Synthesis of an Al/MgAl ₂ O ₄ <i>In situ</i> Metal Matrix Composite from Silica Gel

Abstract

An aluminum/MgAl 2 O 4 in situ metal matrix composite has been synthesized using silica gel containing ∼98% SiO 2 in an Al–5Mg alloy. The thermodynamics and kinetics of MgAl 2 O 4 formation have been discussed in detail. A transition phase of composition between MgO and MgAl 2 O 4 has been detected in the SEM‐EDS analysis of the particles extracted from the composite by a 25% NaOH solution. This confirms the gradual transformation of MgO to MgAl 2 O 4 by the reaction 3SiO 2 ( s )+2MgO( s )+4Al( l )→2MgAl 2 O 4 ( s )+3Si( l ). The stoichiometry, n , of MgAl 2 O 4 has been found to sustain close to 1 and the crystallite growth of MgAl 2 O 4 has been stopped at D ∼30 nm in the composites held at 750°C up to 10 h.