Mechanical properties and hardness gradation of Al/Al₂O₃ composite bodies obtained by reaction between SiO₂ and molten Al

It is known that silica (SiO₂) is converted into Al/Al₂O₃ composite body, according to the following reaction: 3SiO₂(S)+4Al(l)=2Al₂O₃(s)+3Si(in Al) In this study, composite bodies were fabricated at different reaction temperatures and their mechanical properties were investigated. The composite bodies fabricated below 1273 K had high hardness and revealed a brittle nature. On the other hand, in the composite bodies fabricated above 1273 K, hardness decreased and ductility increased a little. It is possible that plastic deformation occurs in the widened Al part in the coarse composite structures, and this is related with the decrease in hardness. The gradation of hardness in the composite bodies was attempted by changing the reaction temperature. A silica rod was dipped into a molten Al bath at 1073 K, and the temperature was increased to 1373 K. There was no hardness variation along the radial direction in the cross section of the rod. When the reaction temperature was decreased form 1373 K to 1073 K, hardness was graded to be larger in the core area. However, the maximum hardness was not high as expected from the non-graded (reacted at constant temperature of 1073 K) specimens. By lowering the reaction temperature together with moving the specimen (gradual plunging), hardness gradation along the longitudinal direction was accomplished and large hardness difference existed. It was found that the cause for high hardness was related with the formation of nano-scaled composite microstructures containing low temperature (θ, γ)-Al₂O₃ phases.