Effects on Mechanical Properties

Radiation-induced defects, being obstacles to disloca­tion glide, increase yield and flow stress and reduce ductility (see Chapter 1.04, Effect of Radiation on Strength and Ductility of Metals and Alloys for

experimental results). Furthermore, ifthe obstacle den­sity is sufficiently high to block dislocation motion, pre­existing Frank-Reed dislocation sources are unable to operate and plastic deformation requires operation of sources that are not active in the unirradiated state. These new sources operate at much higher stress and give rise to new mechanisms such as yield drop, plastic instability, and formation of localized channels with high dislocation activity and high local plastic deforma­tion. Understanding these phenomena is necessary for predicting material behavior under irradiation and the design and selection ofmaterials for new generations of nuclear devices.

Obstacles induced by irradiation affect moving dislocations in a variety of ways, but can be best categorized as one of two types, namely inclusion­like obstacles and those with dislocation properties.

The first type includes voids, bubbles, and preci­pitates, for example. They usually have relatively short-range strain fields and their properties may not be changed significantly by interaction with disloca­tions. (Copper precipitates in iron are an exception to this — see Sections 1.12.4.1.1-1.12.4.1.2.) Those that are not impenetrable are usually sheared by the dislo­cation and steps defined by the Burgers vector b of the interacting dislocation are created on the obstacle — matrix interface. Unstable precipitates, such as Cu in Fe, may also suffer structural transformation during the interaction, which can change their properties. These obstacles do not usually modify dislocations signifi­cantly, although they may cause climb of edge dis­locations (see Sections 1.12.4.1.1-1.12.4.1.2). Their main effect is to create resistance to dislocation glide. Obstacles such as voids and bubbles are among the strongest, and as a result of their high density, they contribute significantly to radiation-induced harden- ing.7 Materials designed to exploit oxide dispersion strengthening (ODS) are produced with a high con­centration ofrigid, impenetrable oxide particles, which introduce extremely high resistance to dislocation motion.8 These obstacles are also considered here as their scale, typically a few nanometers, is similar to that of obstacles formed under irradiation.

The second obstacle type consists of those with a dislocation character, for example, DLs and SFTs, and so dislocation reactions occur when they are encountered by moving dislocations. Loops have rel­atively long-range strain fields and hence interact with dislocations over distances much greater than their size. SFTs are three-dimensional (3D) struc­tures and have short-range strain fields. Loops with perfect Burgers vectors are glissile, in principle, whereas SFTs and faulted loops, for example, Frank loops in fcc metals, are sessile. In addition to causing hardening, the reaction of these defects with a gliding dislocation can modify both their own structure and that of the interacting dislocation. As will be demon­strated in Section 1.12.4.2, their effect depends very much on the geometry of the interaction, that is, their position and orientation relative to the moving dislo­cation, and the nature of the mutual dislocation seg­ment that may form in the first stage of interaction. The contribution of these obstacles to strengthening can be significant, for their density can be high.

Modification of irradiation-induced microstructure due to plastic deformation is an additional possibly important effect. If mechanical loading occurs during irradiation, it can contribute significantly to the overall microstructure evolution and therefore to change in material properties. Accumulation of internal stress during irradiation is unavoidable in real structural materials and so this effect should not be ignored. The effects of concurrent deformation and irradiation on microstructure are far from clear, for only a few exper­imental studies of in-reactor deformation have been performed.9 This area, therefore, provides a good example of how atomic-scale modeling can help in understanding a little-studied phenomenon.