Figure 2

(a) Crack tip plasticity consists of the following subsequent processes: (i) the nucleation of dislocations, (ii) their glide through the crystal and (iii) their interaction with long-range obstacles such as grain boundaries. In this paper we assess the impact of the following three parameters on toughness and the BDT temperature: (i) the mean free path for dislocation glide, \(\delta \), combined with (ii) the obstacle force of the grain boundary, \(\phi \), both visualized in subfigure (a). In subfigures (b) and (c), the dislocation source spacing along the crack front, \(\lambda \), and the process of nucleation of dislocation half-loops and their expansion and coalescence are demonstrated for a large source spacing (b) and a small one (c). After merging of the half-loops, the whole crack front is shielded. Dislocation sources along the crack front, can be discriminated into intrinsic sources, which operate within the grains, and extrinsic sources, as the intersection points of grain boundaries with the crack front. The dislocation source spacing, \(\lambda \), impacts on the distance at which the half loops are merged, \({d}_{shielding}\).