Thermomechanical fatigue of short fibre reinforced aluminium matrix piston alloy

Abstract

The technical potential of short fibre reinforced aluminium matrix composites lies in their higher stiffness and higher strength at elevated temperatures compared with unreinforced matrix alloys. In the present investigation, thermal cycling creep tests were conducted on the piston alloy AlSi12CuMgNi reinforced with 20% Saffil (Al₂O₃) short fibres, to simulate the cold start conditions of combustion engines. After processing of the metal matrix composite (MMC) by direct squeeze casting, four heat treatment conditions were produced. Specimens under constant load were thermally cycled between 50 and 300°C, whereby a heating and cooling speed of 12.5 K s1 was achieved. Series of up to 5000 cycles at tensile stresses between 20 and 80 MPa were executed, comparing reinforced specimens and unreinforced matrix material. The results of these experiments showed that the creep properties of the alloy, especially minimum creep rate and lifetime to fracture, were improved by the reinforcement. Furthermore, the creep rate of the MMC was essentially independent of the heat treatment condition, whereas the minimum creep rate was increased significantly for the matrix material by overaging. It can be concluded that precipitation strengthening influenced the creep properties of unreinforced specimens only, which is in good agreement with theoretical considerations. An analysis of fibre length revealed that the majority of the fibres broke at between 50 and 75% of the lifetime, just before the beginning of tertiary creep. Metallographic investigations using a scanning electron microscope did not show fibre pullout, but multiple fracture of fibres along the whole specimen. Micromechanical models for isothermal creep in short fibre reinforced aluminium alloys confirm the above results, since tertiary creep is assumed to be a consequence of fibre fracture.