Volume 5, Issue 9 (October 2008)
Focused Ion Beam as New Tool for Local Investigations of the Interaction of Microcracks with Grain Boundaries
So far the interaction of microcracks with microstructural elements like grain boundaries or phase boundaries is described qualitatively well by the models of Tanaka and Navarro and De Los Rios. However, an experimentally verified quantitative description is missing due to a lack of systematic experiments. The concept by Marx and Schaef introduced a combination of a focused ion beam technique with high resolution scanning electron microscopy. It is the first method to initiate artificial microcracks with idealized crack parameters relative to selected phase and grain boundaries. This combination of characterizing and manipulating techniques on a micro-scale enables for the first time a systematic investigation of the interaction between short cracks and micro-structural barriers to check the models quantitatively. However, the geometry of the active slip planes in the adjacent grains, their misorientation angle, and the position of the grain boundary is a three-dimensional problem. Therefore the geometry was reconstructed after the crack propagation measurements by focused ion beam tomography to measure the inclination angles between the surface, the crack plane, and the grain boundary. The results should lead to a better understanding of the physical mechanisms of crack propagation through grain and phase boundaries, which is fundamental to explain the huge scatter in the short crack behavior and to develop materials with fatigue resistant grain boundaries.