The effects of crack orientation on the twin formation from the crack tip in γ′-Ni3Al

The effect of crack's orientation on the twin formation from the crack tip in γ′-Ni3Al has been studied by fabricating three types of mode I crack. The simulation results show that (001)<110> type crack produces two slip bands at the crack tip; however, (1̄10)<110> and (11̄1̄)<110> type cracks produce twins rather than slip bands at the crack tip. The formation mechanism of the twin is the slipping of 1/6[112] type Shockley partial dislocations on consecutive {111} planes. The underlying reasons of the twin and slip band formation have been studied in term of the generalized stacking fault energy and shear stress, and the study results show that it can be attributed to the asymmetry of shear stress associated with (111)<112> slip system.