mm/min, the higher amount of discontinuous β-Mg17Al12 phase and α + β eutectic formed in the grain boundaries. By applying the friction stir back extrusion process on the AZ91 rod a random crystallographic texture formed in the extruded wires. In the composite samples extruded at a rotational speed of 1200 rpm, a traverse speed of 20 mm/min, a gradient distribution, and improvement of mechanical properties (ultimate tensile strength of 332.95 MPa, yield strength of 268.33 MPa, and elongation of 9.89%) and corrosion rate (1.879 mm/year) at simulated body fluid were achieved.

Microstructure and Mechanical Properties of Weaving Wire and Arc Additive Manufactured AZ91 Magnesium Alloy Based on Cold Metal Transfer Technique.

Based on the cold metal transfer (CMT) technique, a deposited wall of AZ91 magnesium alloy was fabricated by weaving wire and arc additive manufacturing (WAAM), the shaping, microstructure, and mechanical properties of the sample with the weaving arc were characterized and discussed by compared with the sample without the weaving arc, and the effects of the weaving arc on grain refinement and property enhancement of the AZ91 component by CMT-WAAM process were investigated. After introducing the weaving arc, the effective rate of the deposited wall could be increased from 84.2% to 91.0%, and the temperature gradient of the molten pool could be reduced with an increase in constitutional undercooling. The equiaxed α-Mg grains became more equiaxial due to the dendrite remelting, and the β-Mg17Al12 phases distributed uniformly induced by the forced convection after introducing the weaving arc. Compared to the deposited component fabricated by the CMT-WAAM process without the weaving arc, the average ultimate tensile strength and elongation of the component by weaving the CMT-WAAM process both increased. The weaving CMT-WAAM component showed isotropy and has better performance than the traditional cast AZ91 alloy.

Microstructure evolution and corrosion properties of ECAPed Mg–Pb-9.2Al-0.8B alloys

Effect of grain refinement on corrosion behavior of Mg–Pb-9.2Al-0.8B alloy by equal-channel angular pressing (ECAP). By measuring open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS), the corrosion behavior of as-cast and ECAP samples in 3.5 wt % NaCl solution were compared and analyzed. The results showed that sample obtained fine crystal after ECAP, and grain refining reduced the corrosion resistance of the alloy. In the polarization test, the Icorr value was the largest, and the type changed from local corrosion to uniform corrosion. Two factors weaken the corrosion resistance: The first is that β-Mg17Al12 phase is homogenized by the tissue and disappears, thus losing the barrier of corrosion propagation, resulting in reduced corrosion resistance. The second is that Mg + Pb eutectic phase produced by thermal deformation is the first to be corroded. As a result, the corrosion resistance of the alloy decreases after ECAP.

Influence of hybrid reinforcements on the mechanical properties and morphology of AZ91 magnesium alloy composites synthesized by ultrasonic-assisted stir casting

In the present study, AZ91 magnesium alloys reinforced with a fixed weight fraction of aluminium oxide (Al2O3: 1.5 wt%) and titanium di-boride (TiB2: 1, 3, and 5 wt%) particle hybrid metal matrix composites were developed using the stir casting method assisted with ultrasonic agitations. The microstructural analysis reveals that the structure of monolithic alloy is made up of α-Mg grains with evenly distributed β-Mg17Al12 intermetallic secondary phases near the grain boundaries. Incorporating hybrid reinforcements into the matrix alloy decreases the average grain size and also leads to the breakdown of the β-Mg17Al12 phase from a rounded shape to sharp needle-like profiles. The micro-hardness, ultimate tensile strength, and impact strength outcomes revealed a direct correlation with varying TiB2 particulate contents, the peak values of which have been achieved with the addition of 3 wt% of TiB2 particulates with AZ91 + 1.5 wt% Al2O3 hybrid composite.

Corrosion and discharge performance of AZ61, AZ63, AZ101 and AZ103 alloys as anode in magnesium-dissolved oxygen seawater long term batteries

Distribution of phases, morphologies of discharge products, and concentration of Al and Zn elements in discharge products affect corrosion, anodic efficiency, and discharge performance of AZ61, AZ63, AZ101, and AZ103 alloys. The AZ101 alloy has a low overpotential for the dissolution of the Mg, which provided the highest OCP and the lowest discharge potential (−1.474 V vs SCE). The detachment of a large number of the β-Mg17Al12 phase particles and the loose discharge products with deep cracks on the AZ101 alloy surface provided the highest self-corrosion rate (2.225 g cm−2 year−1), the lowest anodic efficiency (41.76 ± 1.43%) and the lowest overpotential for hydrogen gas evolution. Fine β-Mg17Al12 phase particles, low concentration of Al and Zn within α-Mg, and condensed discharge products of the AZ63 alloy contribute to uniform corrosion. Despite offering a higher potential by the AZ63 alloy (−1.432 V), it has a lower chemical corrosion rate (0.476 g cm−2 year−1) and the highest anodic efficiency (56.07 ± 1.48). It suggests that AZ63 alloy is a good candidate as an anode for long-term seawater batteries.

Investigation of microstructure and corrosion behavior of AZ91/64SiO2-31CaO-5P2O5 composite wire fabricated by friction stir back extrusion

In this study, the AZ91-bioactive glass surface composite wires were created using a unique method called friction stir back extrusion (FSBE). The microstructure, mechanical properties, and corrosion resistance of FSBE-processed wires were studied. The results show that by applying the FSBE process, the amount of β-Mg17Al12 phase decreased compared to the as-cast AZ91 rod. Also, by increasing rotational speed during FSBE, the distribution of bioactive glass powder in the AZ91 matrix become more homogenous, and gradient AZ91 matrix composite is fabricated at a rotational speed of 1200 rev/min. By increasing temperature and plastic strain during FSBE at high rotational speed, the higher amount of discontinuous β-Mg17Al12 phase and α + β eutectic formed in the grain boundaries. By inserting bioactive glass powder using a hole drilling method and applying the FSBE process, a gradient AZ91-bioactive glass composite wire with ultimate tensile strength, yield strength, elongation, and corrosion resistance of 58, 64, 62, and 34 % higher than AZ91 as-cat rod can be achieved. Also, the corrosion resistance of gradient AZ91 matrix composite at simulated body fluid (SBF) increased 48 % compared to as-cast AZ91 base metal.