High-temperature production of AlN in Mg alloys with ammonia gas

Abstract The objective of this study was to produce composites with a uniform distribution of aluminum nitride (AlN) reinforcing particles in the magnesium (Mg) metal matrix composites. In this study, experiments were carried out to evaluate the effect of time and temperature on the nitridation of aluminum to form AlN and its distribution in Mg composites. High-temperature production of AlN in Mg alloys melts using the ammonia gas bubbling method was investigated. The effect of ammonia bubbling time and temperature at a flow rate of 0.1 liters per minute on the amount of AlN formation was studied. Bubbling of ammonia gas resulted in the in-situ formation of AlN in Mg alloys, yielding AlN-reinforced Mg alloy composites. The AlN formation in the alloy was increased with increasing bubbling time. The rate of AlN formation was found to be 0.34 g·min−1 at 1,073 K. An average yield of AlN (wt%) was 6.47, 29.65, and 27.43 at 973, 1,073, and 1,173 K, respectively. An activation energy of 59.57 kJ was determined for the nitridation process. The magnitude of activation energy indicates that the reaction proceeds in the mixed regime with control of both nucleation and interface diffusion. The product was characterized using X-ray diffraction (XRD), optical microscopy, and scanning electron microscopy. The characterization of samples showed that the AlN particles distributed throughout the alloy matrix. The AlN particles formed in-situ are small in size, and uniform dispersion of AlN particles was observed at higher bubbling times and at higher temperatures. The AlN crystallite size increased with an increase in bubbling time and temperature. The XRD characterization results showed that the composite formed in-situ was composed of (Mg), intermetallic γ-(Mg, Al), and AlN phases. The Rockwell hardness of the in-situ composites was higher than the un-reinforced Mg alloy, and the hardness increased with an increase in the AlN wt% in the Mg alloy composites.