fig3

Figure 3. Schematic illustration for the formation and evolution of defects near the crack tips under different mechanisms of dislocation nucleation at the crack tips in FCC Al. (A) The initial dislocation nucleation at the crack tips accompanied by the formation of SF zone and a surface step on the dislocation propagation plane; (B) The back twinning dislocation nucleation at the crack tips, with the dislocation nucleation occurring on the slip plane adjacent to previous dislocation and the corresponding SF zone transforms into the twinning zone with stepwise shape due to the position distinctions in dislocation nucleation at the crack tips; (C) The forward twinning dislocation nucleation at the crack tips, with the dislocation nucleation occurring on the slip plane at the right of the first partial dislocation accompanied by instability of the crack tips and the corresponding twinning zone extended toward the crack tips. FCC: Faced-centered cubic; SF: stacking fault.