The nature of microstructural defects involved in crack deviation and branching was further characterized using in situ SEM imaging of another SENB sample under stress. Similar behaviors and features were observed with fast crack propagation steps separated by arrests. With in situ imaging, the active cracks are open and can be directly visualized (Fig. ) whereas in a posteriori characterization, cracks are closed and their exact location in the complex contrasts of the microstructure is more difficult to determine (Fig. ). Although the sample used for in situ SEM was different from the one used in fluorescence mapping, similar microstructural features are associated to crack deviation and the microstructure are likely to play a role because platelets cannot be broken along a mechanically strong direction of the platelets. Thus the crack has to deviate along an easier path, the weaker aluminosilicate interphase (Figs. and). This is consistent with the results of numerical simulations that used the observed microstructures to show cracks propagate at higher stress around bundles of misoriented platelets, thus contributing to toughening the composite . These defects likely contribute to the increasing toughness (R curve) of the nacre-like alumina. High-stress gradients are clearly associated with defects, in spite of the intrinsic limitation that the observation is performed on the surface of a 3D sample.