The mechanisms of long fatigue crack growth behaviour in Al–Si casting alloys at room and elevated temperature

Abstract

Pistons are commonly made from multicomponent Al–Si casting alloys, which have complex, interconnecting three-dimensional (3D) networks of secondary phase particles. They are non-serviceable parts and so must be able to withstand high cycle fatigue while operating at temperatures between 30 and 80% of T_m. Long fatigue crack growth tests were performed at room temperature (RT) and 350°C to assess the micromechanisms of fatigue. The fracture profiles at low and high da/dN were analysed; at low crack growth rates at both temperatures there is no crack path preferentiality with respect to the microstructure. At high da/dN in the RT sample the crack growth occurs preferentially via hard particles, while at 350°C there is a change in mechanism and the crack appears to avoid hard particles. X-ray tomography has been used to image the crack tips and gain a detailed insight into the mechanisms of fatigue in these complex 3D microstructures.