Evolution of MgAl2O4 crystals in Al-Mg-SiO2 composites

The present study analyzes the morphological transformations of reaction products i.e., MgO, MgAl2O4 occurring during the reaction between SiO2 and Al-Mg alloy in Al-Mg-SiO2 composite processed by the liquid metallurgy technique. Different phases of platelet and hexagonal morphologies are detected and their composition analysis by EDS has confirmed them as being transition phases existing between MgO, MgAl2O4 and Al2O3. This study has also revealed the gradual transformation of (i) MgO needles to octahedral MgAl2O4 through Mg-Al-Si-O and Mg-Al-O transition phases having platelet morphologies and (ii) MgAl2O4 to Al2O3 through hexagonal transition phases on holding of Al-5Mg-SiO2 and Al-1Mg-SiO2 composites respectively at 1023K. Fully developed &alpha;-Al2O3 crystals are not observed under the present experimental conditions, wherein the Mg content is well above the equilibrium Mg content required for the formation of stable Al2O3 (&lt;0.05 wt.&thinsp;%). <br /

Thermodynamics and kinetics of the formation of Al2 O3/ MgAl2O4/MgO in Al-Silica metal matrix composite

The formation of Al2O3, MgAl2O4, and MgO has been widely studied in different Al base metal matrix composites, but the studies on thermodynamic aspects of the Al2O3/ MgAl2O4/MgO phase equilibria have been limited to few systems such as Al/Al2O3 and Al/SiC. The present study analyzes the Al2O3/MgAl2O4 and MgAl2O4/MgO equilibria with respect to the temperature and the Mg content in Al/SiO2 system using an extended Miedema model. There is a linear and parabolic variation in Mg with respect to the temperature for MgAl2O4/MgO and Al2O3/MgAl2O4 equilibria, respectively, and the influence of Si and Cu in the two equilibria is not appreciable. The experimental verification has been limited to MgAl2O4/MgO equilibria due to the high Mg content (&ge;0.5 wt pct) required for composite processing. The study has been carried out on two varieties of Al/SiO2 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 MgAl2O4 at Mg levels &ge;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 MgAl2O4 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.<br /