NITRIDATION REACTIONS OF MOLTEN AL-(MG, SI) ALLOYS

The isothermal nitridation of magnesium- and silicon-doped aluminium melt at 1273 K was investigated. With increasing Mg/Si ratio and decreasing oxygen content in the nitriding atmosphere, four major reaction mechanisms may be separated: (i) a passivating surface nitridation, (ii) a volume nitridation with precipitation of isolated AIN in the aluminium matrix, (iii) a volume nitridation resulting in a three-dimensionally interconnected AIN/AI composite microstructure, and (iv) a break-away nitridation with complete conversion of aluminium to AIN. The behavioural transition of the nitridation mechanism is reflected by the growth direction and the crystal morphology of AIN which change from inward (mechanisms i, ii) to outward (mechanisms iii, iv) growth of the reaction product with [0 0 0 1] as the dominating growth direction. Attempts are made to define the critical magnesium and silicon contents for the regime of controlled AIN/AI composite growth (mechanism iii) at 1273 K, in order to develop novel AIN/AI composite materials.