Low Cycle Fatigue in Model Al-Si Alloys. A Contribution to the Understanding of Cyclic Damage in Two Phase Materials

Abstract

Low cycle fatigue of porosity free Al-Si alloys containing between 11 and 18 wt.% Si and produced through directed solidification has been investigated. All alloys contain acicular Al-Si eutectic as the principal element of microstructure, completed by different amounts of primary (Al) and (Si) phases. Crack initiation and propagation modes have been determined for all alloys: crack initiation is always associated with brittle Si particles while propagation takes place across layers of the ductile aluminium which act as effective microstructural barriers. A simple energetic model allows a semi-quantitative interpretation of experimental results concerning damage evolution at the surface: single or multiple cracking. LCF data are analysed both on the basis of Coffin-Manson relation and taking into account the effect of the maximal stress on the fatigue life. The second approach gives a coherent and complete interpretation of experimental results in all investigated materials. The fatigue life of two phase Al-Si model alloys is determined by a combination of the macroscopic response of alloys to cyclic straining which depends on the overall microstructure, including phases which do not participate directly in fatigue, and of local parameters which act at the level of short crack propagation. Since the damage mechanisms at the microstructure size scale are the same in all investigated alloys, the parameter which really determines the fatigue life is the maximal stress. Concerning the effect of microstructure, it is emphasized that it is necessary to take into account both extreme and average values of parameters associated with microstructure elements which effectively play a role in fatigue. Finally, it is shown that the conclusions of the present work can be easily generalised to the fatigue of various single and two phase materials, the unifying element being the physical nature and the resistance of microstructural barriers to the propagation of short cracks.