| Discontinuously reinforced aluminum alloy composites (DRACs) have potential applications in automotive, electronic packaging, and recreation industries. Conventional processing of DRACs is by incorporation of ceramic particles/whiskers/fibers into matrix alloys. Because of the high cost of ceramic particles, DRACs are expensive.; The goal of this work was to develop a low-cost route of AlN-Al DRACs processing through bubbling and reacting nitrogen and ammonia gases with aluminum alloy melt in the temperature range of 1373–1523 K.; Thermodynamic analysis of AlN-Al alloy system was performed based on Gibbs energy minimization theory. AlN is stable in aluminum, Al-Mg, Al-Si, Al-Zn, and Al-Li alloys over the whole temperature range for application and processing of DRACs.; Experiments were carried out to form AlN by bubbling nitrogen and ammonia gases through aluminum, Al-Mg, and Al-Si alloy melts. Products were characterized with XRD, SEM, and EDX. The results showed that in-situ processing of AlN reinforced DRACs is technically feasible. Significant AlN was synthesized by bubbling deoxidized nitrogen and ammonia gases.; When nitrogen gas was used as the nitrogen precursor, the AlN particles formed in-situ are small in size, (<10 μm). The formation of AlN is strongly affected by the trace oxygen impurities in the nitrogen gas. The deleterious effect of oxygen impurities is due to their inhibition to the chemisorption of nitrogen gas at the interface.; In comparison with nitrogen gas, bubbling ammonia led to formation of AlN particles in smaller size (about 2 μm or less) at a significantly higher rate. Ammonia is not stable and dissociated into nitrogen and hydrogen at reaction temperatures. The hydrogen functions as oxygen-getter at the interface and benefits chemisorption of nitrogen, thereby promoting the formation of AlN.; The overall process of AlN formation was modeled using two-film model. For nitrogen bubbling gas, the whole process is controlled by chemisorption of nitrogen molecules at the gas bubble - aluminum melt interface. For ammonia precursor, the rate of the overall process is limited by the mass transfer of nitrogen atoms in the liquid boundary layer. The models agree well with the experimental results. |
