Dongjun Wang

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.

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.

Mechanical properties of the alumina-coated Al18B4O33w/6061Al composites

Abstract To control the interfacial reactions in squeeze-casting aluminum borate whisker reinforced 6061Al composites, sol-gel γ- and α-Al2O3 coatings were deposited on the whisker surfaces for preparation of the alumina-coated composites. Tensile and bending tests of the coated and uncoated composites were carried out. Experimental results indicated that, although the room-temperature strength of the composite was not improved, the introduction of alumina coatings contributed to the enhancement of elastic modulus. To eliminate the strength-scattering effect and to evaluate the thermal-resistance of alumina coatings, an index of strength-decrease degree (Δσbd) was useful. The alumina coatings were found to improve the composites high-temperature strength and change the fracture mechanisms of the composite at different testing temperatures.

Interface characterization of a β-SiC whisker-Al composite

The nature of the interface, the orientation relationship of β-SiC whisker (β-SiCw)-Al combination, and the misfit dislocation structures at the β-SiCw-Al interfaces in a β-SiCw-Al composite have been observed by a high-resolution transmission electron microscopy (HRTEM). It was shown that quite a good bonding between the whisker and the aluminium was achieved due largely to the lattice match between SiC and aluminium at the interfaces. The orientation relationship between the whisker and the aluminium was {002}SiC∥{111}Al; 〈110〉SiC∥〈110〉Al. The interface was clean, faceted and semicoherent. The misfit dislocation cores were located in the whisker side away from the β-SiCw-Al interfaces.

Fatigue crack propagation of extruded-19Vf%-SiCw/6061Al composites under mixed mode (I + II) loading

In this paper, the fatigue crack propagation threshold and the crack deflection behavior in a SiC{sub w}/6061Al composite under mixed mode (1 + 2) loading are investigated. The fatigue crack propagation direction of both L-T and T-L specimens under mixed mode (1 + 2) loading in the SiCw/6061Al composite is perpendicular to the direction of the maximum normal stress at the crack tip.

In situ transmission electron microscopy observation of the initiation and growth of microcracks in an SiC whiskerAl composite

Abstract The initiation and growth of microcracks in a SiC whisker (SiC w )Al composite have been studied by means of the transmission electron microscope equipped with a tensile stage. It was found that microcrack initiation sources mainly included (1) the whiskers damaged and/or fractured during the preparation of the composite, (2) uninfiltrated zones, (3) SiC w Al interfaces and (4) the matrix. Only the microcrack that nucleated in the matrix grew to become the major crack leading to specimen failure although many microcracks occurred in the early stages of straining. The whiskers and grain boundaries in the composite played an important role in impeding the growth of the cracks. When a crack was perpendicular to the frontal whisker, the crack tip would blunt near the longitudinal interface and it then advanced by bypassing the whisker end; when a crack was parallel to the frontal whisker, the crack would blunt near the whisker end and then propagated along the near-interface region of one side of this whisker.

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

Sol-gel γ-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/γ-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 γ-Al2O3 coating was a nanocrystalline material with an average grain size of 5 nm.

Effects of microstructures on the cyclic deformation behaviour of SiCw/AI composite

The cyclic deformation behaviour of an aluminium matrix composite reinforced with 16% Vf silicon carbide whiskers was studied. The microstructures were designed so that different precipitates existed in the composite. The results indicated that the SiCw/6061 Al composite demonstrated cyclic hardening, however the cyclic response stresses and the cyclic hardening tendency were different under differing ageing conditions. Lower cyclic response stresses and higher hardening tendencies were found under natural ageing conditions, while higher cyclic response stresses and lower hardening tendencies were obtained under artificial ageing conditions. The higher the cyclic response stresses following artificial ageing, the lower the hardening tendencies that reduced fatigue life. The effects of microstructures on the cyclic deformation behaviour of the composite were examined under the TEM and changes in microstructure caused by the cyclic deformation were observed.