5 µm Figure 3. Fracture surface of Fe3%Si indicating discontinuous crack growth during electrolytic hydrogen charging. The yield strength and average fracture toughness values from the previous study [7] are given in Table 1 for five test temperatures. With a c = 48 nm, the yield strengths observed and a kIG of 0.85 MPa-m1/2 as taken from nominal surface energies [7], the agreement is seen to be good between observed and calculated KIc values. TABLE 1 Yield strength and fracture toughness, observed and calculated from Eqn. (2) using a value of c = 48 nm. T, ºK 100 173 233 298 363 σys, MPa 450 395 352 305 285 Obs. KIc, MPa-m1/2 6 23 50 120 > 120 Calc. KIc, MPa-m1/2 11.7 22.7 44.6 118 199 Regarding hydrogen effects, the one simple assumption is that the concentration levels involved do not affect the far field yield strength which dictates pile-up behavior. The second one is ad hoc and assumes that kIG through the surface energy is reduced to 0.70 MPa-m1/2. This is a slightly greater reduction than was found in a previous analysis [5]. This allows an equally good fit to previously published data [17]as seen in Figure 4. Returning to small volume behavior as represented by thin films, in a review of 14 metal/interlayer/substrate combinations, it was found that the dislocation stand-off distance scaled with (kIG/ σys) 2. Taking the Griffith value of 0.85 MPa-m1/2, for this Fe-3wt%Si single crystal, this correlation shown elsewhere [3] would give a value of c ~ 500 nm, an order of magnitude greater than the 48 nm found. This suggests that the longer length of slip bands available in single crystal or coarse grained solids allows greater shielding compared to nanocrystalline thin films. That is, it is well-documented that regular arrays of dislocations can shield the crack tip from the far-field stress. A balance of forces between the stress field of the dislocation array and the stress field of the stress intensity essentially increases the resistance to crack propagation. In the early stages of nanoindentation, this same balance of forces between tip-emitted dislocations and the stress field of the indenter increases the resistance to penetration. In fact, it has been recently shown [5] that
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