TEM study of carbon fibre reinforced aluminium matrix composites: influence of brittle phases and interface on mechanical properties

The microstructure of A357 aluminium alloy (7 wt% Si+0.6 wt% Mg) reinforced by 1D-M40J carbon fibres is characterised using different techniques of transmission electron microscopy (diffraction, HRTEM, EDX, EELS). The microstructure of the high modulus PAN based fibre and of the pyrolytic carbon coating (Cp) is fully characterised. Silicon and Mg2Si grains which determine matrix-reinforcement adhesion are investigated. On the basis of the microstructural features, the mechanical properties of the composites are discussed. The mechanical behaviour of composites prepared with and without Cp interphases corresponds to a brittle matrix reinforced by brittle fibres. In the case of the composite without Cp interphase, the most influent parameter is the high resistance to sliding at the interface between silicon and fibres which leads to a strong fibre-matrix “bonding” and thus, to a weak and brittle material. The interfacial resistance to decohesion and to sliding is lower in the composite with Cp interphase resulting in higher strength and limited pull out. This lower interfacial resistance is due to the successive microporous and layered microstructures of the pyrolytic carbon coating.