New aluminium matrix composites manufactured by an in-situ process

During the last two decades, several researches were devoted to synthetic aluminium matrix composites. 20 years after, their field of applications is very restrictive. Many reasons can explain this fact: (1) the constituents are expensive (mainly the fibres), (2) the quality of fabrication is difficult to get mainly due to the non-wetting behaviour of the reinforcement-matrix couple, (3) fibre and matrix are not in thermodynamic equilibrium and the chemical reaction is always present during the fabrication of the composite or in service. These materials are thus not at all stable and expensive. In this work, we propose the preparation of an Al matrix composite by the in-situ precipitation of the reinforcing phase. This material has the advantage to be stable and recyclable. Due to the low melting point of aluminium, the foundry technique has been chosen for the preparation of these composites. The difficulty was to find the right reinforcement. Thus, a selection of the material procedure has been found to be very powerful and useful to find without any a priori a new reinforcement for the aluminium alloy matrix. Fe and Cr powders were selected for their ability to react with the Al-Si hypereutectic alloy. Elongated precipitates of intermetallic phase Al₄FeSi or Al₉Fe₂Si₂ were in situ formed in the Al-Si matrix by stir casting. The aspect ratio (L/d) is quite important (25 to 100) and the volume fraction of the crystals is in the range of 15% in this work. The platelet shape of the crystals is supposed to improve the creep behaviour and stiffness of the Al alloy. The reaction with Cr leads to the precipitation of large crystals of the cubic Al₁₃Si₄Cr₄ compound. This composite seems to be more appropriate for tribological applications.