(A) study on the growth mechanism and mechanical properties of $Al_2O_3/Al$ composites by directed metal oxidation process

Abstract

$Al_2O_3/Al$ composite produced by directed metal oxidation (DIMOX) process has been a subject of interest because of the ability to produce ceramic composites by itself or with the aid of particulate or fiber reinforcement for favorable structural properties without limitation on size and shape. In this study, the reaction mechanism, microstructure and mechanical properties of the $Al_2O_3/Al$ composite produced by directed metal oxidation of Al alloy were investigated. The Al alloys, Al-Mg and Al-Mg-Si alloy, were used to produce $Al_2O_3/Al$ composite, and surface dopant of $SiO_2$ and NaOH on the above alloys and the pure Al, respectively, were used to highlight the role of surface doping on the growth mechanism of $Al_2O_3/Al$ composite. The weight variations with process time were monitored using TGA (thermo-gravimetric analysis) system to examine the oxidation behaviors of the $Al_2O_3/Al$ composites at various reaction temperatures. The micro- and macro-structure analyses were conducted using X-ray diffractometry, optical microscopy, SEM (scanning electron microscopy), EBSD (Electron back-scattered diffraction) and AES (Auger electron spectroscopy). The mechanical properties were characterized by the indentation-strength method to verify the R-curve behavior of composites. In chapter Ⅲ, the reaction mechanism on the formation of $Al_2O_3/Al$ composite was studied using Al-Mg and Al-Mg-Si alloy surface-doped with $SiO_2$ particles, especially focusing on the oxidation phenomena in the incubation period. It was confirmed that the oxidation consisted of three stages as same as the previous results: initial oxidation with abrupt weight gain forming $MgO/MgAl_2O_4$ surface oxide layers, incubation period with limited weight gain, and bulk oxidation forming $Al_2O_3/Al$ composite. The surface dopant did not significant effects on the initial oxidation stage. However, the dopant reduced incubation time of Al-Mg-Si alloy as well as Al-Mg alloy. Th...