respective pile-ups, as calculated elsewhere [5], are shown in Figure 2. This is of some significance and suggests a connectivity of scale over this range as discussed below. 1 10 100 10 100 1000 70 nm tip 700 nm tip Vickers Averaging 10 areas using total numbers spacing, nm τ R or τ max }cracks , MPa Figure 2. The average dislocation spacing variation with the resolved stress on the slip planes at the crack tips or the maximum shear stress under the indenter. The data range in the 70 and 700 nm tip data are based upon the densest lip band (upper points) and average density of all planes (lower points). RESULTS AND DISCUSSION First, we reexamine some hydrogen embrittlement data in Fe-3wt%Si single crystals. We note that this undergoes discontinuous cracking with single forward crack increments of about 0.74 ± 0.12 µm. Such extensions occurred for both externally supplied (1 atm.) or internally supplied (cathodically charged) hydrogen. An example of the latter, not previously published, is shown in Figure 3. Rather than attempting to reexamine kinetics, which is far too complex [14], we will address threshold in the context of the dislocation model presented above. As discussed elsewhere [5] in this volume, we demonstrate that the lead dislocation for a 22 dislocation pile-up would give a value of 3.2 nm for the “DFZ” which is hardly worth calling a zone. However, Li [15] has recently shown in simulations that additional slip bands emitting can produce a “DFZ” much larger than if a single band is emitting. In fact, a DFZ ranging from 3 to 16 times as large as the distance between the first and second dislocations in front of the DFZ exists. Even though our dislocation arrangements are not identical to the one used [15] we interpreted our 79.5º orientation of slip bands in the context of Li's simulations. From those simulations, it was found that the DFZ could be 15 times as large as our calculated value of 3.2 nm. This would represent a DFZ of 48 nm which is c in Eqn. (2). It is important to note that he used 15 emitting slip planes, quantitatively similar to those previously observed. We used this to determine fracture toughness for ‹100›Fe-3wt%Si crystals tested at temperatures from 100 to 298ºK.
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