Interaction of microcracks with selected interfaces: Focused ion beam for a systematic crack initiation

The newly introduced combination of focused ion beam technique with high resolution scanning electron microscopy is the first method to initiate artificial microcracks with idealized crack parameters in front of selected phase- and grain boundaries. The boundaries of interest are selected after a complete sample characterization by Electron Back Scatter Diffraction and a calculation of the preferred slip systems within each grain. This combination of characterizing- and manipulating techniques on a microscale enables for the first time a systematic investigation of the interaction of short cracks with microstructural barriers to check the models of Tanaka, Navarro and De Los Rios quantitatively. In this paper, different notch geometries from single lines to penny shaped surface cracks for the initiation of artificial microcracks are presented. It is shown that not only the starting point of a crack can be selected. Further, the crack propagation path can be predetermined by initiating the crack directly on a slip system with a high Schmid factor. As a first result the influence of grain boundaries with different misorientation angles on the crack propagation rate for a directionally solidified nickel-based superalloy is shown. Finally this method is discussed as a powerful technique for grain boundary engineering.