Fluctuations of point-defect fluxes to sinks under cascade damage irradiation (original) (raw)
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Journal of Nuclear Materials, 1996
Recent computational and experimental studies have confirmed that high energy cascades produce clustered defects of both vacancy-and interstitial-types as well as isolated point defects. However, the production probability, configuration, stability and other characteristics of the cascade clusters are not well understood in spite of the fact that clustered defect production would substantially affect the irradiation-induced microstructures and the consequent property changes in a certain range of temperatures and displacement rates. In this work, a model of point defect and cluster evolution in irradiated materials under cascade damage conditions was developed by combining conventional reaction rate theory and results from the latest molecular dynamics simulation studies. This paper provides a description of the model and a model-based fundamental investigation of the influence of configuration, production efficiency and the initial size distribution of cascade-produced vacancy clusters.
On the experimental determination of the migrating defect fraction under cascade damage conditions
Journal of Nuclear Materials, 1994
Information on the fraction of defects surviving intracascade recombination, escaping the cascade volume and migrating until their annihilation ("migrating defect fraction", MDF) can be obtained from the analysis of radiation enhanced diffusion (RED) or radiation induced segregation (RIS) and maximum swelling rates. RED and RIS yield the ratio of the MDF over the effective sink strength whereas maximum swelling rates give lower bound estimates of the MDF. The basic assumptions made in the previous analysis of RED (RIS) and swelling are critically examined in the light of the present understanding of defect production in displacement cascades. MDF values deduced previously from RED are found to be clearly below the lower bound estimates obtained from maximum swelling rates. The discrepancy becomes even larger if the conventional monodefect dislocation bias is used in the analysis of swelling. Possible reasons for this discrepancy are discussed: (1) differences in the contribution of mobile defect clusters produced in cascades to RED and swelling, and (2) an underestimation of both the sink strength evolving during cascade damage conditions and the driving force for the swelling. We argue that the conventional method to deduce the sink strength from sink densities observed in TEM and the application of the conventional monodefect dislocation bias indeed yield only lower bound estimates for the sink strength and for the swelling rate, respectively. If the MDF were established by some other method RED (or RIS) could be used to measure the sink strength.
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.
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.
Journal of Nuclear Materials, 2006
The existence of an interplay between the structure of displacement cascades and point defect mobility that influences the long-term evolution of primary damage in a-Fe is revealed by applying an object kinetic Monte Carlo (OKMC) method. The investigation was carried out using different parameter sets, which primarily differ in the description of self-interstitial atom (SIA) cluster mobility. Two sets of molecular dynamics cascades (produced with the DYMOKA and the MOLDY codes, using different interatomic potentials) and one set of cascades produced in the binary collision approximation with the MARLOWE code, using a Ziegler-Biersack-Littmark (ZBL) potential were separately used as input for radiation damage simulation. The point defect cluster populations obtained after reaching 0.1 dpa were analyzed in each case and compared. It turns out that the relative influence of using different input cascade datasets on the damage features that evolve depends on which OKMC parameter set is employed. Published by Elsevier B.V.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2005
Displacement cascades obtained by full molecular dynamics and in its binary collision approximation, as well as random point defect distributions, all having similar overall morphologies, are used as input for long-term radiation damage simulation by an object kinetic Monte Carlo method in a-iron. This model treats naturally point defect fluxes on cascades regions, resulting from cascades generated in other regions, in a realistic way. This study shows the significant effect of the internal structure of displacement cascades and, in particular, SIA agglomeration on the long-term evolution of defect cluster growth under irradiation.
Journal of Nuclear Materials, 2008
Atomic-scale computer simulation has been used to investigate the primary damage created by displacement cascades in copper over a wide range of temperature (100 K 6 T 6 900 K) and primary knock-on atom energy (5 keV 6 E PKA 6 25 keV). A technique was introduced to improve computational efficiency and at least 20 cascades for each (E PKA , T) pair were simulated in order to provide statistical reliability of the results. The total of almost 450 simulated cascades is the largest yet reported for this metal. The mean number of surviving point defects per cascade is only 15-20% of the NRT model value. It decreases with increasing T at fixed E PKA and is proportional to (E PKA ) 1.1 at fixed T. A high proportion (60-80%) of self-interstitial atoms (SIAs) form clusters during the cascade process. The proportion of clustered vacancies is smaller and sensitive to T, falling from 30% to 60% for T 6 600 K to less than 20% when T = 900 K. The structure of clusters has been examined in detail. Vacancies cluster predominantly in stacking-fault-tetrahedron-type configurations. SIAs tend to form either glissile dislocation loops with Burgers vector b = 1/2<1 1 0> or sessile faulted Frank loops with b = 1/3<1 1 1>. Despite the fact that cascades at a given E PKA and T exhibit a wide range of defect numbers and clustered fractions, there appears to be a correlation in the formation of vacancy clusters and SIA clusters in the same cascade. The size and spatial aspects of this are analysed in detail in part II [unpublished], where the stability of clusters when another cascade overlaps them is also investigated.
Relevancy of displacement cascades features to the long term point defect cluster growth
Journal of Nuclear Materials, 2008
Displacement cascades in iron have been generated by means of the MARLOWE binary collision approximation (BCA) code with primary knock-on atom (PKA) energies ranging from 5 to 100 keV. They serve as input for modelling long term evolution by means of the LAKIMOCA Object Kinetic Monte Carlo code. It is found that the size distributions of the fractions of vacancy and interstitial clustered in the long term are not significantly dependent on the PKA energy in this range. Since the subcascade formation, morphology and spatial extension, as well as the spatial correlations between primary point defect positions do depend on the PKA energy, it is concluded that the size distributions of clustered point defects fractions in the long term do not depend on these cascade features. In contrast, the size distributions of clustered point defect fractions in displacement cascades are found to be independent of the PKA energy while their spatial correlations strongly influence the cluster size distributions in the long term. The use of a mean field approximation in cluster growth kinetics predictions is thereby invalidated. Irradiation dose and dose-rate are also found to be determinant factors governing the long term evolution.
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, 1994
Using a composite model of point defect behavior and microstructural evolution, the influence of cascade vacancy cluster formation and vacancy trapping at solute atoms on the point defect fluxes, point defect clustering and extended defect development was investigated. The point defect model calculates the concentrations of isolated and trapped point defects, and of simple or complex clusters. The extended defect model consists of individual rate theory models describing the evolution of cavities, Frank loops and network dislocations. Cascade vacancy clusters were observed to become the dominant sink for point defects in the early stages of irradiation at low to intermediate temperature. Therefore, the sink strength of the vacancy clusters largely determines the point defect flux and agglomeration rates. The calculations also showed that solute traps affect the irradiated microstructure to a large extent for certain combinations of trap concentration and trapping energy. Both the trap concentration and trapping energy have a non-monotonic effect on vacancy clustering though they do not change the interstitial flux significantly.