Growth of nanorods or nanostructured eutectic in the formation of magnesium-based metal matrix composites.

摘要

Magnesium-based metal matrix composites (Mg-MMCs) have immense potential in the development of automobile and aerospace industries because of their remarkable specific properties. In this thesis, we report the fabrication of different types of Mg-MMCs that were reinforced by one-dimensional (1-D) nanorods or nanostructured eutectics. The reinforcements in the matrices were in-situ formed by the internal reactions between Mg and different selected metal oxides. The phase transformation, crystallography and microstructure of the Mg-MMCs were characterized using various techniques such as differential scanning calorimetry, X-ray diffractometry, scanning and transmission electron microscopies, energy dispersive x-ray spectrometry and atomic force microscopy. Based on the results of our studies, reaction models describing the reactions between Mg and the selected metal oxides were established. The growth models of the nanorods and the eutectic in the Mg-MMC were also proposed. Three Mg-MMC systems have been studied, namely Mg-20 wt.% CuO, Mg-20 wt.% NiO, and Mg-12 wt.% Al-1 wt.% CuO.; In the sintering of an Mg-20 wt.% CuO powder mixture at 450°C, MgO nanorods were in-situ formed. After sintering, we identified three regions with distinct microstructures in the reaction zone between the Mg grain and the newly formed Cu grain. Region I contained MgO nanorods and Cu nanoparticles, region II was composed of MgO nanorods, while larger nanometer-sized MgO crystals were found in region III. The MgO nanorods were single crystals with a diameter of ∼20 nm and a length of ∼100 nm. The growth of these nanorods was controlled by the vapor-solid mechanism. The progressive change in morphology of the MgO phase had induced a hardness gradient across the reaction zone. As a result, the interfacial bonding between the major phases in the sintered product was enhanced. When the Mg-20 wt.% CuO green compacts were sintered at 550°C and then rapidly quenched to ambient, nanostructured Mg-Mg ₂Cu eutectic and MgO were formed and were embedded in the Mg matrix. (Abstract shortened by UMI.)