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Interfacial regions in metal matrix composites are important in controlling the mechanical and thermal properties of these materials. An ultrahigh modulus fibre-reinforced magnesium alloy matrix composite has been studied, with particular attention paid to the interfacial and precipitate microstructures. Fibres were surface treated but uncoated prior to composite manufacture. Observations revealed that an interface consisting of polycrystalline magnesium oxide with occasional Mg 17 Al 12 (b) precipitate particles predominates. Discontinuous b particles formed at fibre surfaces, and continuous spherical and lamellar b precipitates nucleated at grain boundaries and fibre surfaces. High dislocation densities exist at the interface indicating matrix-yielding subsequent to manufacture and that a high mean residual compressive stress acts on fibres. The effect that the observed microstructural features has on composite properties and on interfacial bonding is discussed and compared to examples in the literature.

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A randomly orientated short carbon ® bre reinforced magnesium composite consisting of Sigra® l C25 high strength carbon ® bres in a matrix of AZ91D magnesium alloy has been fabricated by indirect squeeze casting. This was subjected to tensile testing at room temperature, and 150, 200, 300, and 350°C. Composite properties have been compared to monolithic AZ91D. Both unreinforced and reinforced materials were heat treated to the T6 condition. Results indicated that composites exhibited good strengthening over unreinforced AZ91D, especially at temperatures above 200°C. Changes in strength with test temperature were re¯ected in fracture surface morphology. Characterisation of the failure modes and composite microstructure was carried out using optical, scanning-and transmission-electron microscopy. The more effective use of the reinforcing phase at elevated temperatures and the general behaviour of the short ® bre composite have been discussed in terms of features observed in the materials tested.MST/5394