Abstract:
The formation of Al(2)O(3), MgAl(2)O(4), and MgO has been widely studied in different Al base metal matrix composites, but the studies on thermodynamic aspects of the Al(2)O(3)/ MgAl(2)O(4)/MgO phase equilibria have been limited to few systems such as Al/Al(2)O(3) and Al/SiC. The present study analyzes the Al(2)O(3)/MgAl(2)O(4) and MgAl(2)O(4)/MgO equilibria with respect to the temperature and the Mg content in Al/SiO(2) system using an extended Miedema model. There is a linear and parabolic variation in Mg with respect to the temperature for MgAl(2)O(4)/MgO and Al(2)O(3)/MgAl(2)O(4) equilibria, respectively, and the influence of Si and Cu in the two equilibria is not appreciable. The experimental verification has been limited to MgAl(2)O(4)/MgO equilibria due to the high Mg content (>= 0.5 wt pct) required for composite processing. The study has been carried out on two varieties of Al/SiO(2) composites, i.e., Al/Silica gel and Al/Micro silica processed by liquid metallurgy route (stir casting route). MgO is found to be more stable compared to MgAl(2)O(4) at Mg levels >= 5 and 1 wt pct in Al/Silica gel and Al/Micro silica composites, respectively, at 1073 K. MgO is also found to be more stable at lower Mg content (3 wt pct) in Al/Silica gel composite with decreasing particle size of silica gel from 180 micron to submicron and nanolevels. The MgO to MgAl(2)O(4) transformation has taken place through a series of transition phases influenced by the different thermodynamic and kinetic parameters such as holding temperature, Mg concentration in the alloy, holding time, and silica particle size.
