Atomic processes during damage production and defect retention (original) (raw)
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Journal of Nuclear Materials, 1996
The consequences of displacement damage produced by energetic particles on physical and mechanical properties of metals and alloys have been investigated both experimentally and theoretically for several decades. Over the years, a number of theoretical models have been proposed to rationalize the rate and magnitude of defect accumulation under different irradiation conditions. In recent years, significant advances have been made in understanding the nature of the damage produced in this form of multi-displacement cascades. The new knowledge regarding the intra-cascade recombination and clustering of self-interstitial atoms and vacancies during the cooling-down phase of cascades makes it necessary to reexamine the appropriateness of the available models for describing the accumulation of damage under cascade damage conditions. In this paper, recent advances in the understanding of damage production and its consequences are reviewed. A historic perspective is adopted. A comprehensive analysis of the effects of temperature, dose rate and particle type on multi-phenomena (swelling, creep, growth, microstructure evolution, RED, RIS) is presented to discuss the strength and weakness of various models, as they have evolved with the understanding of the damage processes. It has been shown that the irradiation damage modeling has progressed from the standard rate theory model to the BEK model to the production bias model with an increasing degree of sophistication as increasingly more realistic features of the irradiation damage production process were incorporated. It is shown that the newly proposed production bias model uniquely includes the necessary features of cascade damage production in its treatment of the damage accumulation.
New perspectives on the theory of radiation damage in metals
International Journal of Pressure Vessels and Piping, 2004
The modelling of irradiation creep is now highly developed but many of the basic processes underlying the models are poorly understood. A brief introduction is given to the theory of cascade interactions, point defect clustering and dislocation climb. The range of simple irradiation creep models is reviewed including: preferred nucleation of interstitial loops; preferred absorption of point defects by dislocations favourably orientated to an applied stress; various climb-enhanced glide and recovery mechanisms, and creep driven by internal stresses produced by irradiation growth. A range of special topics is discussed including: cascade effects; creep transients; structural and induced anisotropy; and the effect of impurities. The interplay between swelling and growth with thermal and irradiation creep is emphasized. A discussion is given on how irradiation creep theory should best be developed to assist the interpretation of irradiation creep observations and the kqirements of reactor designers.
Journal of Nuclear Materials, 2002
The analysis of the available experimental observations shows that the occurrence of a sudden yield drop and the associated plastic flow localization are the major concerns regarding the performance and lifetime of materials exposed to fission or fusion neutrons. In the light of the known mechanical properties and microstructures of the as-irradiated and irradiated and deformed materials, it has been argued that the increase in the upper yield stress, the sudden yield drop and the initiation of plastic flow localization, can be rationalized in terms of the cascade induced source hardening (CISH) model. Various aspects of the model (main assumptions and predictions) have been investigated using analytical calculations, 3-D dislocation dynamics and molecular dynamics simulations. The main results and conclusions are briefly summarized. Finally, it is pointed out that even though the formation of cleared channels may be rationalized in terms of climb-controlled glide of the source dislocation, a number of problems regarding the initiation and the evolution of these channels remain unsolved.
Classical nucleation theory of microstructure development under cascade-damage irradiation
Journal of Nuclear Materials, 2003
Cascade irradiation produces a significant fraction of the damage in the form of small mobile and immobile vacancy and interstitial clusters. This has led to the introduction of the Woo-Singh production bias theory. In the pursuant studies, the predominant effort that has been spent is in investigating the validity of the concept, and in its usefulness in complementing the traditional theory based on the concept of sink bias. Although plenty of theoretical and experimental results supports the concept, relatively little attention has been paid to the important area of microstructure nucleation. Within the framework of the classical theory of nucleation of overcritical precipitates from small subcritical nuclei, the nucleation processes at elevated temperatures of both voids and interstitial loops from the primary clusters are similar, and can be similarly treated. Recognizing the importance of stochastic fluctuations in the evolution of small embryos, a single-component nucleation theory is formulated using the Fokker-Planck equation, to take into account the stochastic effects of the fluxes of mobile defects, arising from the random nature of diffusion jumps and cascade initiation. Analytic solutions for the separate cases of voids and Frank loops are obtained, and the corresponding effects on the evolution of the microstructure are discussed.
Journal of Nuclear Materials, 2011
ABSTRACT Atomic displacement cascades in solids are complex phenomena, the outcome of which can be statistically characterised by properties such as their spatial extent, morphology and the spatial correlation of defects. Some properties scale in a simple way with parameters such as the cascade energy, others have limited variability with energy, for example point defect cluster size distributions. Taking advantage of the latter invariance, we use object kinetic Monte Carlo simulations to demonstrate that most properties of displacement cascade play no significant role in the evolution of point defect cluster size distributions after long enough time. It is suggested that reliable long-term predictions are possible, when using only the self-interstitial and vacancy cluster size distributions from low energy displacement cascades as building blocks to represent the complete spectrum of cascade energies obtained under neutron irradiation conditions. This is shown on the basis of recursive properties of displacement cascades evidenced for the first time and taking only approximately into account the average volumes in which vacancies and self-interstitial atoms are confined.The model has been successfully used to simulate the evolution of point defect clusters in iron for displacement rates in the range of 10−6 dpa/s and doses of the order of 0.1 dpa. The applicability beyond this range and to more complex materials is discussed.
Radiation-induced damage and evolution of defects in Mo
Physical Review B, 2011
The formation of defects in bcc Mo lattice as a result of 50-keV Xe bombardment is studied via atomistic simulation with an interatomic potential developed using the force-matching ab initio based approach. The defect evolution in the cascade is described. Diffusion and interaction of interstitials and vacancies are analyzed. Only small interstitial atom clusters form directly in the cascade. Larger clusters grow only via aggregation at temperatures up to 2000 K. Stable forms of clusters demonstrate one-dimensional diffusion with a very high diffusion coefficient and escape quickly to the open surface. Point vacancies have much lower diffusivity and do not aggregate. The possibility of a large prismatic vacancy loop formation near the impact surface as a result of fast recrystallization is revealed. The mobility of the vacancy dislocation loop segments is high, however, the motion of the entire loops is strongly hindered by neighbor point defects. This paper explains the existence of the large prismatic vacancy loops and the absence of the interstitial loops in the recent experiments with ion irradiation of Mo foils.
Defect accumulation under cascade damage conditions
Journal of Nuclear Materials, 1994
There is now ample evidence from both experimental and computer simulation studies that in displacement cascades not only intense recombination takes place but also efficient clustering of both self-interstitial atoms (SIAs) and vacancies. The size distributions of the two types of defects produced in cascades are not only different but this difference is also temperature dependent because of the different thermal stabilities of SIA and vacancy clusters. This asymmetry in the production of SIAs and vacancies has been termed "production bias". In the present paper, the basic physics of the defect reactions kinetics associated with the specific features of cascade damage is described, with emphasis on asymmetries between SIA and vacancy type defects concerning their production, stability, mobilif and interactions with other defects. Defect accumulation under cascade damage conditions is discussed in terms of this reaction kinetics taking into account cluster production, dissociation, migration and annihilation at extended sinks. Microstructural features which are characteristic of cascade damage and cannot be explained in terms of the conventional single defect reaction kinetics are emphasized, in particular the heterogeneous microstructural evolution at low doses and the dose and temperature dependence of swelling.
Void nucleation at elevated temperatures under cascade-damage irradiation
Physical Review B
The effects on void nucleation of fluctuations respectively due to the randomness of point-defect migratory jumps, the random generation of free point defects in discrete packages, and the fluctuating rate of vacancy emission from voids are considered. It was found that effects of the cascade-induced fluctuations are significant only at sufficiently high total sink strength. At lower sink strengths and elevated temperatures, the fluctuation in the rate of vacancy emission is the dominant factor. Application of the present theory to the void nucleation in annealed pure copper neutron-irradiated at elevated temperatures with doses of 10 Ϫ4-10 Ϫ2 NRT dpa showed reasonable agreement between theory and experiment. This application also predicts correctly the temporal development of large-scale spatial heterogeneous microstructure during the void nucleation stage. Comparison between calculated and experimental void nucleation rates in neutron-irradiated molybdenum at temperatures where vacancy emission from voids is negligible showed reasonable agreement as well. It was clearly demonstrated that the athermal shrinkage of relatively large voids experimentally observable in molybdenum at such temperatures may be easily explained in the framework of the present theory.
Aspects of microstructure evolution under cascade damage conditions
Journal of Nuclear Materials, 1997
The conventional theoretical models describing the damage accumulation, particularly void swelling, under cascade damage conditions do not include treatments of important features such as intracascade clustering of self-interstitial atoms (SIAs) and one-dimensional glide of SIA clusters produced in the cascades. Recently, it has been suggested that the problem can be treated in terms of 'production bias' and one-dimensional glide of small SIA clusters. In the earlier treatments a 'mean size approximation' was used for the defect clusters and cavities evolving during irradiation. In the present work, we use the 'size distribution function' to determine the dose dependence of sink strengths, vacancy supersaturation and void swelling as a function of dislocation density and grain size within the framework of production bias model and glide of small SIA clusters. In this work, the role of the sessile-glissile loop transformation (due to vacancy supersaturation) on the damage accumulation behaviour is included. The calculated results on void swelling are compared with the experimental results as well as the results of the earlier calculations using the 'mean size approximation'. The calculated results agree very well with the experimental results. 0 1997 Elsevier Science B.V.