Fine structure of coated whiskers in Al18B4O33w/γ-Al2O3/6061Al composite

Sol-gel -Al₂O₃ coating was deposited onto surfaces of aluminum borate whiskers to control the spinel reactions between whiskers and matrix alloys. High-resolution electron microscopy (HREM) observations of the whisker defects and coating structure in the squeeze-cast Al₁₈B₄O₃₃w/-Al₂O₃/6061Al composite were carried out. Stacking faults and edge dislocations in the whiskers were directly observed, and zigzag anti-phase boundaries were also found. A deformation of whiskers along sectional direction was observed, which was determined to be a source of in situ fracture of whiskers. The sol-gel -Al₂O₃ coating was a nanocrystalline material with an average grain size of 5 nm.     

The effect of solution temperature on the microstructure and tensile properties of Al–15%Mg2Si composite

The effect of different solution temperatures has been investigated on the microstructure and tensile properties of in situ Al–Mg₂Si composite specimens were subjected to solutionizing at different temperatures of 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C and 580 °C for holding time of 4 h followed by quenching. The microstructural studies of the polished and etched samples by scanning electron microscopy (SEM) in the solution condition indicated that the increase in the temperature changes the morphology of both the primary and secondary Mg₂Si phases. Solutionizing led to the dissolution of the Mg₂Si particles and changed their morphology. Tensile test results indicated that ultimate tensile strength (UTS) gradually decreased upon solutionizing from 300 to 550 °C while further increase in the temperature followed by a sharp decrease in UTS up to 580 °C solutionizing temperature. It was found that the elongation has become three times greater in comparison to the as-cast state. Elongation results showed an increase up to 500 °C and then reduced temperatures of 550 and 580 °C. Fractographic analysis revealed a cellular nature for the fracture surface. On the cellular fracture surface, the features of both brittle and ductile fracture were present simultaneously. As a result of solution treatment the potential sites for stress concentration and crack initiation areas were reduced due to softening of the sharp corners and break up of eutectic network respectively, while increase in the number of fine dimples rendered the nature of fracture to ductile and also increased elongation.     

Effect of alumina coating and extrusion deformation on microstructures and thermal properties of short carbon fibre–Al composites

Short carbon fibres were coated with alumina by sol–gel process. Uncoated and alumina-coated short carbon fibre–Al composites were fabricated by gas pressure infiltration process. The effects of alumina coating and extrusion deformation on microstructures and thermal properties of the composites were studied. The results show that alumina coating is effective to improve the quality of the short carbon fibre preform as well as act as diffusion barrier to impede interfacial harmful chemical reactions between aluminium and short carbon fibres, which would increase the thermal properties of the composites. Extrusion deformation can orient the carbon fibres to the extrusion direction to improve their degree of orientation, meanwhile decreasing their aspect ratio. Extrusion deformation has a beneficial effect on the thermal conductivity of the composites. However, its effect on coefficient of thermal expansion of the composites is small because the effects of the improvement in degree of orientation and the decrease of aspect ratio tend to cancel each other somewhat.     

Fabrication of 6061 aluminium alloy/Al18B4O33(w) composite by using sol–gel alumina binder

Abstract

For fabrication of aluminium borate whisker (Al₁₈B₄O_33(w)) reinforced 6061 aluminium alloy composites, a sol–gel alumina binder instead of conventional silica binder was used for preparing the whisker preforms of the squeeze cast composites. The results show that a sound whisker preform and a uniform composite can be made by this method. Unlike the reactive silica binder, the sol–gel alumina binder is rather stable throughout the entire high temperature fabrication process. Under appropriate conditions, the sol–gel alumina binder can also serve as a thermal barrier for minimising interfacial reactions between aluminium borate whiskers and the matrix alloy. With a binder concentration of 0.6 mol L^-1, the ultimate tensile strength of the composite is as high as 277.6 MPa at room temperature and moderate at elevated temperatures. The tensile fracture of the alumina bound composite shows a mixed mode of dimple fracture and interface debonding.     

Effect of rolling temperature on the evolution of defects and properties of an Al–Cu alloy

The defects and properties of a precipitation hardening Al–Cu alloy 2017 were studied after rolling at room temperature (RT) and cryogenic (liquid N₂) temperature (CT). It is found that CT rolling produced practically the same hardness as RT rolling for a wide range of rolling strains. However, electrical resistivity measurement revealed a clear difference indicating different defect structures in the CT- and RT-rolled samples. This difference led to higher hardness, after subsequent ageing, for samples processed by CT rolling. It is deduced that precipitation occurred during RT rolling, which compensated for the effect of lower dislocation density (evaluated from X-ray diffraction) in RT-rolled sample, and consequently resulted in similar hardness in both RT- and CT-rolled samples. It is noted that after ageing, CT-rolled sample has higher strength (~35%) than the standard T4 treatment.     

（Al18B4O33）w／ZL202复合材料的界面反应

利用ＸＲＤ和ＴＥＭ分析研究了（Ａｌ_（１８）Ｂ_４Ｏ_（３３））_ｗ／ＺＬ２０２复合材料中Ａｌ_（１８）Ｂ_４Ｏ_（３３）晶须同基体合金ＺＬ２０２之间的界面反应。结果表明，在Ａｌ_（１８）Ｂ_４Ｏ_（３３）晶须和ＺＬ２０２之间存在界面反应，界面反应物为具有尖晶石结构的ＣｕＡｌ_２Ｏ_４，并发现反应物同晶须之间存在如下的位向关系：｛１１１｝_（Ａｌ_（１８）Ｂ_４Ｏ_（３３））‖｛１１１｝_（ＣｕＡｌ_２Ｏ_４），＜２１１＞_（Ａｌ_（１８）Ｂ_４Ｏ_（３３））‖＜１１２＞_（ＣｕＡｌ_２Ｏ_４）。文中还对界面反应机制进行了分析。     

ZnAl2O4涂覆Al18B4O33(w)增强6061Al复合材料界面结构与拉伸性能

使用溶胶2凝胶法在硼酸铝晶须表面制备 ZnAl 2O 4涂层 , 采用挤压铸造法制备了 ZnAl 2O 4涂覆的硼酸铝晶须增强 6061Al 复合材料。研究了 ZnAl 2O 4涂覆对复合材料界面润湿性、 室温拉伸性能以及高温热暴露后界面热稳定性的影响。试验结果表明: 纳米 ZnAl 2O 4涂覆能够明显提高复合材料的界面润湿性 , 从而提高复合材料的室温拉伸性能; 均匀的 ZnAl 2O 4涂覆能有效地阻碍界面反应 , 使复合材料具有良好的热稳定性能。研究了涂覆对复合材料在铸态及高温热暴露后拉伸断裂行为的影响。未涂覆硼酸铝晶须增强的铝基复合材料在拉伸变形过程中晶须以折断为主 , ZnAl 2O 4涂覆硼酸铝晶须增强的铝基复合材料在拉伸变形过程中晶须以拔出为主。     

Effect of hot extrusion on wear properties of Al–15 wt.% Mg2Si in situ metal matrix composites

Al–15 wt.% Mg₂Si composites were prepared by in situ casting and characterized in wear tests. Previous to the extrusion of specimens at 470 °C – varying extrusion ratio (7.4, 14.1 and 25), the as-cast composites were homogenized at 500 °C for 5 h, followed by slow furnace cooling. The microstructure, hardness and sliding wear behavior were characterized for both, the as-cast and hot extruded composites. Results show that increasing the extrusion ratio causes a significant improvement in hardness and wear resistance. This is ascribed to the observed decrease in average size and better distribution of Mg₂Si particles, in tandem with a remarkable decrease in porosity percentages, which goes from 5.63 in the as-cast condition, to 0.47 at the extrusion ratio of 25. It was found that abrasion is the dominant wear mechanism in all extruded composites, whilst a combination of adhesion and delamination appears to be the governing mechanism for as-cast composites.     

Effect of Bi4B2O9 addition on the sintering temperature and microwave dielectric properties of BaO–Nd2O3–4TiO2 ceramics

The effects of Bi₄B₂O₉ addition on the sintering temperature, phase transition and microwave dielectric properties of BaO–Nd₂O₃–4TiO₂ (BNT) ceramics have been investigated. With 10 wt% Bi₄B₂O₉ addition, the sintering temperature of the BNT ceramics can be lowered down to about 1,150 °C. The secondary phase was observed at the level of 15 wt% Bi₄B₂O₉ addition. The Bi₄B₂O₉ addition can significantly affects the microwave dielectric properties. The Q × f ₀ value is a function of the sintering temperature and the Bi₄B₂O₉ content. For the samples sintered at 1,150 °C, Q × f ₀ value varies from 6,300 to 3,300 GHz as the Bi₄B₂O₉ addition increases from 5 to 20 wt%. The addition of Bi₄B₂O₉ does not induce much degradation in ε_r but modified the τ_f value to near zero. Typically, When 10 wt% Bi₄B₂O₉ is added, the τ_f of the ceramics could be tuned to a near-zero value (~1.2 ppm/°C), a substantial ε_r (~86) and Q × f ₀ (~4,670 GHz) could also be achieved simultaneously. The Bi₄B₂O₉ is an efficient sintering additive to decrease the sintering temperature and tune the τ_f value of the microwave dielectric materials for the practical microwave applications.     

Effect of CaO-doped in NiMn2O4–LaMnO3 composite ceramics on microstructure and electrical properties

The composite ceramics (NiMn₂O₄)_0.50(La_1?x Ca _x MnO₃)_0.50 (0?≤?x?≤?0.3) consisting of spinel-structured NiMn₂O₄ and perovskite-structured CaO-doped LaMnO₃ were prepared by classical solid state reaction. The X-ray diffraction (XRD) patterns have shown that the major phases presented in the sintered samples are NiMn₂O₄ compounds with a spinel structure, La_1?x Ca _x MnO₃ with a perovskite structure. The Scanning Electron Microscope (SEM) pictures have exhibited that the grain size of the composite ceramics decreases from ca. 6.5 to 2.0?μm as the mole fraction of CaO increases from 0 to 0.3. The ρ _25?°C and B _25/50 constants of the composite samples are in the range of 0.234–8.61?Ω?cm and 2,600–2,962?K, respectively. In particular, CaO-doped leads to a decrease in the resistance drift of the (NiMn₂O₄)_0.50(La_1?x Ca _x MnO₃)_0.50 composite NTC (negative temperature coefficient) ceramics after aging test. This indicates that the CaO-doped (NiMn₂O₄)_0.50(La_1?x Ca _x MnO₃)_0.50 NTC ceramics display high electrical stability in comparison with the Ca-free (NiMn₂O₄)_0.50(LaMnO₃)_0.50 ceramics. The X-ray photoelectron spectroscopy (XPS) analysis verifies that the valence states of the manganese ions have a highly mixed state of Mn²⁺, Mn³⁺ and Mn⁴⁺ at B site. And the electrical conduction of the composite ceramics can be elaborated by the ions migration mechanism.     

Effect of B2O3 on the densification and magnetic properties of Li–Zn ferrite

Densification, grain growth and magnetic properties of Li–Zn ferrite (Li_.30Zn_.4Fe_2.30O₄) doped with B₂O₃ as a sintering aid were investigated. B₂O₃ is a low melting point (460 °C) oxide and forms a liquid phase during sintering which affects the densification and grain growth of ferrites. Results showed that density and grain growth rate were sensitive to the B₂O₃ content and sintering temperature. At low amounts of B₂O₃ (<1 wt.%), an increase in the B₂O₃ content increased density and grain growth rate. The highest density and the maximum magnetization were obtained for the sample containing 1.0 wt.% B₂O₃ which was sintered at a lower temperature (1000 °C) for 1.5 h, in comparison with undoped samples. Higher B₂O₃ contents than 1.0 wt.% caused a decrease in density of samples due to secondary phases formation and evaporation of B₂O₃. The sample with the highest grain size showed the highest permeability and the lowest magnetic loss.     

Effect of casting techniques on tensile properties of cast aluminium alloy (Al–Si–Mg) and TiB2 containing metal matrix composite

Abstract

An analysis of the statistical distribution of the tensile strength of a TiB₂ containing aluminium matrix composite and its matrix alloy (Al-7Si-0.35Mg) was carried out using different casting techniques. The scatter of the tensile strength data was assessed by Weibull statistics. Results for the metal matrix composite (MMC) and the matrix alloy in as cast and heat treated conditions were compared. It was found that a low turbulence casting technique resulted in less scatter of tensile values, confirming the greater reliability of the cast material. Fractographic examination of the fractured faces of lower strength specimens showed that entrained oxide films play an important role in failure of the specimens. Heat treatment causes increased scatter in strength values, reflected in the lower Weibull modulus.     

Effect of precipitates on properties of cold-rolled Al–Mg–Si–Sc–Zr alloy with higher temperature aging

The aging hardening behaviours of the cold-rolled Al–Mg–Si–Sc–Zr alloy were investigated. The microstructure, hardness and electrical conductivity (EC) of the Al–Mg–Si–Sc–Zr alloy were measured. The relationship between the microstructure and the properties of the Al–Mg–Si–Sc–Zr alloy with cold-rolling and aging processes was studied. The result shows that the addition of Sc and Zr elements significantly refines the grains of the Al–Mg–Si alloy during casting. The cold rolling promotes the Al–Sc(Zr) precipitation. The precipitate strengthening increases with increasing roll reduction. The EC of the cold rolling?+?aging Al–Mg–Si–Sc–Zr alloy increases with increasing rolling reductions. The combination effects of the precipitation hardening and DRX softening during the aging process lead to the similar peak hardness values of around 70?HV of the rolled Al–Mg–Si–Sc–Zr alloy with the different reductions.     

Effect of pre-deformation anneal on the microstructure and texture evolution of Mg–3Al–1Zn–0.7Sr alloy during hot extrusion

The effect of pre-deformation annealing on the microstructure and texture of an AZ31 + 0.74 wt% Sr alloy has been investigated. As-cast samples as well as three samples that have been annealed at 400 °C for 10, 30, and 120 min were extruded at 300 °C. Results indicate that annealing transforms the bulky non-equilibrium Al–Mg–Sr precipitates to stable Al₄Sr spheroids. As the extent of this transformation increases before extrusion, there is seen an increase in the amount of uniformly dispersed intermetallic stringers in the extruded material. Texture measurements reveal the alignment of basal poles with the compression axis (perpendicular to the circular cross section of the extruded bar) and the formation of the basal ring texture in all the samples. However, an increase in the duration of the pre-deformation anneal switches the plane facing the extrusion direction from first order prismatic (10-10) to second order prismatic planes (11-20). Annealing decreases the Al solute concentration in Mg and lowers the lattice resistance against dislocation movement. Consequently, the more favorable (0002)[11-20] slip system is activated in grains that see low basal resolved shear stress (τ). As a result, those grains work harden and are consumed by dynamic recrystallization (DRX). However, the (0002)[-1100] slip system with high τ still avoids basal dislocation movement. Hence, the grains with high τ_{(0002)[-1100]}, which need to move dislocations in the (0002)[-1100] system to fulfill the strain compatibility conditions across the microstructure would be prevented from work hardening and DRX. This specific orientation has a (11-20) plane facing the extrusion direction.     

Effect of Mg/nano-Al2O3 interaction time during stirring on microstructure and mechanical properties of Mg–Al2O3 composite

Abstract

In the present study, magnesium based composites containing 1·1 vol.% of alumina particulates at nanolength scale were synthesised using disintegrated melt deposition technique. The main focus was to vary interaction time between molten magnesium and nano-Al₂O₃ particulates during stirring and to study its effects on the microstructure and mechanical properties. With an increase in stirring time, microstructural characterisation of the composite materials revealed reasonably uniform distribution of Al₂O₃ reinforcement, marginal grain refinement and presence of minimal porosity. Mechanical properties characterisation revealed that an increase in stirring time led to a simultaneous increase in hardness, 0·2% yield strength and ultimate tensile strength while ductility was compromised beyond 7·5 min stirring when compared to pure Mg. The optimal stirring time is observed to be in 5–7·5 min range in order to realise the best combination of strength, hardness, ductility and work of fracture.     

界面反应对（Al18B4O33）w／ZL109复合材料冲击性能的影响

研究了（Ａｌ１８ｂ４Ｏ３３）（ｗ）ＺＬ１０９Ａｌ合金界面反应对冲击韧性的影响，结果表明，界面反应与挤压铸造温度密切相关，随挤压铸造温度的提高，界面反应程度增加，冲击韧性先升后降，７６０℃挤压铸造时，冲击韧性值最高，就冲击韧性而言，存在一个最佳的界面反应程度。     

ZK60和Al18B4O33w/ZK60高温压缩流变应力行为的研究

为了分析ZK60镁合金和Al18B4O33w/ZK60复合材料的高温热变形行为,研究其高温流变应力与应变速率、变形温度之间的关系和组织情况,确定其应变速率敏感指数m和表观激活能Q.利用Gleeble-1500D热模拟试验机,在变形量为60%和不同温度、不同应变速率的条件下对其进行高温热压缩变形.研究表明:在温度为573～673 K和应变速率为0.001～0.1 s-1范围内,镁合金的应变速率敏感指数m值为0.14,复合材料的m值为0.12;合金的表观激活能Q值为226～254 kJ/mol;复合材料的Q值为254～283 kJ/mol.     

Effect of the alpha grain size on the deformation behavior during isothermal compression of Ti–6Al–4V alloy

The effects of alpha grain size on the flow stress, the apparent activation energy for deformation (Q) and the processing maps of Ti–6Al–4V with an equiaxed microstructure are thoroughly investigated using isothermal compression tests, and detailed explanation is given based on the microstructure observation and quantitative analysis. The shapes of flow curves are dependent on the microstructure characteristic of the alloy before deformation and during the deformation process. The flow stress increases with increasing equiaxed alpha phase, but decreases with increasing alpha grain size. The Q-values for d_r1 and d_r2 are smaller than those for d_r3 and d_r4, respectively, which is possibly attributed to that Ti–6Al–4V alloy for d_r1 and d_r2 which exhibits a very strong grain-boundary sliding (GBS) mode besides dominant dislocation glide/climb mechanism. The local efficiency maxima and unstable regions in processing maps change with the alpha grain size, which implies that proper hot-working domains should be modified in different grain size range so as to meet the precision forging process.     

Effect of grain refinement on the mechanical properties of Cu–Al–Be–B shape memory alloy

In this work, the mechanical properties of equal channel angular processing (ECAP)-processed fine- and coarse-grained Cu–11.42Al–0.35Be–0.18B shape memory alloys (wt.%) were evaluated using tensile testing. After eight passes of ECAP and subsequently quenching from 600 °C to RT, the mean grain diameter was refined from 227 μm to 42 μm with grain boundaries purified. The fine-grained alloy exhibited good mechanical properties with a high tensile strength (703 MPa) and featured deeper and closer dimples on its fracture surface. The micro cracks were more refined, and the cracks extension along the grain boundaries was improved in the fine-grained alloy. These changes can be attributed to improvement of martensite morphology, structural refinement and grain boundary purification.