Thermal fatigue behaviour of a 2014/Al2O3-SiO2 (Saffil Fibre) composite processed by squeeze casting

Thermal fatigue behaviour of a 2014/Saffil composite has been investigated. This composite was produced by infiltration of preforms of Saffil fibers (Al2O3-SiO2 fibers) with a 2014 aluminium alloy (Al-4.7Cu-1.0Si-0.6Mg). The composite samples, containing 13 vol.% of fibers, were sectioned and their microstructure was investigated by optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). Tensile tests, hardness and microhardness measurements were carried out. The fracture surfaces were examined by SEM. Dilatometry was used in order to measure the thermal expansion coefficient (CTE) and to evaluate the strength of matrix/fiber bonds. Thermal cycling tests were performed on specimens (either as fabricated or in the T6 temper) in the temperature range between 25 and 220°C. After 1000 cycles both the microstructure and the mechanical characteristics of the composite samples were investigated. The thermal fatigue resulted in a damage of the composite because of the partial fragmentation of the fibers. In addition, other phenomena involving the modification of composite matrix microstructure, such as over-aging of T6 treated samples and precipitation of strengthening phases within the matrix of as-cast samples, were observed. Thermal cycling also resulted in interfacial matrix/fiber reaction, which probably strengthened the composite and caused a CTE decrease. All these phenomena concurred to change both tensile behaviour and material hardness.