  We report the mechanism of cleavage crack deviation at grain boundaries. A
fracture mechanics based model has been developed to combine the effect of tilt
and twist angles between cleavage planes on the reduction in normalized energy
release rate. Furthermore, electron back-scatter diffraction analysis on
cleavage fracture surface has been carried out. The microstructural features
associated with the presence of tilt and twist angle at the grain boundary have
been identified and found to be consistent with the prediction from the
analytical model.
