Mechanical properties of Al/Al2O3 composites fabricated by reaction between SiO2 and molten Al, Al-Cu

Abstract

Generally, fabrication costs of composite materials are high because several processes are involved, such as embedding ®bres or particles within the matrix materials followed by various heat treatments. In situ composites have recently attracted attention [1], as their fabrication processes are expected to be effective for reducing the fabrication costs and the desired micro=macro structures of the composite material are obtained during the fabrication process. In the case of Al=Al2O3 composites, a process of direct oxidation of molten Al has been developed as the DIMOX process [2], and fundamental studies have been reported on their reaction kinetics [3, 4] and the microstructure [4, 5]. On the other hand, it has been reported that the Al=Al2O3 composite materials can also be obtained [6±8] by means of a substitutional reaction between SiO2 (or mullite [9]) and molten Al, which have similar microstructure to that obtained by direct oxidation. However, the reported studies have mainly dealt with the relationships between the process conditions and the microstructure [6, 7]. The detailed relationships between the process conditions, microstructures and the properties are still not clari®ed. In this study, Al=Al2O3 composite materials were fabricated between SiO2 and molten Al at different reaction temperatures and their microstructures and mechanical properties are investigated. Moreover, modi®cation of the hardness was attempted, by utilizing molten Al-Cu for the reactant, followed by heat treatment. The Al sections utilized for the reactions was cut from an ingot of 99.99% purity, etched with 0.1 N NaOH solution for 1.8 ks then cleaned with puri®ed water. Commercial fused silica rods having diameters of 5 mm (Toshiba Ceramics Inc.) were dipped into molten Al in air. After suf®cient time for completion of the reaction (investigated for the reaction kinetics [10], in advance) the specimens were cut and the microstructures were observed by optical microscope and scanning electron microscope (SEM). Compositional analysis was carried out with energy dispersive X-ray analysis (EDX), attached to the SEM, and with wavelength dispersive X-ray analysis (WDX). The Vickers hardness was measured by applying a load of 5 kg. Compression tests were conducted for the specimen rods having the following dimensions: length 7 mm, diameter 5 mm. Compressive fracture strength (of ) was also measured. Optical micrographs of the specimen microstructures are shown in Fig. 1. The bright regions in the photographs correspond to Al and the dark regions to Al2O3. Microstructures of the specimens fabricated at temperatures higher than 1373 K were coarse, compared with those fabricated at 1073 K. Microstructures coarsened as the reaction temperature increased.