Thermodynamics and kinetics of the formation of Al(2)O(3)/MgAl(2)O(4)/MgO in al-silica metal matrix composite

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.