C. K. Yao

Wear behaviour and microstructural changes of SiCw-Al composite under unlubricated sliding friction

Abstract The transition of wear mechanisms and the microstructural changes of the SiCw-Al composites under unlubricated sliding friction were studied by X-ray diffraction, scanning electron spectroscopy and transmission electron spectroscopy techniques. The results show that with the increase of normal load and/or speed during dry sliding, the wear behaviour of the SiCw-Al composites transforms from mild wear to severe wear and then to seizure. The mild wear is characterized by mild oxidation and severe oxidation assisted slightly by adhesion, while the severe wear is dominated by abrasion or adhesion and delamination. Plastic deformation occurs in the surface layers of the tested specimens and the SiC whiskers realign along the sliding direction in the deformation layers. Oxide, β-Al 2 O 3 , forms on the surfaces; matrix metal grains fragmentation, fracture and fragmentation of SiC whiskers occur in the subsurfaces. Cracks usually initiate along whisker-matrix interfaces and propagate into the matrix.

Evaluation of the microstructure of in-situ reaction processed Al3Ti-Al2O3-Al composite

Abstract 1. 1. Differential thermal analysis (DTA) of squeeze-cast TiO2/Al bulk indicates that an exothermic reaction occurs after the aluminium melts. A high temperature combustion synthesis process was achieved to produce a fully dense Al3Ti + α-Al2O3+ Al in situ composite. 2. 2. Very fine-scale dispersion of the Al3Ti and α-Al2O3 reinforcing phases, ranging from 0.4 to 1.0μm, was obtained in the Al matrix. The Al3Ti phase, some containing Al2Ti precipitates, form in the shape of block and whisker. The nanometer scale α-Al2O3 particles are found near the Al3Ti phase and in the Al matrix, and, occasionally, within the Al3Ti phase. 3. 3. Analysis of the microstructural evolution indicates that the in situ processing involves two steps: (1) a reaction process between the molten aluminium and TiO2 to produce alumina particulates and liquid Ti. (2) a subsequent solidification process of remained Al and Ti displaced from the reaction with decreasing temperature to produce Al + Al3Ti blocks and then the smaller Al3Ti precipitates form as the solidcools.

Compression testing of a SiCw/Al composite at temperatures close to and above the solidus of the matrix alloy

Hot compression behaviour of a 20 vol.% SiCw/6061Al composite was determined at temperatures close to and above the solidus of the matrix. Work hardening behaviour of the composites was analyzed by whisker redistribution and breakage during deformation. Temperatures above the solidus of the matrix were suggested for the plastic forming of the composites.

Microstructure evolution of alumina borate whisker reinforced AA2024 composite after T6 treatment

Abstract The microstructure of alumina borate whisker reinforced AA2024 composite, subjected to different aging states, was studied by transmission electron microscopy. The characteristics of interfacial phases and the morphology of precipitates in the matrix were investigated. The morphology and distribution of precipitates near the interface between the whiskers and matrix in the composite were also studied. The results indicated that two typical interfacial phases existed at the interface between the whisker and the matrix. These two interfacial phases were different in composition and shape. The morphology and distribution of precipitates were also different near the interface with the different interfacial phases and the geometric features of whisker surface. The precipitates in the matrix were also investigated.

Effect of interfacial reaction on the Young's modulus of aluminium borate whisker reinforced aluminium composite

The interface plays a very important role in the properties of composites. One of the effects of the interface on the properties of the composite is that it may affect the load transfer from the matrix to the reinforcement. As it is well known that the Youngs modulus of composites is influenced mainly by the load transfer [1, 2], the interfacial reaction has a great effect on the bonding and strength of the interface [3], which can affect the Youngs modulus of the composite. Recently, a low cost aluminium matrix composite reinforced by aluminium borate whisker (AIIsB4 O33W » denoted by ABOw) has been studied by many researchers [4 8]. Although the interfacial reaction, mechanical properties and ageing behaviours of the composite have been investigated extensively, there have been few studies on the effect of the interfacial reaction on the Youngs modulus of ABOw/A1 composite. This letter aims to report this effect. The ABOw/AC8A-A1 composite used was fabricated by the squeeze casting method. The volume fraction of the whisker was 25%. The composition of the matrix alloy was Si: 12.0wt%, Mg: 1.0wt%, Cu: 0.8 wt%, NJ: 1.0 wt%, AI: 85.2 wt%. To obtain different degrees of the interfacial reaction, the composites were fabricated at various temperatures: 660°C, 720°C, 760°C, 800°C and 840°C. The specimens for tensile testing were solutionized at 500 °C for 5 h then quenched into water maintained at 80 °C, to prevent the effect of precipitation on the properties of the composite. The microstructure and the interfacial reaction were studied by transmission electron microscopy (TEM). The specimens for TEM observation were thinned by the ion milling method. The Youngs modulus of the composite was measured by the slope of the tensile stress-strain curves. The dependence of Youngs modulus of the composites on the casting temperature is shown in Fig. 1. It can be seen that when the casting temperature is lower than 800°C, the Youngs modulus of the composite increases with the casting temperature. When the casting temperature is higher that 800 °C, however, the Youngs modulus of the composite decreases with increasing casting temperature. That is to say, there exists an optimal casting temperature for the Yourigs modulus of the ABOw/AC8A composite. The microstructure of the ABOw/AC8A composite was studied using TEM. As shown in Fig. 2, an

A study on an aluminum matrix composite reinforced by both β-eucryptite particle and aluminum borate whisker

Low thermal expansion coefficient and high tensile strength were attained simultaneously in an aluminum matrix composite reinforced by both β-eucryptite particle and aluminum borate whisker by squeeze casting technique. The thermal expansion coefficient of the composite is much lower than that of aluminum alloy. The ultimate tensile strength (UTS) of the compounds of the composite 375 MPa. Optical microscope (OM), scanning electron microscopy (SEM) and transmission electron microscope (TEM) were used to observe the fractograph and microstructure of the composite. The effect of heat treatment on the thermal expansion coefficient (CTE) of the composite was also studied.

Sol-gel alumina coatings for whisker reinforced metal matrix composites

There exist serious interfacial reactions in squeeze-casting aluminum borate whisker reinforced 6061Al composites. To control the spinel reactions a sol-gel coating method was used to deposit gamma-Al2O3 and alpha-Al2O3 ceramic coatings on aluminum borate whiskers. TEM observations of coating morphology indicate that nanostructured alumina coatings are uniformly deposited on whisker surfaces, with a thickness of about 30-50 nm. Ball-milling has an important effect on final mechanical properties of the composites. gamma-Al2O3 and alpha-Al2O3 coatings were found to be effective in controlling the interfacial reactions, by completely preventing the diffusion of magnesium from matrix/coating interface to coating/whisker interface

Interfacial reactions and mechanical properties of 6061Al matrix composites reinforced with alumina-coated Al18B4O33 whiskers

Abstract Sol–gel γ-Al 2 O 3 and α-Al 2 O 3 ceramic coatings were deposited onto surfaces of aluminum borate whiskers to control the spinel reactions between whiskers and matrix alloys. Interface observations, tensile experiments and hardness measurements of the composites were conducted to evaluate the coating effect. By comparison with γ-Al 2 O 3 coating, the α-Al 2 O 3 coating was found to be more effective in controlling the interfacial reactions, and it helped to enhance the elastic modulus of the composite. The introduction of alumina coatings to the composite postponed the peak-aging process, due to a smaller depletion of magnesium in the matrix of the coated composites.

Effect of annealing treatment on the microstructure of matrix in SiC whisker reinforced aluminum composite

Residual stress is an intrinsic feature of SiCw/Al composites due to the difference of coefficients of thermal expansion between the ceramic whiskers and the aluminum matrix [1–3]. When composites cool down from higher temperature, the residual stress increases with temperature changing (1T ), if1T is sufficiently high, the residual stress can exceed the yield strength of the matrix of composite [4], which results in residual stress relaxation, plastic deformation of the matrix and a high density of dislocations in the matrix. Certainly, the residual stress relaxation and microstructure of matrix have an important influence on the properties of composites. In the present study, the effect of annealing treatment on the matrix microstructure of SiCw/Al composites was investigated. The composites used were SiC whisker with a volume fraction of 20% reinforced pure aluminum (SiCw/p-Al) and 2024Al (SiCw/2024Al), the composites were fabricated by squeeze casting. The annealing techniques are list in Table I. The dislocation state was examined by transmission electron microscopy (TEM), and the specimens for TEM observation were prepared by ion milling. The dislocation states of pure aluminum matrix composites are shown in Fig. 1. The dislocation density in the matrix of the as-cast composite was very high, and dislocation tangles can be seen in the matrix (see Fig. 1a). The high density of dislocations in the matrix suggests that heavier plastic deformation had been induced in the matrix during the quenching process, due to residual stress relaxation. As shown in Fig. 1b, a low dislocation density can be found in the matrix of SiCw/p-Al composite annealed at 200 ◦C for 4 h then cooled with a low cooling rate, and some sub-grain boundaries can also be found. Because the high residual stress in the matrix may reduce the recovery and recrystallization temperature of the

Effect of heat treatment on dislocation states and work hardening behaviors of SiCw/6061Al composite

Abstract Dislocation states in the matrix and work hardening behaviors of SiCw/6061Al composite with different treatments were studied by means of transmission electron microscopy (TEM) and true stress–strain curves. The results showed that the dislocations and their states in the matrix played very important roles in prompting the work hardening ability and yield strengths of SiCw/6061 Al composite. The work hardening ability at the micro deformation stage was much higher than that at the macro deformation stage. The quenching treatment was an effective heat treatment technique to prompt the micro yield strength and yield strength.