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Focuses on a theory of radiation damage formation in alkali halides based on a mechanism of dislocation climb. Discussion on the mechanism of dislocation climb; Bias driven evolution of extended defects; Material parameters of sodium chloride and sodium colloids used in calculations.

Early experimental data on void swelling in electron-irradiated materials disagree with the dislocation bias models based on the dislocation-point defect elastic interactions. Later, this became one of the factors that prompted the development of models based on production bias (PBM) as the main driver for swelling, which assumed that the dislocation bias was much lower than that predicted by theoretical analyses of dislocation bias. However, the PBM in its present form fails to account for important and common observations, namely, the indefinite void growth often observed under cascade irradiation and the swelling saturation observed under high-dose irradiation and in void lattices. In this paper, we show that these contradictions can be naturally resolved in the framework of the rate theory that accounts for the radiation-induced vacancy emission from extended defects, such as voids, dislocations and grain boundaries. This modification introduces a new bias type in the theory, namely, the emission bias. This modified rate theory agrees well with the experimental data and demonstrates that the original dislocation bias should be used in rate theory models along with the emission bias in different irradiation environments. The modified theory predictions include, but are not limited to, the radiation-induced annealing of voids, swelling saturation under high-dose irradiation, generally, and in void lattices, in particular. [ABSTRACT FROM AUTHOR]

Abstract: In the present paper we consider mechanisms of recovery from radiation damage, based on the radiation-induced formation of highly localized and very persistent anharmonic lattice excitations (called alternatively Intrinsic Localized Modes, Discrete Breathers or Quodons) that propagate great distances in atomic-chain directions and interact with crystal defects such as voids and dislocations. The rate theory of microstructure evolution in solids is modified with account of quodon-induced production of Schottky defects and is applied for description of the low-temperature radiation-induced “annealing” of voids. The quodon propagation length deduced from comparison of the model with experimental data appears to be within the micron range, which is consistent with independent data on the irradiation-induced diffusion of interstitial ions in austenitic stainless steel. Experimental evidence of larger quodon ranges obtained recently for copper is also discussed along with some other manifestations of the quodon interaction with extended defects. [Copyright &y& Elsevier]

Abstract: Void ordering has been observed in very different radiation environments ranging from metals to ionic crystals bombarded with energetic particles. The void ordering is often accompanied by a saturation of the void swelling with increasing irradiation dose, which makes an understanding of the underlying mechanisms to be both of scientific significance and of practical importance for nuclear engineering. We show that both phenomena can be explained by the original mechanism based on the anisotropic energy transfer provided by self-focusing discrete breathers or quodons (energetic, mobile, highly localized lattice solitons that propagate great distances along close-packed crystal directions). The interaction of quodons with voids can result in radiation-induced “annealing” of selected voids, which results in the void ordering under special irradiation conditions. We observe experimentally radiation-induced void annealing by lowering the irradiation temperature of nickel and copper samples pre-irradiated to produce voids or gas bubbles. The bulk recombination of Frenkel pairs increases with decreasing temperature resulting in suppression of the production of freely migrating vacancies (the driving force of the void growth). On the other hand, the rate of radiation-induced vacancy emission from voids due to the void interaction with quodons remains essentially unchanged, which results in void dissolution. The experimental data on the void shrinkage and void lattice formation obtained for different metals and irradiating particles are explained by the present model assuming the quodon propagation length to be in the micron range, which is consistent with independent data on the irradiation-induced diffusion of interstitial ions in austenitic stainless steel. [Copyright &y& Elsevier]

The irradiation hardening of reactor pressure vessel steels due to the formation of dislocation loops is analyzed. The analysis is based on the original model for the nucleation and subsequent evolution of dislocation loops in irradiated materials. The loop formation in displacement cascades is taken into account, along with the homogeneous clustering of point defects. The loop evolution is shown to contribute mainly to the athermal component of the yield stress, which is determined by interaction of gliding dislocations with strong barriers. Irradiation-induced hardening is evaluated as a function of irradiation dose and temperature, dose rate, material parameters and initial microstructure. The model results are compared with experimental data for neutron irradiated pressure vessel steels of various grades and with empirical low power expressions of the yield stress increase with increasing irradiation dose. [ABSTRACT FROM AUTHOR]

Abstract: In the conventional theory of radiation damage, it is assumed that the main effect of irradiation is due to formation of Frenkel pairs of vacancies and self-interstitial atoms (SIAs) and their clusters. The difference in absorption of vacancies and SIAs by primary or radiation-induced extended defects (EDs) is thought to be the main reason of microstructural evolution under irradiation. On the other hand, the recovery of radiation damage is thought to be driven exclusively by thermal fluctuations resulting in the vacancy evaporation from voids (void annealing) or dislocations (thermal creep) and in the fluctuation-driven overcoming of obstacles by gliding dislocations (plastic strain). However, these recovery mechanisms can be efficient only at sufficiently high temperatures. At lower irradiation temperatures, the main driving force of the recovery processes may be due to nonequilibrium fluctuations of energy states of the atoms surrounding EDs arising as a result of scattering of radiation-induced excitations of atomic and electronic structure at EDs. In the present paper, the mechanisms of nonequilibrium fluctuations that result in such phenomena as the void shrinkage under irradiation at low temperatures (or high dose rates), irradiation creep and irradiation-induced increase of plasticity under sub-threshold irradiation was considered. [Copyright &y& Elsevier]

Abstract: We propose a mechanism of void growth in di-atomic ionic crystals due to agglomeration of divacancies produced by interactions between dislocations and excitons. An exciton can cause movement of nearby dislocation jogs, resulting in the creation of equal numbers of anion and cation vacancies (Schottky defects). Owing to the heat generated locally during the exciton annihilation, the jog can be displaced while a divacancy arises in the lattice. Subsequent diffusion and agglomeration of divacancies can result in void formation and growth. We evaluate the void nucleation and growth rates in electron irradiated NaCl. [Copyright &y& Elsevier]

Abstract: Recent results of MD simulation have shown that vacancies can be emitted from voids not only by thermal fluctuations (as it is generally conceived) but also by focusing collisions produced under irradiation. A subsequent diffusion of the ejected vacancies away from the void may result in the radiation-induced void dissolution. In the present paper we derive analytical expressions for the rate of the radiation-induced void dissolution under different irradiation conditions and compare it with the rate of void growth due to the biased absorption of vacancies from the bulk. It is shown that the radiation-induced vacancy emission from voids may be responsible for the low temperature (or high dose rate) dissolution of voids and for the saturation of swelling with increasing irradiation dose observed experimentally. [Copyright &y& Elsevier]

Abstract: We have performed numerical calculation of the capture efficiency of a dislocation loop in a finite toroidal reservoir, which is a more appropriate model for a dislocation loop than a spherical or cylindrical reservoir adopted in the previous models. It allows a direct evaluation of the capture efficiency and the bias for a loop of any size with a full account of the stress field in the loop region of influence. It is shown that the loop bias depends on the loop size, dislocation density and the interstitial to vacancy dilatation ratio. With increasing loop size its bias decreases or increases to the straight dislocation bias value if the dislocation density is low or high, respectively. The bias difference of loops of different sizes is shown to be the reason of a coexistence of vacancy and interstitial loops under irradiation. In the conventional case of the dislocation bias for interstitials, interstitial loops are expected to grow to larger sizes than vacancy loops, while in a special case of dislocation bias for vacancies, the opposite tendency is expected. [Copyright &y& Elsevier]

The kinetics of nucleation and coarsening of vacancy clusters in irradiated crystals are considered with account of their elastic interaction with point defects resulting in the biased absorption of vacancies and interstitial atoms. It is shown that in the technologically important range of high dose rate (or low temperature) irradiation, the nucleation rate and the final number density of clusters are determined by the bias parameters rather than by irradiation conditions. The model is applied to the evolution of sodium colloids and chlorine bubbles in NaCl resulting in the formation of voids followed by a sudden fracture of the material, which presents a potential problem in rock salt nuclear waste repositories. The number densities and mean sizes of colloids, bubbles and voids are evaluated and compared with experimental data. [Copyright &y& Elsevier]

Abstract: Recent experimental and molecular dynamics results give evidence for the existence of the discrete solitons (DS’s), a.k.a. intrinsic localized modes (ILM’s), discrete breathers (DB’s) and quodons, which are stable, highly localized lattice excitations that can transfer energy along close-packed crystal directions. The DS interaction with crystal defects results in the amplification of the reaction rates, such as atom ejection and diffusion. In this paper, recent results on the DS detecting in three-dimensional crystal structures by means of molecular dynamics are discussed, and a new method of the rate theory modification in solids under irradiation is presented. [Copyright &y& Elsevier]

Abstract: Vacancy voids have been produced in Ni by 1.2MeV Cr ion irradiation at 873K up to the ion fluence of 1021 m−2. Subsequent irradiation of specimens containing voids at 798 and 723K has resulted in the reduction of the void size and number density. Accordingly, the void swelling has decreased by a factor of ∼5. The experimental results are explained in the framework of an original model taking into account the interaction of voids with radiation-induced excitations of atomic structure such as focusing collisions and long-propagating self-focusing breathers. [Copyright &y& Elsevier]

Rate equations are employed to the kinetic description of lattice defects, created in lithium fluoride by ion irradiation. The F centers are assumed to be created in a mobile state (anion vacancies or excited F* centers), having a certain relaxation time to become stable at room temperature. Concentrations of the F centers and their aggregates are analyzed depending on the irradiation conditions and model parameters. Comparison of the swift ion and electron/gamma irradiations using the suggested model captures the prominent experimental results of a higher complex defect fraction, which created under irradiation with the heavier projectiles. [ABSTRACT FROM AUTHOR]

By means of molecular dynamics simulations it is demonstrated that moving discrete breathers (DBs) can be excited in pure bcc metals such as V, Fe and W. For each of the three metals, the following properties of the DBs are estimated: the frequency as the function of amplitude, the degree of spatial localization and the maximum propagation velocity. For the three studied metals, DB frequency lies above the phonon spectrum and increases with increasing DB amplitude. DBs in vanadium are much more spatially localized and robust than in iron and tungsten. Application of the presented results to the DB-induced mechanism of the void ordering under irradiation is discussed. [ABSTRACT FROM AUTHOR]

Ions arriving at a semiconductor surface with very low energy (2–8 eV) are interacting with defects deep inside the semiconductor. Several different defects were removed or modified in Sb-doped germanium, of which the E -center has the highest concentration. The low fluence and low energy of the plasma ions imply that the energy has to be able to travel in a localized way to be able to interact with defects up to a few microns below the semiconductor surface. After eliminating other possibilities (electric field, light, heat) we now conclude that moving intrinsic localized modes (ILMs), as a mechanism of long-distance energy transport, are the most likely cause. This would be striking evidence of the importance of ILMs in crystals and opens the way to further experiments to probe ILM properties both in semiconductors and in the metals used for contacts. Although most of the measurements have been performed on germanium, similar effects have been found in silicon. [ABSTRACT FROM AUTHOR]