Thermodynamics and Synthesis of AlN-Reinforced Mg Alloy Composites

In situ synthesis of aluminum nitride (AlN)-reinforced magnesium alloy composites using ammonia (NH₃) gas was investigated. Feasibility of in situ formation of AlN reinforcement was analyzed based on Gibbs energy minimization method. Thermodynamic analysis showed that AlN forms in Mg–Al alloy in the temperature range from 923 K to 1223 K and the oxygen that may be present in the system was detrimental for the formation of nitrides. The thermodynamic analysis also shows that Mg aids in decreasing the oxygen content from the system. Injecting ammonia gas through Mg alloy caused bulk nitridation of the alloy, which led to the formation of AlN-reinforced Mg alloy composites. Presence of hydrogen, which was generated by the dissociation of ammonia, in the alloy getters the trace oxygen in the gaseous precursor, enabling the formation of AlN. For the ammonia injection through the Mg–Al alloy containing 30 wt pct Al at 1173 K and injection time of 70 minutes, the yield of AlN was found to increase with an increase in the flow rate of ammonia. The AlN formed in the melt was 15.42 wt pct AlN for a gas flow rate of 0.04 LPM (liters per minute), and 27.43 wt pct AlN at a flow rate of 0.1 LPM. AlN particles formed in situ are surrounded by the γ-(Mg, Al) phase. X-ray diffraction characterization showed that the composites formed in situ were composed of (Mg), intermetallic γ-(Mg, Al), and AlN phases. Scanning electron microscopy and optical microscopy analyses revealed that the AlN particles formed in situ, dispersed uniformly, and the increase in flow rate increases the dispersion density of AlN particles in the composites. The experimental compositions and phases obtained are included in the constructed Mg–Al–N ternary phase diagram at 1173 K. All experimental data fall in the region of AlN and Mg–Al alloy, which is in validation with the thermodynamic prediction. The experimental obtained amount of AlN in composites as a function of Al content in alloy are in good agreement with the thermodynamic modeled results, which indicates that the formation of AlN in Mg–Al alloy is close to an equilibrium process.