Effect of holding pressure on densification and mechanical properties of Cf/Mg composites

Carbon fibre reinforced magnesium alloy matrix composites were fabricated by using liquid–solid extrusion directly following vacuum infiltration technique. The experimental results showed that the microstructures of C_f/Mg composites depended on the holding pressure. The porosity was reduced gradually, and the densification was improved obviously, respectively, with the increase of the holding pressure. The densification, hardness and Ultimate tensile strength of C_f/Mg composites were significantly improved as the holding pressure increased in the range of 0.1–15 MPa. The densification was not obvious, but the UTS of the C_f/Mg composites decreased gradually as the holding pressure increased in the range of 25–45 MPa. The C_f/Mg composites presented a good performance when the holding pressure was about 15 MPa.     

Heat-treatable Mg-9Al-6Sn-3Zn extrusion alloy

Mg-9Al-6Sn-3Zn (wt%) alloy was extruded and heat treated in T5 and T6 conditions, and its mechanical properties and microstructures were investigated. The extruded product can be slightly strengthened by the T5 treatment as a result of sparse and heterogeneous precipitation. Significant increase in strength is achieved by the T6 treatment, and this is mostly attributed to the formation of lamellar discontinuous Mg₁₇Al₁₂ precipitates. The segregation of Al and Zn at grain boundaries is responsible for the discontinuous Mg₁₇Al₁₂ nucleation. The T6-treated alloy exhibits a tensile yield strength of 341 MPa and an ultimate tensile strength of 409 MPa, together with an elongation to fracture of 4%.     

Corrosion characteristics of hybrid Al/SiCp/MgAl2O4 composites fabricated with fly ash and recycled aluminum

The corrosion characteristics of Al/SiC_p/spinel composites fabricated with SiC_p, fly ash (FA) and recycled aluminum were experimentally assessed. For type A composites prepared with the alloy Al–8Si–15Mg (wt.%), the Mg₂Si intermetallic precipitated during solidification acted as a microanode coupled to the matrix (in the presence of condensed humidity) and led to catastrophic localized corrosion. Although the potential attack of SiC by liquid aluminum was successfully avoided by the presence of SiO₂ in the FA, Al₄C₃ was still formed due to the reaction of carbon in the FA with aluminum. For type B composites, processed with the alloy Al–3Si–15Mg (wt.%) and calcinated FA, the silicon content was low enough to avoid formation of Mg₂Si. Moreover, chemical degradation by Al₄C₃ hydrolysis did not happen either because of the absence of carbon and the presence of SiO₂. This explains the physical integrity of type B composites even after 11 months of exposure to humid environment.     

Microstructure,mechanical properties and yield asymmetry of Mg–4Al–2Sn–xY alloys

The effect of 0, 0.3, 0.6, 0.9?wt-% Y addition on the microstructure and mechanical properties of extruded Mg–4Al–2Sn alloys were investigated. The results show that α-Mg, Mg₁₇Al₁₂, Mg₂Sn and Al₂Y phases form in the extruded Y-containing alloys. Mg₁₇Al₁₂ phase, containing trace amounts of Y, tends to distribute on the grain boundaries in the form of needles. When the Y content is 0.6?wt-%, the alloy has the best combination mechanical properties. Its tensile yield strength, ultimate tensile strength and tensile elongation are 172?MPa, 270?MPa and 11.2%, respectively. As the Y content increases, the tensile and compressive asymmetries in the Mg–4Al–2Sn–xY alloy decrease, due to grain refinement and the weakening of texture.     

Microstructure and mechanical properties of Mg–6Li–xAl–0.8Sn alloys

As-cast and as-extruded Mg–6Li–xAl–0.8Sn (x?=?0, 1, 3 and 5?wt-%) alloys were prepared. The microstructure and mechanical properties were investigated and discussed. The experimental results show that the Mg–6Li–0.8Sn alloy is composed of three phases: α-Mg, Mg₂Sn and Li₂MgSn. With the addition of Al, the test alloys display typical α-Mg?+?β-Li duplex structures. The new Mg₁₇Al₁₂ and LiMgAl₂ phases were found in the Mg–6Li–1Al–0.8Sn alloy. The lamellar-type AlLi phase was formed whereas the Mg₁₇Al₁₂ phase disappeared in Mg–6Li–3Al–0.8Sn alloy. The LiMgAl₂ phase vanished in the Mg–6Li–5Al–0.8Sn alloy. The mechanical properties of as-extruded alloys were remarkably improved. The as-extruded Mg–6Li–3Al–0.8Sn alloy exhibited the best mechanical properties, with a yield strength, tensile strength and elongation of 209.8?MPa, 242.6?MPa and 15.5%, respectively.     

The interface,microstructure and mechanical properties of Cf/LAS glass-ceramic composites

Carbon fibre (C_f)-reinforced lithium aluminium silicate (LAS) glass-ceramic matrix composites were prepared by using LAS ultrafine powders and LAS sol as starting materials and binder, respectively. The effects of fibre content, hot-pressing temperature and pressure on the mechanical properties of the composites were studied. By means of SEM and theoretical calculation, the effects of thermal mismatching between fibre and matrix, and the microstructure on the mechanical properties of the composites were analysed and discussed. The flexural strength and fracture toughness of C_f/LAS glass-ceramic matrix composite prepared were 740 MPa and 19.5 MPa m^1/2, respectively. The wettability of carbon fibre with matrix was also investigated.     

Effect of Pd on microstructures and tensile properties of as-cast Mg-6Al-1Zn alloys

The effects of Pd on the microstructure and mechanical properties of Mg-6Al-1Zn alloys were investigated. Mg-6Al-1Zn-xPd (x = 0-6 wt.%) alloys were prepared using a permanent mould casting method. The microstructure of the as-cast alloys was characterized by the presence of Mg₁₇Al₁₂ and Al₄Pd phases. The volume fraction of the Al₄Pd phase was increased by the addition of 1-6 wt.%Pd but the volume fraction of the Mg₁₇Al₁₂ phases decreased. At room temperature, the tensile strength increased with increasing Pd addition up to 2 wt.%Pd, and the elongation to fracture decreased with a concomitant increase in the aggregation of the coarse Al₄Pd phase. At 150 °C, the tensile strength increased with the addition of Pd. Therefore, the room and elevated temperature tensile properties of as-cast Mg-6Al-1Zn alloys can be improved by Pd addition.     

Investigation of interfacial structure of Mg/Al vacuum diffusion-bonded joint

The interfacial structure of the Mg/Al diffusion-bonded joint was studied by means of SEM, EPMA and TEM. The test results indicated that the interface zone of Mg/Al diffusion-bonded joint included the transition region on Mg side, the mid-diffusion region and the transition region on Al side. The concentration distribution of Mg and Al atoms in the diffusion zone includes three different regions. The three regions are intermetallic compounds Mg₂Al₃, Mg₃Al₂ and MgAl, respectively. The location of Mg₂Al₃, Mg₃Al₂ and MgAl phases in the diffusion zone can be determined. The Mg₃Al₂ phases were observed in the transition region on Mg side by means of TEM.     

化学镀锡层对镁铝异种金属超声波焊接的影响

为提高镁铝异种金属超声波焊接接头强度,预先在铝合金表面镀锡后进行镁铝异种金属超声波点焊,并对接头的微观组织和力学性能进行分析.研究表明:无镀锡层的镁铝超声波焊接接头界面出现了大量的Mg_3Al_2和Mg_(12)Al_(17)相,其接头的最大拉伸剪切强度为27.5 MPa;含镀锡层的铝镁超声波焊接结合区由镁锡反应扩散层、残余锡层和铝锡反应扩散层组成,其中,铝锡反应层是固溶体层,镁锡反应层主要是过饱和的固溶体基体及弥散析出的中间相Mg_2Sn,其接头的最大拉伸剪切强度为32.9 MPa.镀锡层的加入有效阻止了镁铝的相互扩散,抑制了硬脆的Mg-Al系金属间化合物的生成,提高了镁铝超声波焊接接头强度,与镁铝超声波焊接相比最大拉伸剪切强度提高了19.6%.     

Effect of Ca additions on microstructure and microhardness of an as-cast Mg-5.0 wt.% Al alloy

The effects of Ca additions (0.5-2.0 wt.%) on the microstructure and the microhardness of an as-cast Mg-5.0 wt.% Al alloy have been investigated. The coarse microstructure of the base alloy can be refined through adding Ca. DSC and TEM results showed that, as Ca additions increased up to 1.5 wt.% Ca, the β-Mg₁₇Al₁₂ phase was completely replaced by a (Al, Mg)₂Ca phase. The Vickers microhardness of the as-cast Mg-Al-Ca alloys increased with increasing Ca content. Tests on the Mg-5.0Al-2.0Ca (wt.%) alloy showed an indentation size effect, which was well described by Meyer's Law.     

Influence of the combined addition of Y and Nd on the microstructure and mechanical properties of Mg–Li alloy

Y and Nd are simultaneously added into Mg–5Li–3Al–2Zn alloy. It is found that there exist the phases of α-Mg, AlLi, Al₁₁Nd₃ and Al₂Y in the alloys. When the contents of Y and Nd are 1.2% and 0.8%, respectively, the grain is the finest with an average size of 30 μm, and the tensile strength of the alloy reaches 231 MPa, the elongation reaches 16%. When the ratio of Y to Nd is 1.2:0.8, there is a synergistic strengthening effect.     

Formation of spinel (MgAl2O4), MgO and pure Cu particles in Al-2Mg alloy-CuO particle composites

CuO particles were introduced into liquid Al-2Mg alloy by the vortex method to prepare an Al alloy-MgAl₂O₄ in situ particle composite, by reaction between CuO particles and the Al-2Mg alloy melt. Pure Cu, MgAl₂O₄ and MgO particles were detected in the particles extracted from the composites. DTA study showed partial dissolution of Cu in the matrix. Microhardness and hardness of the composites are higher than those of the base alloy. Both microhardness and hardness are higher for the Al-2Mg-2CuO composite than those of the Al-2Mg-5CuO composite. The hardness of the Al-2Mg-2CuO composite is remarkably high. The increase in microhardness has been attributed to the solid solution hardening effect with Cu as well as to the difference in CTE between the Al matrix and the particles. On the other hand, the improvement in hardness resulted from both solid solution hardening as well as the presence of hard particles such as MgAl₂O₄ and MgO.Retired.     

Influence of Pd addition on the creep behavior of AZ61 magnesium alloy

The effect of Pd addition (0, 2, and 4 wt%) on the microstructure and creep properties of permanent mold AZ61 (Mg-6Al-1Zn) alloy has been studied. The results indicate that Pd addition introduces a lamella-shaped Al₄Pd phase at the grain boundary, in addition to the Mg₁₇Al₁₂ (β) phase. The addition of Pd also suppresses the precipitation of the Mg₁₇Al₁₂ phase and residual Al at grain boundaries during solidification. These effects lead to an improvement in the creep behavior of AZ61. Moreover, extended steady-state creep and reductions in both the minimum creep rate and total creep strain are also observed in the case of 4 wt% Pd addition.     

Effect of Cu addition on microstructures and tensile properties of high-pressure die-casting Al-5.5Mg-0.7Mn alloy

In this study, Cu was added into the high-pressure die-casting Al-5.5Mg-0.7Mn (wt%) alloy to improve the tensile properties. The effects of Cu addition on the microstructures, mechanical properties of the Al-5.5Mg-0.7Mn alloys under both as-cast and T5 treatment conditions have been investigated. Additions of 0.5 wt%, 0.8 wt% and 1.5 wt% Cu can lead to the formation of irregular-shaped Al₂CuMg particles distributed along the grain boundaries in the as-cast alloys. Furthermore, the rest of Cu can dissolve into the matrixes. The lath-shaped Al₂CuMg precipitates with a size of 15–20 nm × 2–4 nm were generated in the T5-treated Al-5.5Mg-0.7Mn-xCu (x = 0.5, 0.8, 1.5 wt%) alloys. The room temperature tensile and yield strengths of alloys increase with increasing the content of Cu. Increasing Cu content results in more Al₂CuMg phase formation along the grain boundaries, which causes more cracks during tensile deformation and lower ductility. Al-5.5Mg-0.7Mn-0.8Cu alloy exhibits excellent comprehensive tensile properties under both as-cast and T5-treated conditions. The yield strength of 179 MPa, the ultimate tensile strength of 303 MPa and the elongation of 8.7% were achieved in the as-cast Al-5.5Mg-0.7Mn-0.8Cu alloy, while the yield strength significantly was improved to 198 MPa after T5 treatment.     

Effects of samarium on microstructures and tensile properties of Mg-5Al-0.3Mn alloy

Microstructures and tensile properties of Mg-5Al-0.3Mn-xSm (x = 0, 1, 2 and 3 wt.%) alloys prepared by metal mould casting method were investigated. It was demonstrated that Mg-5Al-0.3Mn alloy was mainly composed of α-Mg and β-Mg₁₇Al₁₂ phases. However, the other two precipitates (Al₁₁Sm₃ and Al₂Sm) were observed along grain boundaries in the alloys containing Sm. The amount of Al₁₁Sm₃ and Al₂Sm precipitates was increased with the increment of Sm content. Meanwhile, volume fraction of β-Mg₁₇Al₁₂ phase was decreased. Moreover, the morphology of β-Mg₁₇Al₁₂ was altered from bulk bone-like shape to spherical one. Tensile results showed that Mg-5Al-0.3Mn-2Sm alloy exhibited the highest tensile properties both at room temperature and 150 °C. Compared with ultimate tensile strength (UTS), yield strength (YS) and elongation (?) of Mg-5Al-0.3Mn alloy, UTS, YS and ? of Mg-5Al-0.3Mn-2Sm alloy were enhanced by 30%, 45% and 35% at room temperature, and by 17%, 48% and 96% at 150 °C, respectively. The improvement of tensile properties was attributed to the decreased amount of β-Mg₁₇Al₁₂ and its refined morphology, and high thermal stable Al₁₁Sm₃ and Al₂Sm precipitates which effectively prohibited dislocation movement and grain boundary sliding during deformation process.     

Creep deformation mechanism of magnesium-based alloys

Two heat-resistant magnesium alloys AJC421 and Mg-2Nd were prepared. Both as-cast Mg-2Nd and AJC421 alloys exhibited good creep resistance in comparison with commonly used magnesium alloys. The improvement in creep properties through Nd addition to pure magnesium is attributed to both solid solution and precipitation hardening. The stress exponents of 4.5–5.5 and activation energies of 70.0–96.0 kJ/mol obtained from the as-cast Mg-2Nd alloy at low temperatures and low stresses indicate the five power law can be used for predicting the creep mechanism. The additions of alkaline earth elements Sr and Ca into Mg–Al alloys suppress the discontinuous precipitation of Mg₁₇Al₁₂ and form thermal-stable intermediate phases at grain boundaries, leading to effective restriction to grain boundary sliding and migration. However, the mechanism responsible for creep deformation of Mg–Al based alloys with Ca and Sr additions is not consistent with the results of microstructure observations performed on the alloys before and after creep tests.     

Microstructure and mechanical properties of graphite fiber-reinforced high-purity aluminum matrix composite

Industrial pure aluminum (0.5 wt% impurity elements) was utilized in many investigations of aluminum matrix composites at home and abroad. However, impurity elements in industrial pure aluminum may influence the interface during fabrication of composite at high temperature. Thereby, it is necessary to use high-purity aluminum (impurity elements less than 0.01%) as matrix to enable study the interface reaction between reinforcement and matrix. In this study, stretches of brittle Al₄C₃ at the fiber/matrix interfaces in Gr_f/Al composite were observed. The fracture surface of the composite after tensile and bending tests was flat with no fiber pull-out, which revealed characteristic of brittle fracture. This was related to Al₄C₃, as this brittle phase may break before the fiber during loading and become a crack initiation point, while the corresponding crack may propagate in the fiber and the surrounding aluminum matrix, finally resulting in low stress fracture of composites.     

Fabrication of Al2O3 continuous fibre reinforced Al–Cu alloys by axial infiltration process

Abstract

This paper describes the fabrication of Al₂O₃ continuous fibre reinforced Al-Cu alloys by an axial infiltration process which is expected to be used in the production of stick, bar, or platelike composites. A discussion on the infiltrating process gave equations for the critical infiltration pressure and the size of composite defects. Microscopic observations and microprobe analyses on Al-4.43Cu, Al-6.48Cu, Al-10.11Cu, and Al-4.45Cu-1.54Mg (wt-%) matrix composites identified the solidification process of matrix alloys in the presence of Al₂O₃ fibres. The approximate relationships between microstructure, interspace size, and the matrix composition are described schematically. Microsegregation of Cu and Mg in the composites are also analysed quantitatively.     

Interface analysis in Al and Al alloys/Ni/carbon composites

Nature of fibre/matrix interfaces existing in Al/C composites were investigated depending on the presence of a nickel interlayer deposited on carbon fibres and on the composition of the aluminium matrix. Auger and electron microprobe analyses were used. The role of the nickel layer on the chemical evolution of the system after a 96 h heat treatment at 600°C is discussed. The presence of this nickel layer limits the diffusion of carbon into aluminium, and thereby, eliminates the formation of a carbide interphase, Al₃C₄, which is known to lower the mechanical properties of Al/C composites. The mechanisms differ according to the composition of the matrix. In the case of pure aluminium, an Al-Ni intermetallic is formed after thermal annealing. It does not react with the carbon fibre and so inhibits the growth of Al₃C₄. In the case of the alloyed matrix (AS7G0.6), the dissolution of the Ni sacrificial layer, after annealing, does not lead to the same Al-Ni intermetallic but a thin nickel layer remain in contact with the carbon fibre avoiding formation and growth of Al₃C₄ carbide. This difference of behaviour is tentatively ascribed to the presence of silicon that segregates at the fibre/matrix interface.     

5A06/TA2 diffusion bonding with Nb diffusion-retarding layers

The structure and performance of 5A06/TA2 diffusion bonding joints with or without Nb diffusion-retarding layers were studied by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and shear strength measurement. The results showed that a diffusion reaction occurred and Al₁₈Ti₂Mg₃ was formed, which markedly decreased the joint strength, and the highest shear strength of 5A06/TA2 joint in direct bonding was 83 MPa. The Nb interlayer impeded the diffusion of Mg atoms from the Al side to the Ti side and also retarded the diffusion of Ti atoms from the Ti side to the Al side, which was acting as a diffusion-retarding layer. The joint strengths were increased by the Nb diffusion-retarding layers, and the highest shear strength reached 105 MPa. When Ti diffused across the Nb layer and achieved saturation nearby the interface with Al alloy, the diffusion reaction of Ti, Al and Mg occurred and Al₁₈Ti₂Mg₃ appeared which decreased the joint strength.