Influence of Magnesium on Nitridation of Molten Aluminum from Nitrogen Bubbling Gas

This investigation deals with the influence of magnesium on in-situ formation of AIN reinforcing particles in aluminum alloy melt from the bubbling nitrogen gas. Thermodynamically, addition of magnesium could remove the trace deleterious oxygen in the nitrogen atmosphere and benefits the formation of AIN. However, the practical effect may change from system to system. Unlike in nitridation of molten aluminum using direct metal oxidation (DIMOX) and infiltration methods, addition of magnesium in the present gas-bubbling method showed no significant beneficial effect on formation of AIN. When nitrogen gas of commercial grade was bubbled, no significant AIN reinforcing particles were formed for either pure aluminum or Al-5 at.% Mg alloy melt. When purified nitrogen gas was used, significant amounts of AIN particles were formed in both the melts with analogous morphology and comparable rate of formation. In the gas-bubbling system, AlN is formed from the heterogeneous reaction between nitrogen gas in the gas bubbles and molten aluminum in the matrix melt. The whole process is controlled by chemisorption of nitrogen at the interface, which could occur only when the oxygen content is below a threshold limit. In the gas bubbling system, magnesium evaporates at the gas bubble-matrix melt interface as well as the surface of melt. Vaporization of magnesium at the gas bubble-matrix melt interface contributes to the lowering of the oxygen content at the reaction sites. However, the lowering of the oxygen content is limited by short residence of the gas bubbles. Addition of magnesium led to no significant improvement in formation of AIN.