Interactions with defects, like dislocations and inter­faces, play a dominant role in the fate of He in complex multiconstituent, multiphase structural alloys. MD, MS, and ab initio calculations have been carried out to characterize the interactions of He with Fe SIA and their clusters.159,270,271 These simula­tions revealed various reactions-interactions involving (a) a spontaneous recombination-replacement reac­tion, where a single SIA replaces a Hes atom leaving a Hei; and, (b) SIA cluster trapping-detrapping reac­tions with single Hes and Hei atoms, as well as with small He clusters. The simulations also showed that small SIA clusters are strongly bound by a single Hei and Hei clusters with high Eb from 1.3 to 4.4 eV. Such trapping significantly retards the primarily one dimensional motion of SIA clusters, which oth­erwise are highly mobile with a migration energy of less than 0.1 eV in pure Fe.

Interactions of He with microstructural features such as dislocations, GBs, and nanometer-scale pre­cipitates have also been modeled.136 The Dimer method272 was used to efficiently identify saddle point activation energies that were then used in MD simulations to observe interaction mechanisms and reaction paths. Energy landscapes for He around dis­locations were modeled for (a/2)[111] and [—1-12] edge dislocations as well as (a/2)[111] screw disloca — tions.273,274 Similar calculations have also been carried out for four symmetric tilt boundaries with a common (101) axis (£3(112), £11(323), £9(114), and S3 {111}) using a two-part rectangular cell.275 The interactions of He with coherent nanometer-scale Cu precipitates have also been modeled.136

Figure 39(a) shows the Hes Eb and excess volume per unit area (Vex) for edge and screw dislocations as a function of the distance from the core. The maximum Eb is much larger for edge («0.5 eV) compared with screw dislocations («0.15 eV). The Eb closely corre­late with the Vex. As shown in Figure 39(b), a similar relationship is found for different GBs. The Eb of substitutional and interstitial He varies from «0.2 to «0.8 eV and «0.6 to «2.7 for screw and edge disloca­tions, respectively, increasing linearly with increasing Vex. Figure 39(c) shows the Eb for both a single vacancy and a Hes atom as a function of distance from a 2 nm coherent Cu precipitate.136 Both the Hes and a single vacancy have very similar energy — distance relations, with the maximum Eb « 0.6 eV at the precipitate surface. Table 4 summarizes the results for these extended defect models. The qualitative implications of these results are that Hei is strongly trapped at various common microstructural features, while Hes is more weakly trapped. Thus, it is likely that detrapping of Hei involves a Hei + V! Hes reaction.