V iet Duc Le

Arts et Métiers Institute of Technology, Angers, France



The objective of this work is to understand the effect of grain crystallographic orientation, the grain size and the porosity on the crack initiation mechanisms in torsion, not only on the surface but also in the volume. In order to achieve this objective, in-situ fatigue tests have been conducted in a synchrotron beamline at the ESRF in Grenoble, France. These experiments made it possible, on one hand, to obtain high resolution 3D tomography images of the microstructure in which we can observe the eutectic Si particles or micro-cracks. On the other hand, thanks to the Diffraction Contrast Tomography technique, the 3D images have been rendered which include information about the grains size, the grain morphology and orientation.

Two crack initiation mechanisms were observed. The first one concerns crack initiation inside grains from the PSBs. The second mechanism concerns crack initiation from pores. For the first mechanisms, crack initiation occurs in a shear mode and is greatly influenced by the grain orientation. A slight effect of the grain size is observed. The presence of a pore diminishes this effect and reduces the resolved stress threshold activating this mechanism. The crack initiation mechanism due to pores occurs in the opening mode. No link to grain orientation was found while the pore size is the key factor. In spite of the presence of a large number of pores, the number of cracks initiated in this mode is smaller than the ones initiated from PSBs and the crack growth rate is also lower.


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