Correlation between microstructure and fatigue cracks in cast aluminium alloy A205

Abstract: Materials research in the aerospace industry is, among other things, focused on reducing the weight and increasing the life of components. Materials with high strength and long lifespan are therefore examined. A205 is an aluminium alloy with promising properties and the work in this thesis aims to gain an increased understanding of fatigue properties for the material in question. This has been done by studying how fatigue cracks are initiated and grow relative to the current microstructure. Four specimens of A205 have been tomographed before and after the initiation of a fatigue crack. The tomography imaging is performed with high resolution so that the microstructure can be studied. Using the tomography images, the crack has been characterised in three dimensions regarding initiation and propagation and its relation to the microstructure. Furthermore, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) have been used to study the fracture surfaces of the specimens and to determine the chemical composition of different precipitates and phases to further deepen the understanding of fatigue properties for the alloy. Correlation technique based on tomography images, so-called digital volume correlation (DVC), enables examination of small displacements and strains in the structure of the specimens, which provides an understanding of the plastic zone size at the tip of the fatigue crack. The thesis discusses how the experimental methods can be combined and provide an increased understanding of the initiation and propagation of the fatigue crack in A205. Results is related to previous research and in combination with experimental study this can develop the general understanding of the aluminium alloy A205. Fractography analysis shows that the crack is initiated by TiAl3 in two out of four specimens. To summarize, the results of the study indicate that defects highly influence the initiation of the cracks compared to propagation that are less sensitive. Crack propagation are controlled by stress intensity, low stress intensity promotes transgranular propagation and high stress intensity promotes intergranular propagation.