Synchrotron X-ray 3D characterisation of fatigue crack initiation during in-situ torsion cyclic tests

This paper focuses on the characterisation of fatigue crack initiation mechanisms under fully reversed torsional loads for the porosity-containing cast AlSi7Mg0.3 aluminium alloy by synchrotron X-ray tomographic imaging and Diffraction Contrast Tomography (DCT). The aim is to analyse the relation between crack initiation and the microstructure, in the fatigue regime close to the fatigue limit of the material (fatigue lives of approximately 2 x 106 cycles). In-situ torsion fatigue tests have been conducted at the European Synchrotron Radiation Facility. A large number of cracks (approximately 80 cracks) were analysed, at the surface and in the bulk, highlighting the role of the surrounding microstructure on the crack initiation mechanisms. Contrary to uniaxial loads in which fatigue crack initiation from casting pores in an opening mode (mode I) is generally only observed for this material, it is shown in this work that multiple crack initiations mechanisms can appear simultaneously under torsional loads. This is very interesting because it is possible to analyse different mechanisms (and the effect of the surrounding microstructure) using the same specimen, that is under identical loading conditions. More precisely, two different crack initiation mechanisms were observed. The first mechanism involves intra-granular fatigue crack initiation from Persistent Slip Bands (PSBs) formed on the slip system with the highest Schmid factor. For this mechanism, the grain orientation is the key parameter. A shear stress threshold of approximately 80 MPa is observed to trigger this mechanism. The presence of a pore, in the crack initiation zone reduces this threshold. The second mechanism concerns crack initiation from pores, in mode I, on planes of maximum principal stress. No link to the local grain orientation was found in that case. The pore size is the key factor governing this crack initiation mechanism. Regarding the competition between these mechanisms in the formation of the final crack, it was observed that at high applied stress, the cracks initiated from PSBs are dominant due to their high density and high growth rates resulting from coalescence. At lower stress, cracks initiated from pores are dominant, mainly because of the little effect of grain boundaries and eutectic zones, while most of cracks initiated from PSBs are arrested within the grain where they have initiated.