ON THE ROLE OF MAGNESIUM AND SILICON IN THE FORMATION OF ALUMINA FROMALUMINUM-ALLOYS BY MEANS OF DIMOX PROCESSING

This article deals with the reaction mechanisms of the DIMOX (Directed Melt Oxided) processing of aluminum alloys. An orthogonalized experim ental procedure was introduced to stipulate the effects of the reactio n temperature, reaction time, and additional metallic elements, magnes ium and silicon, on the oxidation process of aluminum alloys. Emphasis is placed on the distribution of magnesium and silicon in the product s so that the behaviors of these two crucial elements for the formatio n of alumina from directed oxidation of aluminum alloys could be revea led. Alterative methods, including optical and scanning electron micro scopy (SEM), electron probing, and wave spectrum analysis were applied to specify the microstructure characters of the products and locate t he position of both magnesium and silicon in the reaction products. Ju dged by the weight gain after reaction, the results indicated that the temperature is the most influential factor in controlling the oxidati on kinetics. Silicon is more effective than magnesium in accelerating the process, although magnesium is indispensable for the process to ta ke place. While judged by the morphology of the reaction products, an excessive amount of silicon is harmful to the DIMOX process in that th e final products consist of a large amount of porosity. Both magnesium and silicon are rather concentrated in specific regions than homogene ously distributed in the whole products. The contents of magnesium and silicon in the surface region are not as high as expected, with most of the magnesium being concentrated in the region directly neighboring the bulky metals and most of the Si in the residual bulky metals, alt hough the contents of these two elements in the surface region are a l ittle higher than the regions next to the surface. These characteristi cs, combined with other investigations, suggest that the decisive role of the slight amount of magnesium and silicon in the nucleation and g rowth of Al2O3 could be explained by the proposed circulated reaction.