Microstructural factors of low cycle fatigue damage in two phase Al–Si alloys

Low cycle fatigue (LCF) tests in air have been carried out on smooth specimens of several model binary Al–Si alloys with microstructures characterised by a high degree of homogeneity and the absence of casting defects, owing to the processing involving directed solidification. The elementary damage events, namely short crack nucleation at Al–Si interfaces and propagation across microstructural barriers (layers of eutectic Al), have been identified. The microstructure dependent fatigue damage evolves through single or multiple cracking, with significantly longer fatigue lives in the second case. The microstructural parameter responsible for the activation of one of two observed damage modes has been identified and quantified. This parameter is related to the maximal linear extension of Si particles at the surface and to the distance between Si particles, rather than to the average parameters given by conventional image analysis. The validity of this approach is enhanced by extending an existing model of tensile fracture in Al–Si–Fe alloys to the case of low cycle fatigue.