Once dislocation sources are unlocked from their decoration atmospheres causing a yield drop, they additionally interact with the surrounding random field of defect clusters (e. g., vacancy loops or SFTs). We investigate here the interaction between emitted F-R dislocations and vacancy-type defect clusters as a possible mechanism of radiation softening imme­diately beyond the yield point. Numerical computer simulations are performed for the penetration of undissociated slip dislocation loops emitted from active F-R sources (i. e., unlocked from defect dec­orations) against a random field of SFTs or sessile Frank loops of the type: 1(111){111}. At the present level of analysis, there is no distinction between SFTs and vacancy loops as they are modeled as point obsta­cles to dislocation motion that can be destroyed once an assumed critical force on them is reached. The long-range elastic field of these small obstacles is ignored. The random distribution of SFTs is gener­ated as follows: (1) The volumetric density of SFTs is used to determine the average 3D position of each generated SFT; (2) A Gaussian distribution function is used (with the standard deviation being 0.1-0.3 of the average spacing) to assign a final position for each generated SFT around the mean value. (3) The inter­section points of SFTs with glide planes are com­puted by finding all SFTs that intersect the glide plane. For simplicity, we perform this procedure assuming that SFTs are spherical and uniform in size.

Initially, one slip dislocation loop is introduced between two fixed ends and a search is performed for all neighboring SFTs on the glide plane. Subsequent nodal displacements (governed by the local velocity) are adjusted such that a released segment interacts with only one SFT at any given time. The interaction scheme is a dynamic modification to Friedel statis — tics,36 where the asymptotic maximum plane resis­tance is found by assuming steady-state propagation

of quasistraight dislocation lines. While Friedel cal­culates the area swept as the average area per particle on the glide plane, we adjust the segment line shape dynamically over several time steps after it is released from an SFT When a segment is within 5a from the center of any SFT, it is divided into two segments with an additional common node at the point of SFT intersection with the glide plane. The angle between the tangents to the two dislocation arms at the com­mon node is then computed, and force balance is performed. When the angle between the two tangents reaches a critical value of Fc, the node is released, and the two open segments are merged into one. If the force balance indicates that the segment is near equilibrium, no further incremental displace­ments of the node are added, and the segment of the loop is temporarily stationary. However, if a net force acts on that segment of the loop, it is advanced and the angle recomputed. It is possible that the angle between tangents will reach the critical value even though the segment is out of equilibrium. Sun eta/.37 have shown that the elastic interaction energy between a glissile dislocation and an SFT is not suffi­cient to transform the SFT into a glissile prismatic vacancy loop. They proposed an alternate mechanism for the destruction of SFTs by passage of jogged and/ or decorated dislocations close to the SFT. The energy released from recombination of a small frac­tion of vacancies in the SFT was estimated to result in its local rearrangement.

In the present calculations, we assume that vacan­cies in the SFT are absorbed in the dislocation core of the small contacting dislocation segment, forcing it to climb and form atomic jogs. With this mechanism, the entire SFT is removed from the simulation space, and jogged dislocations continue to glide on separate planes, thus dragging atomic-size jogs with them. Successive removal of SFTs from nearby glide planes can easily lead to channel formation and flow locali­zation in the channel, because the passage of consec­utive dislocation loops emitted from the F—R source is facilitated with each dislocation loop emission. The matrix density of SFTs in irradiated copper at low temperature (0.22-0.27 Tm) is taken from experimen­tal data (Singh et a/.38). Figure 5 shows the results of computer simulations for propagation of plastic slip emanating from a single Frank-Read source in cop­per irradiated and tested at 100°C (Singh et a/.38). The density of SFTs is 4.5 x 1023 m~3 and the aver­age size is 2.5 nm. In this simulation, the crystal size is set at ^1.62 pm, while the initial F-R source length is 1600a (^576 nm). A uniaxial applied tensile stress

along [100] (ffn) is incrementally increased, and the dislocation line configuration is updated until equilibrium is reached at the applied stress. Once full equilibrium of the dislocation line is realized, the stress is increased again, and the computational cycle repeated. At a critical stress level (flow stress), the equilibrium dislocation shape is no longer sustainable, and the dislocation line propagates until it is stopped at the crystal boundary, which we assume to be impene­trable. All SFTs interacting with the dislocation line are destroyed, and plastic flow on the glide plane is only limited by dislocation-dislocation interaction through the pileup mechanism. During the initial stages of deformation, small curved dislocation seg­ments unzip, forming longer segments, which are stuck until they are unzipped again by increasing the applied stress. It is noted that, particularly at higher stress levels, the F-R source dislocation elongates along the direction of the Burgers vector, as a result of the higher stiffness of screw dislocation segments as compared to edge components. The F-R source con­figuration is determined by (1) the character of its

initial segment (e. g., screw or edge), (2) the distribution of SFTs intersecting the glide plane, and (3) any other dislocation-dislocation interactions. This aspect is illu­strated in Figure 6, where two interacting F-R sources in copper are shown for a displacement dose of 0.01 dpa, an SFT density of 2.5 x 1023 m~3, and an average size of 2.5 nm. All other conditions are the same as in Figure 5. The two F-R sources are sepa­rated by 20a (~7.2 nm).