Thermodynamics and Kinetics of the Formation of Al2O3/ MgAl2O4/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.

Prospects of In-Situ α-Al2O3 as an Inoculant in Aluminum: A Feasibility Study

“…However, new AlB 2 precipitates in the alloy may act as nucleants in the Al-Si alloys containing more than 4 wt.% Si (liquidus is <659°C). The dissolution-precipitation mechanism explained for AlB 2 doesnÕt hold true for AlB 12 as the compound remains undissolved at longer period of time, which is substantiated in the present study. AlB 2 seems to be the rarest compound in the present study, which negates the possibility of it being an active nucleant for grain refinement.…”

Microstructural Evolution and Property Evaluation of In Situ Al–MgAl2O4 Nanocomposite Prepared by γ‐Al2O3 and Ultrasonication‐Assisted Impeller Mixing

“…[3] Important thermodynamical calculations and experimental studies verified the formation of MgAl 2 O 4 in Al at varying Mg content as low as 0.1 to a maximum of 10 wt%. [17] Following reaction is thermodynamically feasible at a range of temperatures: [18] The tetrahedral vacancies are largely unoccupied due to high activation energy required to move Al 3þ cations from octahedral to tetragonal vacancies. In α-Al 2 O 3 , O 2À anions are hexagonal close packed (ABAB) and all the vacancies are occupied by Al 3þ cations.…”

“…[ 3 ] Important thermodynamical calculations and experimental studies verified the formation of MgAl 2 O 4 in Al at varying Mg content as low as 0.1 to a maximum of 10 wt%. [ 17 ] Following reaction is thermodynamically feasible at a range of temperatures:Crystallographically, MgAl 2 O 4 consists of a cubic closed packed (ABCABC) O 2− anion sublattice and the octahedral and the tetrahedral vacancies are filled by Al 3+ and Mg 2+ cations, respectively. The γ‐Al 2 O 3 demonstrates a spinel‐like crystal structure where octahedral vacancies (65% of total vacancies) are occupied by Al 3+ cations on ccp O 2− anion sublattice.…”

Role of matrix alloy chemistry on the interfacial reaction and solidification of metal matrix composites

“…x Al and x Si represent the atom fraction and J Al and J Si represents the activity coefficient of aluminum and silicon respectively. In [11,12].…”