Interaction between a 1/2(110){111} edge disloca­tion and a 1/2(110) SIA loop in Ni was first studied by Rodney and Martin,70 and then followed a series of more detailed investigations involving glissile and sessile SIA loops and screw and edge disloca — tions.16,70-74 Reaction R3 was observed with small loops having bL = 1/2(110) intersecting the disloca­tion slip plane.70,75 Glissile clusters were attracted by the dislocation core and absorbed there ather — mally, creating a pair of superjogs. Superjog seg­ments have different structure, as depicted in Figure 14, and to accommodate this a few vacancies are formed, as in the case of the R3 reaction with an SFT (see Figure 12). Each superjog has different mobility, for example, the Lomer-Cottrell segment on the left of Figure 14(b) has high Peierls stress. Therefore, although the jogged dislocation con­tinues gliding under applied stress, it has a signifi­cantly lower velocity because it now experiences higher effective phonon drag.71

An interesting example of an R2 reaction was observed during interaction between a 1/2(110) screw dislocation and a 1/3(111) Frank loop.16 If the loop is not too large and the dislocation is not too fast, the dislocation can absorb the whole loop into a helical turn (see stages in Figure 15). The turn can expand only along the dislocation line and, therefore, if applied stress is maintained, the helix constricts; finally a perfect DL, with the same b as the dislocation, is released as the screw breaks away.

Note that the same mechanism occurs when a screw dislocation intersects a glissile 1/2(110) loop with Burgers vector different from that of the dislocation, that is, absorption into a helical turn and release of a loop with the same Burgers vector as the disloca — tion.76 In the case when the Frank loop is large or the dislocation (either screw or edge) is fast, the dislocation simply shears the loop and creates a step on its surface (see Figure 8 in Rodney16). The probability of this reaction is higher for an edge dislocation, whereas transformation of the loop into a perfect loop is more probable for the screw dislocation.73

The strengthening effect due to dislocation-SIA loop interactions can be significant, especially when a helical turn is formed on a screw dislocation. The total contribution of dislocation-SIA loop interaction to the flow stress under irradiation has not been considered so far because of the large number of possible reactions and the sensitivity of their out­comes in terms of mechanism and strengthening effect to parameters such as interaction geometry, loop size and Burgers vector, strain rate, and temper­ature. An estimate of the flow stress contribution for the case of reaction R2 in Ni was made in Rodney and Martin.75

1.12.4.2.2.1 Body-centered cubic metals

Many atomic-scale simulations of interaction between a 1/2(111){110} dislocation and an SIA loop in bcc metals have been made, particularly using EAM potentials for Fe. They have shown that when the defects come into contact, a new dislocation segment is formed with one of two possible Burgers vectors 1/2(111) and (100), and further evolution depends on features such as loop size, b, position, dislocation character, temperature, and strain rate. Thus, many different outcomes have been observed by atomic scale modeling, but here we can present only a few common examples.

Reaction R3 is most common for small SIA loops, that is, loop absorption on the dislocation line resulting in a pair of superjogs. It occurs when the loop is initially below the dislocation slip plane (tension region of the dislocation strain field) and bL = 1/2(111) is inclined to the slip plane. An important feature of the interaction is the ability of such loops to glide quickly toward the core of the approaching dislocation. SIA loops up to 5 nm in diameter have been simulated.77-80 When small loops (a few tens of SIAs) reach the core, they are fully absorbed athermally creating a double superjog of the size equivalent to the number of SIAs in the

Figure 15 Unfaulting of a 1/3(111) Frank loop by interaction with a screw dislocation in Ni at 300 K and transformation into a helical turn on the dislocation line: (a) initial configuration, (b) first cross-slip, (c) and (d) successive cross-slip events,

(e) configuration at the end of unfaulting, (f) configuration after relaxation and elongation of the helical turn. From Rodney, D. Nucl. Instrum. Meth. Phys. Res. B 2005, 228, 100. Copyright (2005) with permission from Elsevier.

loop. This process does not pin the dislocation and these loops are very weak obstacles to dislocation glide.77 Large loops (more than ^100 SIAs) also glide to make contact with the dislocation line but are not absorbed athermally. Instead, a new disloca­tion segment is formed due to the following energet­ically favorable Burgers vector reaction between the dislocation and loop:

2[111]-1[1T1] = [010] [2]

Five stages of the interaction are presented in Figure 16 for a 5-nm loop containing 331 SIAs. The Burgers vectors are indicated and Figure 16(b) corresponds to the occurrence of the reaction of eqn [2]. The new segment with b = [010] cannot glide in the dislocation slip plane (110) and therefore acts as a strong obstacle to further glide of the dislo­cation. Under increasing applied strain, the dislocation bows out until two long segments of 1/2[111] screw dislocations are formed as shown in Figure 16(c). Figure 16(d) shows the same configuration in [111]

projection. High stress at junctions connecting the dislocation line and loop, the remainder of the origi­nal loop, and the new segment induces the latter to slip down on the (101) plane, and glide of this [010] segment over the loop surface results in the following reaction with the remaining loop:

2[1І1] + [010] = 2[111] [3]

This concludes with the formation of a pair of super­jogs on the original dislocation and results in com­plete absorption of the 5 nm loop. Large loops are strong obstacles in this reaction, stronger than voids with the same number of vacancies.4,80

It should be noted, however, that the interaction just described depends rather sensitively on temper­ature because of the low mobility of screw segments in the bcc metals. Cross-slip of the screw segments is required to allow the [010] segment to glide down. Simulations at T = 100 K showed that although the stages in Figure 16(a)-16(c) still occurred, the [010] segment did not glide under the strain rate imposed in MD and the screw dipole created by the bowing dislocation was annihilated without the loop being transformed according to eqn [3]. The resulting reac­tion was of type R1, for both dislocation and loop were unchanged after dislocation breakaway.78 More details and examples can be found.4,77-82

Competition between reactions R1, R2, and R3 for a 1/2(111){110) edge dislocation and 1/2(111) and (100) SIA loops has been considered in detail.83,84 We cannot describe all the reactions here but some pertinent features are underlined; note that the favorable Burgers vector reaction between a 1/2(111) dislocation and a (100) loop results in a 1/2(111) segment, for example,

І[111]+[Г00]=І[Г11] [4]

Thus, a perfect loop with bL = [100] can be converted into a sessile complex of 1/2 [111] and 1/2 [111] loop segments joined bya [100] dislocation segment.83 Similar conjoined loop complexes were observed in simulations of interactions between two glissile 1/2(111) loops85,86 in bcc Fe. Competition between reaction R3 on one side and R1 and R2 on the other was discussed in Bacon and Osetsky. 2 The earlier conclusion that small loops (< 1 nm) can be absorbed easily and not present strong obstacles to dislocation glide was confirmed. The strength and reaction mechanism for larger loops de­pends on their size and the loading conditions. At low T< 100K, both 1/2(111) and (100) loops are strong obstacles that are not absorbed by 1/2(111) dislocations. As with obstacles that result in the true Orowan mecha­nism, the dislocation unpins by recombination of the screw dipole, and the critical stress is determined by the loop size, similar to eqn [1].At higher Tand/or low strain rate (<107s_1), the mechanism changes from Orowan — like bypassing to complete loop absorption, irrespective of bL. The absorption mechanism for R3 involves propa­gation of the reaction segment over the loop surface. This requires cross-slip ofthe arms ofthe screw dipole drawn out on the pinned dislocation and involves dislocation reactions. Thermally activated glide and/or decomposi­tion of the pinning segment, in turn, depends on loop size, temperature, and bL. Therefore, the obstacle strength of a 1/2(111) loop is, in general, higher than that of a (100) loop because a (100) dislocation segment associated with the former (see eqn [3], Figure 16 and related text) is much less mobile than a 1/2 (111) segment involved in reactions with the latter (eqn [4]).

Much less is known about interaction mechanisms involving screw dislocations in bcc metals. We are aware of only two studies reported to date that con­sidered 1/2(111) and (100) SIA loops.87,88 The main feature in these cases is the ability of a 1/2(111) screw dislocation to absorb a complete or part loop into a temporary helical turn before closing the turn by bowing forward and breaking away. As in the case of fcc metals described above, this provides a power­ful route to reorienting the Burger vectors of differ­ent loops to that of the dislocation. An example of such a reaction is visualized in . Other

reactions observed in these studies87,88 include a reforming of the original glissile loop into a sessile complex of two segments having different b, as described above (reaction R2), and complete restora­tion of the initial loop (reaction R1).

As is clear from the examples discussed above, the obstacle strength associated with different mechan­isms can depend strongly on parameters such as loop size and Burgers vector, interaction geometry, Є and T Unlike the situation revealed for inclusion­like obstacles discussed in Section 1.12.4.1, there does not appear to be a simple correlation between size and strength for loops. Some data related to par­ticular sets of conditions can be found in Terentyev et a/.,83,84,88 as well as a comparison of the obstacle strength of voids and DLs.84 There are cases when loop strengthening is compatible to or even exceeds that of voids containing the same number of point defects, making DLs an important component of radiation-induced hardening.