Development of short alumina fibre and in situ Mg2Si particle reinforced magnesium alloy hybrid composite with high temperature strength

Abstract

Short alumina fibre and in situ Mg₂Si particle reinforced hybrid composites were fabricated by infiltration with the molten magnesium alloy into the preforms consisting of the fibres having Si particles attached to their surfaces. P or CaF₂ particles were used as the refiners of the Mg₂Si particles. All of Si particles reacted with the Mg alloy and melt to form Mg₂Si particles. As the melting temperature decreased or the cooling rate after the infiltration increased, the Mg₂Si particles became finer. The introduction of P or CaF₂ further promoted the refinement of the Mg₂Si particles. When the composite was fabricated by squeeze casting in the permanent mould, fine Mg₂ Si particles with a grain size of ～5 μm were formed regardless of introducing the refiners due to the rapid solidification. The strength of the hybrid composites was higher than that of the conventional fibre reinforced composite at both room temperature and high temperature.     

Effects of silicocalcium on microstructure and properties of Mg-6Al-0.5Mn alloy

1 Introduction Magnesium alloys are attractive for applications in the automobile, aerospace and electronic industries due to their light mass, high stiffness, high specific strength, good dimensional stability and damping capacity. It is the lightest sp…     

Centrifugal casting processes of manufacturing <Emphasis Type="Italic">in situ</Emphasis> functionally gradient composite materials of Al-19Si-5Mg alloy

Cylindrical components of in situ functionally gradient composite materials of Al-19Si-5Mg alloy were manufactured by centrifugal casting. Microstructure characteristics of the manufactured components were observed and the effects of the used process factors on these characteristics were analyzed. The results of observations shows that, in thickness, the components possess microstructures accumulating lots of Mg₂Si particles and a portion of primary silicon particles in the inner layer, a little Mg₂Si and primary silicon particles in the outer layer, and without any Mg₂Si and primary silicon particle in the middle layer. The results of the analysis indicate that the rotation rate of centrifugal casting, mould temperature, and melt pouring temperature have evidently affected the accumulation of the second phase particles. Also, the higher the centrifugal rotation rate, mould temperature, and melt pouring temperature are, the more evident in the inner layer the degree of accumulation of Mg₂Si and primary silicon particles is.     

固溶处理对原位合成Mg_2Si/AZ91D复合材料组织及性能的影响

研究固溶处理对Mg-SiO2体系原位合成制备的Mg2Si/AZ91D复合材料的组织和性能的影响。结果表明：AZ91D镁合金在加入SiO2（其中Si占合金质量的1.5%）后,出现了粗大的汉字状Mg2Si相,固溶处理改变了Mg2Si相的形貌与分布,并使β-Mg17Al12相溶入到基体中;随着固溶时间的增加,汉字状的Mg2Si相熔断、球化,在420℃保温16h时Mg2Si相最为细小、弥散;固溶处理后复合材料的抗拉强度增加了14.9%,伸长率增加了38.9%。固溶处理时在Mg2Si/Mg界面间的界面张力作用下,Mg2Si相不断熔断、聚集、扩散,最终获得球化的Mg2Si相。     

Synthesis and properties of bulk nanocrystalline Mg2Si through ball-milling and reactive hot-pressing

1 Introduction The intermetallic compound Mg2Si is useful as strengthening phase in the metal matrix composites (MMC) due to its low density (1.99 g/cm3) and high strength-to-mass ratio[1?5]. It is also promising as a basic material for thermoelectric ene…     

Nanocrystallization of Zr61Al7.5Cu17.5Ni10Si4 metallic glass

The primary nanocrystallization behavior and microstructural evolution of the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy during annealing were investigated by isothermal differential scanning calorimetry, X-ray diffractometry and transmission electron microscopy. During continuous heating of the 4Si and the base (contains no Si) amorphous alloys at a heating rate of 10 K/min, the saturation point of nucleation for the 4Si amorphous alloy occurs at a crystallization fraction of 78%, which is significantly higher than 65% for the base alloy, implying that these metalloid atoms would extend the nucleation stage and refine crystalline particles. The sequence of crystallization phase from the amorphous matrix for the isothermally annealed 4Si amorphous alloy at 703 K is observed to be Zr₂Cu and Zr₂Ni at the early stage, Zr₃Al at an intermediate stage, and Zr₂Si at the final stage. Moreover, enrichment of Si atoms at the interface between Zr₂Cu crystal and the amorphous matrix is detected. This may result in increasing the thermal stability of the remaining amorphous phase and retardation of the crystal growth of Zr₂Cu particles.     

Mechanical and Tribological Characterization of Al-Mg2Si Composites After Yttrium Addition and Heat Treatment

In this study, the effect of heat treatment and yttrium additions on the microstructure, mechanical properties, and tribological behavior of Al-15% Mg₂Si cast composites was investigated. The microstructural study revealed the presence of both primary and secondary Mg₂Si phases in all composite specimens and also Y-containing intermetallics (Al₂Y phases) at higher concentrations. It was also found that Y addition does not change the size and morphology of primary Mg₂Si particles considerably, but the pseudo-eutectic Mg₂Si changed from a flake-like morphology to fine fibrous or rod-like one. The results show that proper content of Y additions can reduce the amount of Mg₂Si phase through dissolving it into the matrix, lead to the precipitation of Al₂Y phase and improve the mechanical properties. Modified composites with 0.5% Y exhibited an ultimate tensile strength (UTS) of 290 MPa with an elongation of 4.3%. After exposing the composite to solution treatment at 520 °C for 4 h, the tensile strength of the composite continuously increased with the increase of Y content, and reached the maximum at 1% Y. The maximum UTS and elongation at room temperature for the heat-treated composites are 294 MPa and 7.4%, respectively. In the cast specimen, fracture surfaces are covered by packets with coarse steps, suggesting a brittle mode of failure. Modified composites with 0.5 wt.% Y contain several cracked particles together with a few decohered primary Mg₂Si particles. In solution heat-treated state, dimples present at the fracture surface are rather coarse but homogenous, showing a semi-ductile mode of fracture. Wear test results showed that the wear resistance of all specimens increases with the addition of Y up to 0.3 wt.%. Scanning electron microscopic observations of the worn surfaces revealed that the dominant wear mechanism was abrasive wear accompanied by some delamination wear mode.     

Centrifugal casting of in-situ Al-Mg2Si(1-x)Snx composites: a study of radial segregation

During centrifugal casting of Al-Mg₂Si hypereutectic cylinders, the light Mg₂Si particles are commonly segregated in centripetal direction towards the inner periphery. In order to change this inevitable centripetal segregation of Mg₂Si particles, some amounts of tin up to 8 wt pct were added to Al-15wt.% Mg₂Si alloy to enhance the formation of Mg₂Si_(1?x)Sn_x particles with variant stoichiometry thus variant density. XRD, EDS, optical metallography and hardness measurements were used to evaluate the stoichiometry and segregation patterns of in situ formed Mg₂Si_(1?x)Sn_x particles. According to the results, the cylinder containing 6 wt pct Sn illustrates a rather uniform distribution of particles thus hardness in all radial sections. However, the cylinders with higher amounts of Sn exhibit a radial segregation of Mg₂Si_(1?x)Sn_x particles in centrifugal direction towards the outer periphery. The results are interpreted in view of the Stokes’ law and the difference in density of the particles and the melt.     

Localized Corrosion Behavior of Al-Si-Mg Alloys Used for Fabrication of Aluminum Matrix Composites

The relationship between microstructure and localized corrosion behavior in neutral aerated chloride solutions was investigated with SEM/EDAX, conventional electrochemical techniques, and with scanning Kelvin probe force microscopy (SKPFM) for two custom-made alloys with Si/Mg molar ratios of 0.12 and 0.49. In this order, Al₃Fe, Al₃Mg₂, and Mg₂Si intermetallics were identified in the first alloy and Al(FeMn)Si and Mg₂Si particles in the second one. Anodic polarization curves and corrosion morphology showed that the alloy with higher Si/Mg molar ratio exhibited a better corrosion performance and evidence was shown that it had a more corrosion-resistant passive film. The corrosion process for both alloys in aerated 0.1 M NaCl solutions was localized around the Fe-rich intermetallics. They acted as local cathodes and produced dissolution of the aluminum matrix surrounding such particles. Mg₂Si and Al₃Mg₂ exhibited anodic behavior. SKPFM was successfully used to map the Volta potential distribution of main intermetallics. The localized corrosion behavior was correlated with a large Volta potential difference between the Fe-rich intermetallics and the matrix. After immersion in the chloride solution, such Volta potential difference decreased.     

Role of second-phase particles in chip breakability in aluminum alloys

To evaluate chip breakability, turning tests for various wrought aluminum (Al) alloys were carried out with a cemented carbide tool (K10) in dry and wet cutting conditions. In the alloys, Al₂Cu, Al₆Mn, Mg₂Si, Al–Fe–Si system compounds and eutectic Si were observed as second-phase particles. Chip breakability of the alloys containing Al₂Cu or Si were superior to those of those containing Mg₂Si or Al–Fe–Si. This tendency was more prominent in wet cutting than in dry cutting. The second-phase particles of Al₂Cu and Si in these alloys fractured during the machining process. It was estimated that the fractured second-phase particles acted as the cause of chip breaking. Moreover, the increase of the chip breakability in wet cutting was attributed to the Rehbinder effect due to the penetration of cutting oil into the micro-cracks.     

Bi对铸造Al-Mg2Si金属基复合材料组织与干摩擦磨损性能的影响（英文）

研究不同Bi含量的添加对Al-15Mg2Si原位金属基复合材料组织与干磨擦磨损性能的影响。结果表明,Bi的适量添加对复合材料中的初生和共晶Mg2Si相具有明显的变质效果。含Bi复合材料的磨损率和摩擦系数都小于无Bi复合材料的。Bi改变了复合材料的磨损机理,1%Bi的添加使其从磨粒磨损、粘着磨损和疲劳磨损转变为微弱磨粒磨损和粘着磨损,4%Bi的添加有效地抑制了摩擦表面初生Mg2Si颗粒裂纹的产生,避免了粘着磨损和疲劳磨损,使复合材料的磨损成为单一的微弱磨粒磨损。含Bi复合材料耐磨性能的改善是Bi对Mg2Si相的变质作用和Bi颗粒自润滑作用的综合结果。     

Microstructure characterization on the formation of in situ Mg2Si and MgO reinforcements in AZ91D/Flyash composites

5 wt.% and 100 μm in size of fly ash cenosphere particles were incorporated in AZ91D Mg alloy to fabricate in situ Mg₂Si and MgO reinforced AZ91D/Flyash composites by means of compocasting method. The in situ Mg₂Si and MgO were identified using X-ray diffraction (XRD), energy dispersive spectrometer (EDS), and thermodynamic analysis. The results show that the cenosphere particles distributed homogeneously in the matrix alloy. Moreover, most of the cenosphere particles were filled with the matrix alloy. The preliminary discussion was carried out on the growth mechanism of the in situ Mg₂Si.     

Passivation behaviors of 6082 aluminium alloy under stress corrosion process

We systematically studied the passivation process of 6082 aluminium alloy under the bending stress situation by combining electrochemical measurement techniques with three-point bending stress fixture designed by our lab, and then examined the microstructures of corroded specimens to analyze electrochemical corrosion mechanism in 1.5% NaCl solution. The results show that secondary Mg₂Si phase acts as the anodic electrode, leading to the self-corrosion of Mg₂Si phase. As a result, the spots of self-corroded Mg₂Si phase within grains act as initial pitting corrosion site, combined with tiny, massive precipitated Mg₂Si particles at the grain boundaries and bending stress, leading to the failure of surface of the 6082 aluminium alloy. The corrosion current density increases from 3.422 × 10⁻⁷ to 13.77 × 10⁻⁷ A/cm² when bending stress level was increased from 0% up to 100% of yield stress. Passive film formation process occurred between polarization potential area of −1.05 and −0.65 V. Sectional microstructural investigations show that the corrosion starts penetrating vertically into the material before it develops corrosion paths extending parallel to the surface, leading to massive stress-induced corrosion cracks. The maximum corrosion depth increases from ∼24 μm on specimen without any stress applied to 85 μm when bending stress of 100% yield strength is applied.     

Mg2Si的微波固相合成及其热电性能

为了解决Mg2Si传统制备方法中Mg的氧化、挥发等问题,采用微波低温固相反应法合成Mg2Si热电材料。用XRD分析手段研究合成产物的结构及相组成。在300到700K的温度范围内,对材料的电导率、Seebeck系数和热导率随温度的变化进行测量。结果表明,当Mg过量8%、加热功率为2.5kW时,于853K保温30min,可以得到单相Mg2Si热电化合物。在测试温度范围内,Mg2Si具有较高的品质因数ZT值,在600K温度下达到0.13。     

Effect of mixed rare earth oxides and CaCO<Subscript>3</Subscript> modification on the microstructure of an <Emphasis Type="Italic">in-situ</Emphasis> Mg<Subscript>2</Subscript>Si/Al-Si composite

The effects of mixed rare earth oxides and CaCO₃ on the microstructure of an in-situ Mg₂Si/Al-Si hypereutectic alloy composite were investigated by optical microscope, scanning electron microscope, and energy dispersive spectrum analysis. The results showed that the morphology of the primary Mg₂Si phase particles changed from irregular or crosses to polygonal shape, their sizes decreased from 75 μm to about 25 μm, and the compound of both the oxide and CaCO₃ was better than either the single mixed rare earth oxides or CaCO₃.     

Influence of silicon on the corrosion behaviour of Al–Zn–In–Mg–Ti sacrificial anode

The influence of Si on the corrosion behaviour of Al–5Zn–0.03In–1Mg–0.05Ti (wt.%) alloy was investigated by the microstructure observation and electrochemical measurements in order to improve its corrosion non-uniform and electrochemical properties. The main precipitates in Al–5Zn–0.03In–1Mg–0.05Ti–0.1Si (wt.%) alloy is Mg₂Si phase, which decrease the galvanic corrosion because the potential difference between Mg₂Si and a-Al is smaller than that between MgZn₂ and a-Al. The addition of Si improves the corrosion uniformity of the anode due to the fine equiaxed grains and grain boundaries where Mg₂Si particles uniformly distributed. The results indicate that the microstructure, electrochemical characteristics and corrosion uniformity can be improved significantly after adding 0.1 wt.% Si into Al–5Zn–0.03In–1Mg–0.05Ti (wt.%) alloy.     

Phase selection and performance of Mg–Cu–Y amorphous composite with different Mg/Cu ratios

In this article, Mg–Cu–Y alloys with two different Mg/Cu ratios(in at%) were prepared using a watercooled copper mold. Scanning electron microscopy and X-ray diffraction were applied to analyze the microstructure and phase composition. Moreover, corrosion resistance and wear resistance were studied systematically. The results show that both Mg65 Cu25 Y10 and Mg60 Cu30 Y10 alloys could form a composition of crystalline and amorphous phases. Although the microstructure of Mg65 Cu25 Y10 consists of an amorphous phase and a-Mg, Mg2 Cu, and Cu2 Y crystalline phases, the microstructure of Mg60 Cu30 Y10 alloy mainly consists of the amorphous phase and a-Mg, Mg2 Cu. With reducing Mg/Cu ratio, the alloys have better corrosion resistance and wear resistance. The mechanism has also been discussed in detail.     

Effects of temperature on fracture behavior of Al-based in-situ composites reinforced with Mg2Si and Si particles fabricated by centrifugal casting

An aluminum-based in-situ composites reinforced with Mg₂Si and Si particles were produced by centrifugal casting Al–20Si–5Mg alloy. The microstructure of the composites was examined, and the effects of temperature on fracture behavior of the composite were investigated. The results show that the average fraction of primary Si and Mg₂Si particles in the composites is as high as 38%, and ultimate tensile strengths (UTS) of the composites first increase then decrease with the increase of test temperature. Microstructures of broken specimens show that both the particle fracture and the interface debonding affect the fracture behavior of the composites, and the interface debonding becomes the dominant fracture mechanism with increasing test temperature. Comparative results indicate that rich particles in the composites and excellent interface strength play great roles in enhancing tensile property by preventing the movement of dislocations.