Microstructure and phase composition of AlN/Al composite fabricated by directed melt nitridation

AIN/Al ceramic composite was fabricated by directed melt nitridation of pure Al block covered with 10wt% Mg powder at 1300 degrees C in a high purity flowing N-2. Microstructure and phase composition of the composite were investigated by scanning electron microscopy with energy dispersive spectroscopy and X-ray diffraction. Results showed that AlN is the main phase in the composite and its lattice parameters of a and c are 3.1110 angstrom and 4.9806 angstrom, respectively. The phase composition of the composite changes along the growth direction and a gradient sandwich structure forms. The surface of the composite is made up of a dense and thin nodular AlN layer, underneath which an AIN/Al layer appears, followed by an AlN/Al/MgAl2O4 layer. Thermodynamic calculations predicted the formation of possible phases with the addition of Mg. It suggested that the content of Mg at the reaction frontier of nitridation is considerably lower to 0.15wt% where MgAl2O4 was stable, because of escape and reaction exhaustion of Mg. Once Mg is lower than 0.05wt%, only a dense AlN layer can exist, which prevents the further nitridation of Al melt.