Abstract
New aluminium alloys are required with strengths comparable with those of conventional precipitation hardened Al alloys, but with improved high temperature stability. Evaporation, because of the intimate mixing in the vapour stream, offers a possible way of producing such alloys. This thesis describes a study of the vapour codeposition of aluminium and oxygen, and investigations of the structures and properties of the deposits. First, a study of the nucleation and growth of pure Al films showed that the mobility of atoms on the surface had a marked effect on the observed structures. An investigation of the initial oxidation of freshly deposited Al films gave a quantitative measure of the oxygen sticking probability and the surface roughness, both of which affect the oxygen content of codeposited Al-0 films. Al-0 films, 200nm thick, deposited between 77°K and 573°K, were examined by electrical resistivity measurements and electron microscopy. The grain size decreased with temperature, to approximately 4nm at 77°K. Four annealing stages were identified, associated with the removal of defects and the redistribution of oxygen. Deposits, 20?m thick, showed that the extremely small grain size was maintained with increasing thickness at 77°K. with 300°K deposits, however, the grains grew to about 1um in diameter as the thickness increased, and they developed straight, oxygen decorated boundaries. Heating in vacuo to above 673°K produced small Al[2]O[3] particles in the grains. Microhardness changes coincided with the annealing stages observed in the thin films. The production of the observed structures is discussed in terms of the mobilities of Al and O atoms and O[2] molecules on the growing surface. Hardnesses comparable with those of strong Al alloys could be maintained for 12h at 673°K, a temperature at which most Al alloys rapidly soften, but because of internal stresses and oxidized boundaries, these harder deposits were brittle.