Damage mechanisms of cast Al–Si–Mg alloys under superimposed thermal–mechanical fatigue and high-cycle fatigue loading

Out-of-phase thermal–mechanical fatigue (TMF) tests with superimposed higher frequency mechanical fatigue (HCF) loading were performed at three cast Al–Si cylinder head alloys. All materials show for maximum temperatures of the TMF cycle between 200 and 250 °C a threshold of the amplitude of the superimposed HCF loading below of which the number of TMF cycles to failure is not significantly affected by the superimposed HCF loading. Above this threshold the TMF life shows a strong decrease with increasing HCF loading. Extensive microscopic analyses revealed that independent from the HCF loading first cracks are initiated at eutectic Si-particles detached from the surrounding α-Al matrix. Crack propagation occurs at small HCF amplitudes along the eutectic regions. At higher superimposed HCF loadings an accelerated crack propagation along slip bands within the α-Al matrix is observed.