INVESTIGATION OF THE DISPLACEMENT REACTION IN MIXED ALN + TIO2 POWDERS .1. MICROSTRUCTURAL CHANGES AT 1 ATM N2

Micrometre-size AIN + TiO2 powders in the molar ratio 2/1.5 were mixed, cold-pressed and heat-treated to 1400-1800 K in an AIN + N2 = 1 atm buffer environment. Coarse AIN particles in fine TiO2 powder, and TiO2 single-crystal chips in fine AIN powder were also processed similarly. From X-ray diffraction and direct microstructural observations it is described and discussed that to a first approximation the overall reaction proceeds in three major steps: (i) reduction of TiO2 to Ti4O7 or Ti3O5 with concurrent Al2O3 dissolution, (ii) formation of Al(x)Ti(y)O5 phases of either monoclinic or orthorhombic structure depending chiefly on the initial TiO2 particle size, (iii) decomposition of such mixed oxide into Al2O3 + TiN. Simultaneously with these steps, the progressive oxidation of AIN to Al2O3 takes place at a rate which does not limit the overall reaction kinetics. Evidence is also provided that the reaction takes place in part via the transport of gaseous Al- and Ti-containing species, which calls for further investigation to be reported in the companion paper.