Whisker Reinforced Aluminum Matrix Composites Research Articles

Effect of stress shot peening on the microstructure and residual stress of Al18B4O33 whisker reinforced aluminum matrix composite

This work systematically studied the effects of stress shot peening (SSP) on the microstructure, residual stress and hardness of the ultrafine-grained Al18B4O33 whisker reinforced Al-Mg-Si matrix composite (Al18B4O33w/Al-Mg-Si) by the modified Warren-Averbach method, X-ray diffraction stress equipment, transmission electron microscopy and hardness tester. The results showed that conventional shot peening generated a gradient deformation layer of the Al18B4O33w/Al-Mg-Si with ultrafine grains (200–400 nm) near the surface layer. By contrast with conventional SP, SSP effectively induced superior microstructures at the deformed zone with finer crystalline size and more dislocations. In parallel, SSP effectively enhanced the compressive residual stress (CRS) distribution with a higher magnitude of surface CRS (−83 MPa) and maximum CRS (−152 MPa) value and deeper affected depth along the loading direction. The fragmentized whisker during SSP acted as the pinning role on the dislocation movements, which was beneficial for grain refinement and CRS improvement. It is concluded that SSP technique can be regarded as an effective process of improving the surface properties of ultrafine-grained Al18B4O33w/Al composite.

Research on the Secondary Forgeability of High Volume Fraction Whisker Reinforced Aluminum Matrix Composites of Original Squeeze Casting

The poor formability of high volume fraction whisker reinforced aluminum matrix composites of original squeeze casting is an important factor restricting its further development and application. Currently, there are no reports on the secondary forgeability of aluminum matrix composites of original squeeze casting, although some papers on its first forgeability are published. The secondary forgeability is very important for most metals. This study aims to investigate the secondary forgeability of aluminum matrix composites. In this study, the secondary upsetting experiments of 20 vol% SiCw + Al18B4O33w/2024Al composites, treated by the original squeeze casting and extrusion, were carried out. The first upsetting deformation is close to the forming limit, the secondary upsetting deformation under the same deformation conditions was carried out to investigate the secondary forgeability. The experimental results show that, unlike aluminum alloys, the 20 vol% SiCw + Al18B4O33w/2024Al composites at the original squeeze casting and extrusion states have no secondary forgeability due to the whisker rotating and breaking during the secondary upsetting. The high volume fraction whisker reinforced aluminum matrix composites of original squeeze casting cannot be formed by the multiple-forging method since the cavities and cracks caused by whisker fracture continue to expand during secondary processing, which leads to further extension of macroscopic cracks.

Effect of Volume Fraction Whiskers on Tensile and Thermal Expansion Properties of Mg2B2O5 Whisker-reinforced Aluminum Composite

This paper prepared magnesium borate whisker reinforced aluminum matrix composites by extrusion casting, and studied the effects of the volume fraction of the reinforced phase on the mechanical properties, thermal expansion properties and microstructure of the composites. The test results show that the reinforced magnesium borate whisker in the composite can be uniformly dispersed in the aluminum matrix to improve the strength. With the increase of the volume fraction, the strength of the composite material increases and the plasticity decreases. When the whisker volume fraction is 30%, the strength of the composite material reaches the maximum, with its tensile strength reaching 244 MPa and elastic modulus 107.6 GPA. When the whisker volume fraction is 20%, the composite has the best plasticity and the fracture elongation is 0.47%. The thermal expansion curve of the composite shows that the increase of whisker volume fraction will reduce the thermal expansion coefficient of the composite.

High strength-ductility synergy of MgAlB4 whisker reinforced aluminum matrix composites achieved by in situ synthesis

For ceramic reinforced metal matrix composites, it is a critical issue to improve the limited ductility to achieve a combination of high strength and high toughness. Herein, in-situ synthesized MgAlB4 whisker reinforced Al (MgAlB4w/Al) composites with rational distribution of the reinforcement were successfully prepared by an efficient process combining vacuum pressureless sintering and hot-extrusion. The results demonstrate that the MgAlB4 whiskers noticeably coarsen with increasing sintering temperature, while the sintering duration has little effect on the size of the whiskers. The as-extruded composite with 15 wt% MgAlB4w has excellent comprehensive properties with the ultimate tensile strength of 380 MPa and the fracture elongation of ~9.6%. Fracture analysis reveals that the whiskers bear most loads and fracture firstly to form crack sources with the load increasing during drawing. In addition, the alignment distribution of MgAlB4w along extrusion direction is the main reason for the improved ductility of the MgAlB4w/Al composite.

Microstructure, mechanical and wear properties of aluminum borate whisker reinforced aluminum matrix composites

Abstract The microstructural features and the consequent mechanical properties were characterized in aluminium borate whisker (ABOw) (5, 10 and 15 wt.%) reinforced commercially-pure aluminium composites fabricated by conventional powder metallurgy technique. The aluminium powder and the whisker were effectively blended by a semi-powder metallurgy method. The blended powder mixtures were cold compacted and sintered at 600 °C. The sintered composites were characterized for microstructural features by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis. Porosity in the composites with variation in ABOw contents was determined. The effect of variation in content of ABOw on mechanical properties, viz. hardness, bending strength and compressive strength of the composites was evaluated. The dry sliding wear behaviour was evaluated at varying sliding distance at constant loads. Maximum flexural strength of 172 MPa and compressive strength of 324 MPa with improved hardness around HV 40.2 are obtained in composite with 10 wt.% ABOw. Further increase in ABOw content deteriorates the properties. A substantial increase in wear resistance is also observed with 10 wt.% ABOw. The excellent combination of mechanical properties of Al−10wt.%ABOw composites is attributed to good interfacial bonds, less porosity and uniformity in the microstructure.

Tensile properties of an aluminum matrix composite reinforced by SnO2-coated Al18B4O33 whisker

Al18B4O33 whisker was coated by SnO2 particles using a chemical precipitation method, and an aluminum matrix composite reinforced by the coated whisker was fabricated by squeeze casting technique. It is found that the SnO2 coating can react with aluminum matrix during squeeze casting process, and Sn particles are induced near the interface between Al18B4O33 whisker and matrix. The tensile test at room temperature indicated that the tensile strength of Al18B4O33 whisker reinforced aluminum matrix composite can be enhanced by suitable content of SnO2 coating. The composites with various whisker coating contents exhibit maximum tensile plasticity at about 300 °C, and the composite with a suitable whisker coating content could enhance its tensile plasticity evidently, which suggest that an Al18B4O33 whisker-Al composite with both high strength at room temperature and high formability at elevated temperature can be designed.

The effect of reinforcement clustering on the steady-state creep behaviors of discontinuous metal matrix composite: A possible origin of ‘anomalously high’ stress exponent

The effect of reinforcement clustering on the steady-state creep behaviors of unidirectional SiC whisker reinforced aluminum matrix composite was investigated using finite element models with two different microstructures: Homogeneous microstructure was made up of a random dispersion of whiskers and the heterogeneous (clustered) one was idealized using hierarchical multiscale technique with 2D microstructure image of this material. Based on the numerical results, a possible mechanism for the ‘anomalously high’ creep stress exponent was proposed in the light of current knowledge.

Influence of Extrusion Temperature on Microstructures and Mechanical Properties of Bi<sub>2</sub>O<sub>3</sub>-Coated Al<sub>18</sub>B<sub>4</sub>O<sub>33</sub> Whisker Reinforced Al Composites

Extrusion experiments have been carried out on Bi2O3-coated Al18B4O33 whisker reinforced aluminum matrix composite. The effects of the extrusion temperature on the microstructures, the formability and the mechanical properties of ABOw reinforced aluminum (ABOw/Al) composites with and without Bi2O3 coating were investigated. The results show that the steady-state extrusion load and the probability of whisker fracture of the extruded two composites decrease with the extrusion temperature increasing. The mechanical properties of the composites attain highest values at extrusion temperature of 673K.

Effect of changing rate of residual stress on thermal expansion behavior of magnesium borate whisker-reinforced aluminum composite

Magnesium borate whisker-reinforced aluminum matrix composite is a new aluminum matrix composite with high properties and low cost. In the study, the residual stress changing behaviors on heating and cooling of the composite were investigated through the coefficient of thermal expansion (CTE) curves. On heating, the tensile stress relaxation and compressive stress generation in the matrix of the composite results in the CTE of composite is smaller than that predicted by mixture rule, but the compressive stress relaxation in the matrix of composite causes the CTE of composite is larger than that predicted by mixture rule. The tensile and compressive stress changing rates with temperature are linear on heating in the matrix of the composite at lower and higher temperature ranges. The CTE of the composite on cooling is smaller than that on heating.

Effect of Reinforcement Nonuniformity on Effective Thermal Conductivity of Composite

In composites reinforced by whiskers or fibers, clustering of the reinforcements often occurs. In this work, we studied the effect of these clustering phenomena on effective thermal conductivity of the composites. An analytical model has been developed based on effective medium theory to calculate the effective thermal conductivity of composite with consideration of the nonuniformity of reinforcement distribution. With this model the thermal conductivities of SiC whisker reinforced aluminum matrix composites have been calculated, and the results are in good agreement with the experimental data. The dependences of composite thermal conductivity on parameters such as the shape of the reinforcement cluster, the volume fractions inside and outside cluster, etc. have been investigated.

Experimental and numerical studies of the effect of whisker misalignment on the hot compressive deformation behavior of the metal matrix composites

Abstract The combined experimental and numerical approaches are utilized to investigate the effects of the whisker misalignment and rotation on the stress–strain response of discontinuous SiC whisker reinforced aluminum matrix composites during the hot compressive deformation at 300 °C. It is found from the numerical results that the whisker orientation has significant effects on the stress and rotation of whiskers, but little influences on the flow stress and work hardening of the matrix. Both numerical and experimental results indicate that the whisker misalignment and rotation play an important role in hot deformation behavior of the composites. During the hot compressive deformation, the composites with whisker misalignment angles less than 30° exhibit strain-softening behavior due to the whisker rotation or breakage. However, the composites with whisker orientation angles more than 45° show work hardening phenomenon due to the matrix work hardening. The predicted stress–strain behavior of the composites is in qualitative agreement with the experiment results.

Load transfer mechanism and mechanical properties of SiCw/Al metal matrix composites

The load transfer mechanism and mechanical properties of SiC whisker reinforced aluminum matrix composites were investigated. A generalized shear-lag model was proposed to analyze the effects of the aspect ratio, misorientation angle and volume fraction of SiC whisker on the mechanical properties of SiCw/Al composites. A new concept on effective aspect ratio was suggested, combining the effects of aspect ratio and alignment of whiskers, to calculate the effective stress acting on the aluminum matrix in SiCw/Al composites. The magnitude and anisotropy of the yield strength of the SiCw/2124Al composite can be estimated more accurately by substituting the average aspect ratio with the effective aspect ratio based on the generalized shear-lag model. The SiCw/2124Al composite with the largest effective aspect ratio showed the lowest minimum creep rate which decreased with increasing effective aspect ratio of SiC whisker, but were found to be almost similar under the same effective stress acting on the Al matrix. It is suggested that the role of SiC whisker is to improve the mechanical properties by reducing the effective stress acting on the Al matrix in the SiCw/Al metal matrix composite.

Effect of interfacial reaction on bending strength of Al18B4O33 whisker-reinforced aluminum composites

The present study deals with the mechanism of the interfacial reaction and the microstructure of the interface in aluminum borate whisker-reinforced pure aluminum matrix composites prepared by a squeeze casting process. By means of X-ray diffraction analysis (XRDA), differential thermal analysis (DTA), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM), it has been found that no interfacial reaction occurred under the as-cast condition, whereas aluminum reacted with the whiskers when the composite was reheated at a temperature higher than 726 °C. The reaction produced γ-Al2O3. Based on HRTEM observation, it is considered that the whiskers bond to the aluminum matrix directly after squeeze casting. The observation also shows that there is a specific orientation relationship between the reaction product and whisker, {002} Al18B4O33 ‖ {220} γ-Al2O3, Al18B4O33 ‖ γ-Al2O3, which leads to a coherent interface with a mismatch of less than 1 pct. The interfacial bonding state changed after heating under different conditions. Propagation of cracks in each interfacial state was observed by SEM, and the effects of the interfacial reaction on the bending strength were studied with the microstructure of the interface.

特集 粉末の成形・加工 酸化チタンウィスカー強化Ａｌ基複合材料の疲労特性

Titanium oxide whisker reinforced aluminum matrix composites were fabricated by a powder metallurgical method. Our previous study on tensile properties of TiO2(N)/A1 composite indicated that the interfacial reaction occured considerably on the fabrication process, however, their tensile properties were nearly equal with those of SiC(W)/ Al composite.In this study, fatigue strength was examined by rotating bending test, and the whisker /matrix interface structure was investigated by scanning electron microscope and X-ray diffraction method. For the TiO2(w)/A1 composite, TiO2 whisker changed to Ti2O3 after hot pressing and A1, Ti and A12O3 were produced after hot extrusion at the interfaces. Fatigue limit for the TiO2(R)/A1 composite increased about 50% compared with that of monolithic aluminum. Fatigue crack growth direction was diflected by the whisker and propagated in the matrix along the interface. This fact means that the resistance to crack initiation and propagation increased because the adhesion force between whisker and matrix was improved by the existence of reactants at the interface.

Mechanical Properties of Powder Metallurgical Aluminum Matrix Composites Reinforced with Oxide Whiskers

Tensile properties of oxide whiskers-reinforced pure aluminum matrix composites produced by power metallurgy technique were investigated in connection with their microstructures. Reinforcements were aluminum borate, titanium oxide, and potassium titanate whiskers that were added independently to the aluminum matrix. Transmission electron microscopy characterization and the microanalysis clearly showed the interfacial reactants for every composite. The reactivities of aluminum borate and potassium titanate whiskers with pure aluminum are lower than that of titanium oxide whiskers. The interfacial reactants act as anchors to enhance the cohesive force between matrix and fillers. Therefore oxide whiskers reinforced aluminum matrix composites revealed nearly equal tensile properties to SiC whisker-reinforced aluminum matrix composite in the wide temperature range of 300 to 823 K

High temperature deformation behavior of 20 vol.% metal matrix composite

The SiC whisker reinforced aluminum matrix composites are attractive materials for aerospace or automotive structural components due to high specific strength and high specific modulus with good thermal stability. However, the SiC whisker reinforced composites generally have the major drawback of poor formability because of low ductility and poor fracture toughness in comparison with matrix alloys. Therefore, the enhancement of ductility in metal matrix composites remains as a major obstacle to the expansion of the industrial application. In this paper, the high temperature deformation behavior of a 20 vol. % SiC[sub w]/2024Al composite, fabricated by the vacuum hot pressing process, has been investigated. The tensile deformation behavior of the SiC[sub w]/2024Al composite was observed at temperatures ranging from 475--575 C with initial strain rates ranging from 10[sup [minus]3]--1.7s[sup 1]. The effect of extrusion on high temperature deformation behavior was investigated. The microstructural parameters, such as the orientation and aspect ratio of SiC whiskers and the grain size of the 2024Al matrix, are related with the high temperature deformation behavior of SiC[sup w]/2024Al composites.

Finite Element Analysis of Whisker-Reinforced SiC/Al Composites Subjected to Cryogenic Temperature Thermal Cycling

Thermo-elastoplastic finite element methods are used to investigate the thermal cycling of SiC whisker-reinforced aluminum-matrix composites. In a previous investigation, the development of residual stresses and plastic deformation during cooling from a typical solution-treatment or annealing temperature to room temperature was studied. In the present paper we investigate changes in these residual quantities during thermal cycling, including cycling to cryogenic temperatures, and the effect of these changes on subsequent mechanical behavior. The effective coefficients of thermal expansion and convergence to a stable cyclic loop are also examined.

Abrasive wear of silicon carbide particulate— and whisker-reinforced 7091 aluminum matrix composites

The abrasive wear behavior of silicon carbide particulate- and whisker-reinforced aluminum matrix composites was investigated under two-body abrasion conditions utilizing a pinon-disk apparatus. Both SiC particulate- and SiC whisker-reinforced composites exhibited greater wear resistance than the unreinforced 7091 alloy. The whisker reinforcement was more effective than the particulate reinforcement in improving the wear resistance of the matrix alloy. Further, whisker reinforcements oriented normal to the contact surface were, at small abrasive sizes, more effective than whiskers oriented within the contact surface. At larger abrasive sizes the opposite effect was observed. It was found that, at constant reinforcement volume fraction, the ratio of the average abrasive penetration depth to the characteristic size of the reinforcement was the critical parameter controlling the relative abrasive wear resistance of composites reinforced with SiC.

FABRICATION OF SiCp-AI COMPOSITE MATERIALS

ABSTRACT Liquid phase fabrication methods for aluminum matrix composites reinforced with SiC whiskers, or SiC particles have been investigated and the mechanical properties of fabricated composites have been evaluated. Three kinds of liquid phase fabrication methods; hot extrusion, hot pressing and pressure infiltration, were studied. Commercial SiC whiskers and SiC powders of alpha type and beta type were used as the reinforcements for an aluminum matrix. Among the fabrication methods investigated, the best results were achieved by the pressure infiltration. The mechanical properties and the wear resistance of the fabricated composites were measured. The SiC whisker reinforced aluminum matrix composites have high strength, so that they can be used as high specific strength materials. The SiC particulate reinforced aluminum matrix composites are not strong as the SiC whisker reinforced composites. However, the SiC particulate reinforced aluminum matrix composites have a good potential for use as wear resi...