Metal matrix composite reinforced with co-continuous Al–Al 2 O 3 fibres produced in situ by reaction of an aluminium matrix with fossil SiO 2 spicules

Aluminium metal matrix composite (MMC) reinforced with fossil silica fibres was produced by a powder metallurgy extrusion route. The fibres, silica rich spicules known as spongilites, come from abundant natural geological deposits in Brazil. Further processing the MMC by heat treating at high temperatures converts the silica fibres into an interlaced (Al–Si)/Al₂O₃ microcomposite structure, retaining the original fibre morphology. The new co-continuous microstructure of the fibres is a result of a reduction or displacement reaction, where the silicon released from the silica reduction forms at first a liquid Al–Si phase around the fibres and later on diffuses into the solid matrix. The fine internal microstructure of the fibres was studied by FEG-SEM and optical microscopy both on polished and fractured surfaces. Tensile properties of the MMC before and after heat treatment were measured, showing a considerable increase in UTS. Analysis of the fracture surface of the heat treated ruptured specimens showed necking (bridge formation) in the metallic phase of the fibre and no visible pullout.