On the role of brittle interfacial phases on the mechanical properties of carbon fibre reinforced Al-based matrix composites

High-resolution microfractography performed in combination with transmission electron microscopy shows that the mechanical behaviour of carbon fibre reinforced Al-based matrix composites is related to the presence of brittle interfacial phases. Attaining the required specifications in the unreinforced part of cast aluminium aeronautical components locally reinforced with a carbon preform necessitates the use of a highly alloyed Al-based matrix. Moreover, the precipitate hardening heat treatment appropriate to the unreinforced alloy must be applied to the whole component. The two composites M40J/A357 and M40J/Cpyr/A357 (pyrolytic carbon coated fibres), either as-cast or after the usual T6 heat treatment of the A357 alloy (16 h solution treatment at 540°C followed by water quenching and 6 h ageing at 160°C) have been investigated. The formation of Si and Al4C3 interfacial brittle phases resulting from both the dendritic solidification and the solution treatment leads to the premature failure of the composite.