Ductile–brittle behavior of (0 0 1)[1 1 0] nano-cracks in bcc iron

We carry out atomistic simulations and stress analysis of crack behavior in bcc iron under Mode I, quasi-static loading. We observe that the formation of unstable stacking faults at the crack tip of a relatively long crack (∼16 nm) cannot prevent unstable crack extension at the Griffith level. A crack of smaller dimensions (∼4 nm) produces twinning on the 〈1 1 1〉{1 1 2} slip system ahead of the tip, which gives rise to a slow ductile growth during subsequent quasi-static loading. The influence of the T-stress on the shear processes at the crack tip and crack stability is discussed.