Stress-induced Preferred Absorption Research Articles

Irradiation creep behavior of V–4Cr–4Ti alloys irradiated in a liquid sodium environment at the JOYO fast reactor

Irradiation experiments on V–4Cr–4Ti alloys with sodium-enclosed irradiation capsules in the JOYO fast reactor were conducted using pressurized creep tubes (PCTs). The irradiation creep strain was significantly larger than the thermal creep strain below 686°C, but there was no swelling of the neutron-irradiated V–4Cr–4Ti alloys. At temperatures below 500°C, the irradiation creep was found to be proportional to the square root of the neutron dose and linear with the stress level. Above 500°C, it was expected to be proportional to the stress level to a power greater than unity, because the irradiation creep mechanism could change from the stress-induced preferred absorption mechanism (SIPA) to the preferred absorption glide mechanism (PGA). By comparing annealed PCT specimens with cold-worked specimens, the cold-worked V–4Cr–4Ti alloys exhibited a larger irradiation creep strain compared with the annealed alloys. The irradiation creep compliance of the V–4Cr–4Ti alloys were ∼10×10−6MPa−1dpa−1 below 500°C and 50–200×10−6MPa−1dpa−1 above 500°C, a value greater than that of commercial V–4Cr–4Ti alloys, austenitic steels and ferritic steels.

In-reactor deformation of cold-worked Zr–2.5Nb pressure tubes

Over forty years of in-reactor testing and over thirty years of operating experience in power reactors have provided a broad understanding of the in-reactor deformation of cold-worked Zr–2.5Nb pressure tubes, and an extensive data-base upon which to base models for managing the life of existing reactors and for designing new ones. The effects of the major operating variables and many of the metallurgical variables are broadly understood. The deformation is often considered to comprise three components: thermal creep, irradiation growth and irradiation creep. Of the three, irradiation growth is best understood – it is thought to be driven by the diffusional anisotropy difference (DAD). It is still not clear whether the enhancement of creep by irradiation is due to climb-plus-glide (CPG), stress-induced preferred absorption (SIPA) or elasto-diffusion (ED). The least understood area is the transition between thermal creep and irradiation where the fast neutron flux may either suppress or enhance the creep rate. The three components are generally treated as additive in the models, although it is recognized that this is only a crude approximation of reality. There are still significant gaps in our knowledge besides the thermal- to irradiation-creep transition, for example, the effect of Mo which is produced from Nb by transmutation in the thermal neutron flux is not known, and on-going work is required in a number of areas. This paper reviews the current state of knowledge of the in-reactor deformation of cold-worked Zr–2.5Nb pressure tubes, and highlights areas for further research.

Low-temperature irradiation creep of fusion reactor structural materials

Irradiation creep has been investigated in the Oak Ridge Research Reactor in an assembly spectrally tailored to achieve a He : dpa ratio of 12–14:1 appm/dpa in austenitic stainless steels. Temperatures of 60–400°C were investigated to address the requirements of near term fusion devices. It was found that austenitic alloys, especially PCA, have higher creep rates at 60°C than at 330 and 400°C. Since this phenomenon could not be explained by existing theoretical models, a new mechanism was proposed and a corresponding theoretical model was developed. Since vacancy migration times can be a few orders of magnitude longer than the irradiation times in this temperature regime, the immobile vacancies do not cancel climb produced by mobile interstitials absorbed at dislocations. The result is a high climb rate independent of stress-induced preferred absorption (SIPA) mechanisms. Preliminary calculations indicate that this mechanism coupled with preferred-absorption-driven glide at higher temperatures predicts a high creep rate at low temperatures and a weak temperature dependence of irradiation creep over the entire temperature range investigated.

Shape effect in the drift diffusion of point defects into straight dislocations

Effects on point-defect drift diffusion in the strain fields of straight edge or screw dislocations, due to anisotropic diffusion arising from the anisotropy of the point defect in its saddle-point configuration, are investigated. Expressions for sink strength and bias that include the saddle-point shape effect are derived, both in the absence and presence of an externally applied stress. These are found to depend on instrinsic parameters such as the relaxation volume and the saddle-point shape of the point defects, and on extrinsic parameters such as temperature and the magnitude and direction of the externally applied stress with respect to the line direction and Burgers-vector direction of the dislocation. The theory is applied to fcc copper and bcc iron. It is found that the stress-induced bias differential for edge dislocations depends much more on the line direction than on the Burgers-vector direction. The origin of this strong line-direction dependence is shown to be unrelated to the dislocation strain field, but is solely a consequence of elastodiffusion and the translational symmetry of the dislocation line. Comparison is made with the stress-induced bias differential due to the usual stress-induced preferred absorption effect. It is found that the shape effect causes a bias differential that is more than an order of magnitude larger.

Stress-induced preferred absorption due to saddle-point anisotropy: The case of an infinitesimal dislocation loop

The effect of an external stress on point-defect migration into an edge dislocation loop in an Isotropic linear elastic medium is studied. The effects of the saddle-point anisotropy are considered. It is found that an external stress changes the bias of a dislocation loop in a way somewhat similar to the usual SIPA in having the same linear stress dependence and stress orientation dependence. Application of the theory to copper and iron shows that the magnitude of the change is much larger than that due to the usual SIPA.

Internal stress buildup in SIPA creep

A discrete simulation of internal stress effects, arising during stress-induced preferred-absorption (SIPA) irradiation creep, has been conducted for randomly oriented linear-elastic fcc polycrystals. Of particular interest are the intercrystalline incompatibility stresses arising during the development of faulted interstitial loops (Frank loops). These simulations suggest that a unique state of internal stress can develop at a sufficiently large loop volume-fraction. As this state is approached the deviatoric creep-rate reduces to zero. Existing microstructural data suggest, however, that unfaulting processes limit loop volume-fractions to levels consistent with a maximum reduction in creep-rate of ∼18%. A substantial coupling effect is predicted between faulted loop SIPA and other SIPA mechanisms, such as preferred absorption at perfect dislocations (SIPAD), and climb controlled glide (CCG). Faulted loop SIPA coupled with CCG creep leads to an enhanced creep-rate. The opposite effect is predicted for the coupling with SIPAD creep.

Mechanisms of radiation induced creep and growth

Irradiation creep occurs primarily because the applied stress causes the evolving microstructure to respond in an anisotropic fashion to the interstitial and vacancy fluxes. On the other hand, irradiation growth requires the response to be naturally anisotropic in the absence of applied stress. Four fundamental mechanisms of irradiation creep have been conjectured: stress induced preferred absorption (SIPA) of the point defects on the dislocations, stress induced preferred nucleation (SIPN) of point defects in planar aggregates (edge dislocation loops), stress induced climb and glide (SICG) of the dislocation network and stress induced gas driven interstitial deposition (SIGD). These mechanisms will be briefly outlined and commented upon. The contributions made by these mechanisms to the total strain are not, in general, mutually separable and also depend on the prevailing (and changing) microstructure during irradiation. The fundamental mechanism of irradiation growth will be discussed: it is believed to arise by the preferred condensation of point defects and climb of dislocation loops and network on certain crystallographic planes. The preferred absorption and nucleation is thus a consequence of natural crystallographic anisotropy and not due to any external stresses. Again the effectiveness of this mechanism depends on the prevailing microstructure in the material. In this connection attention will be particularly drawn to the significance of solute trapping, segregation at grain boundaries, dislocation bias for interstitials and transport parameters for an understanding of irradiation growth in materials like zirconium and its alloys; the relevance of recent simulation studies of growth in such materials using electrons to the growth under neutron irradiation will be discussed in detail and a consistent model of growth in these materials will be presented.

Irradiation creep by climb-enabled glide of dislocations resulting from preferred absorption of point defects

Abstract A mechanism of irradiation creep arising from the climb-enabled glide of dislocations due to stress-induced preferred absorption of radiation-produced point defects is proposed. This creep component is here termed preferred absorption glide, PAG. PAG-creep operates in addition to the previously studied components of creep from climb by stress-induced preferred absorption, (SI)PA-creep, and the climb-enabled glide due to excess absorption of interstitials on dislocations during swelling, I-creep. A formulation of the various climb and climb-enabled glide processes which includes earlier results is presented. PAG-creep is comparable in magnitude to PA-creep in the parameter range of applications. While the PA-creep rate and the I-creep rate are linear in stress, the PAG-creep rate is quadratic in stress and thus dominates at high stresses.

Irradiation-creep due to point defect absorption

A rate theory model for the analysis of the stress-induced preferred absorption (SIPA) mechanism of irradiation-creep is outlined and used to devise a simple design formula for the expected creep. This result is compared with an accurate numerical solution and with the relevant experimental data for solution-treated and cold-worked M316 steel. The importance of the high dislocation density arising from the athermal vacancy loop formation is emphasized and used to demonstrate the insensitivity of the predicted irradiation-creep to the usual metallurgical and physical variables.