Spatial ordering of nano-dislocation loops in ion-irradiated materials (original) (raw)
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Spatial instabilities and dislocation-loop ordering in irradiated materials
Physical Review B, 1989
The formation of inhomogeneous distributions of vacancy loops in irradiated materials is discussed in the framework of a dynamical model based on the rate theory of radiation damage. Dislocation structures are associated with dynamical instabilities due to the competition between defect motion and interactions. The dependence of the critical wavelength of the microstructures on material variables, such as the displacement-damage rate, network-dislocation density, or temperature, is obtained. The postbifprcation analysis is performed in the weakly nonlinear regime, where the selection and stability properties of three-dimensional structures are investigated.
Elastic trapping of dislocation loops in cascades in ion-irradiated tungsten foils
Journal of physics. Condensed matter : an Institute of Physics journal, 2014
Using in situ transmission electron microscopy (TEM), we have observed nanometre scale dislocation loops formed when an ultra-high-purity tungsten foil is irradiated with a very low fluence of self-ions. Analysis of the TEM images has revealed the largest loops to be predominantly of prismatic 1/2〈111〉 type and of vacancy character. The formation of such dislocation loops is surprising since isolated loops are expected to be highly mobile, and should escape from the foil. In this work we show that the observed size and number density of loops can be explained by the fact that the loops are not isolated-the loops formed in close proximity in the cascades interact with each other and with vacancy clusters, also formed in cascades, through long-range elastic fields, which prevent the escape of loops from the foil. We find that experimental observations are well reproduced by object Kinetic Monte Carlo simulations of evolution of cascades only if elastic interaction between the loops is...
Journal of Nuclear Materials, 2002
During irradiation, mobile defects, defect clusters and impurity atoms segregate on dislocations. When an external stress is applied, plastic flow is initiated when dislocations are unlocked from segregated defects. Sustained plasticity is achieved by continuation of dislocation motion, overcoming local forces due to dispersed defects and impurities. The phenomena of flow localization, post-yield hardening or softening and jerky flow are controlled by dislocation-defect interactions. We review here computational methods for investigations of the dynamics of dislocation-defect interactions. The influence of dislocations on the motion of glissile self-interstitial atoms (SIAs) and their clusters is explored by a combination of kinetic Monte Carlo and dislocation dynamics. We show that dislocation decoration by SIAs is a result of their 1-D motion and rotation as they approach dislocation cores. The interaction between dislocations and immobilized SIA clusters indicates that the unlocking mechanism is dictated by shape instabilities. Finally, computer simulations for the interaction between freed dislocations and stacking fault tetrahedra in irradiated Cu, and between dislocations and microvoids in irradiated iron are presented, and the results show good agreement with experimental observations.
Point-defect-driven dislocation patterning during deformation under irradiation
Applied Physics A Solids and Surfaces, 1993
Dislocation patterning driven by interactions of dislocations with deformation-induced point defects is considered. The effect of concurrent irradiation-induced production of point defects is also included. The uniform time-dependent solution of the set of equations describing the evolution of the system is "probed" by small periodic perturbations. A linear stability condition obtained in this way as well as the preferred wavelength of the emerging pattern depend on the values of the parameters reflecting biases in the production and annihilation of vacancies and interstitial atoms. It is proposed that by studying the effect of different types of radiation and different irradiation intensities on the occurrence and the wavelength of the dislocation pattern information about the deformationinduced point defect production bias may be obtained.
Philosophical Magazine A, 2001
Plastic instability associated with formation of narrow¯ow channels results from dislocation pinning±unpinning by defect clusters. We investigate the dynamics of dislocation interaction with radiation-induced defect clusters, and speci®cally with, ®rstly, sessile self-interstitial atom clusters in dislocation decorations and, secondly, stacking-fault tetrahedra (SFTs) in the matrix. It is shown that the critical stress to free trapped dislocations from pinning atmospheres can be a factor of two smaller than values obtained on the basis of rigid dislocation interactions. The unpinning mechanism is a consequence of the growth of morphological instabilities on the dislocation line. Dislocation sources are activated in spatial regions of low SFT density, where their destruction by glide dislocations leads to subsequent growth of localized plasticity in dislocation channels. We show that removal of SFTs is associated with simultaneous dislocation glide and climb. Jogs of atomic dimensions are formed when a fraction of SFT vacancies are absorbed by pipe di usion. The width of a¯ow channel is explained in terms of two length scales: the size of an individual SFT, and the dislocation source-to-boundary distance …d of the order of micrometres). While dislocation segments climb by a few atomic planes with each SFT destruction event, d determines the total number of such events. Numerically computed channel widths (about 70±150 nm), and the magnitude of radiation hardening in copper are consistent with experimental observations.
Dislocation decoration and raft formation in irradiated materials
Philosophical Magazine, 2005
Experimental observations of dislocation decoration with self-interstitial atom (SIA) clusters and of SIA cluster rafts are analysed to establish the mechanisms controlling these phenomena in bcc metals. The elastic interaction between SIA clusters, and between clusters and dislocations is included in kinetic Monte Carlo (KMC) simulations of damage evolution in irradiated bcc metals. The results indicate that SIA clusters, which normally migrate by 1D glide, rotate due to their elastic interactions, and that this rotation is necessary to explain experimentally-observed dislocation decoration and raft formation in neutronirradiated pure iron. The critical dose for raft formation in iron is shown to depend on the intrinsic glide/rotation characteristics of SIA clusters. The model is compared with experimental observations for the evolution of defect cluster densities (sessile SIA clusters and nano-voids), dislocation decoration characteristics and the conditions for raft formation.
On Homogeneous Nucleation of Dislocation Loops in Nanocrystalline Materials
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science
In this article, we readdress the question of homogeneous nucleation of dislocation loops in the context of nanocrystalline materials. In this case, the commonly adopted assumption of an infinite medium is no longer valid, and image forces on dislocations must be accounted for in the analysis. An additional energy term associated with the presence of finite boundaries may act to promote homogeneous nucleation and growth of dislocation loops. Based on a simplified consideration of a circular dislocation loop in a spherical nanoparticle or nanosized grain in a polycrystal, energy calculations are carried out to estimate the activation energy for homogeneous nucleation of a dislocation loop in such a system. Two different cases are considered: (1) a single nanoparticle and (2) a grain in a polycrystalline nanomaterial. Based on simulations for aluminum, it is shown that this mechanism may be plausible in both cases, albeit only for small particles and grains in the nanometer range.
Communications Materials
Dislocation loops are ubiquitous in irradiated materials, and dislocation loop bias plays a critical role in void swelling. However, due to complicated interactions between dislocation loops and point defects, it is challenging to evaluate the bias factors of dislocation loops. Here, we determine the bias of sessile < 100 > loops in α-iron using a recently developed atomistic approach based on the lifetime of point defects. We establish a mechanistic understanding of the loop interaction based on the diffusion tendency of point defects near the loop core region. Mobile self-interstitial atoms tend to be absorbed from the edge of the loop, and a trapping region perpendicular to the habit plane of the loop exists. The dislocation loop bias is found to be substantially lower than those of straight dislocations in α-iron and should be included in swelling rate estimates. With the obtained sink strength and bias values, agreement is achieved with experimental results for both absol...
Overview on dislocation-point defect interaction: the brownian picture of dislocation motion
Materials Science and Engineering A, 2004
The interactions between dislocations (D) and point defects (PD) are one of the most important causes of mechanical damping in metals. In the past 40 years, many experimental results have been obtained and published, from which it appears that two fundamental behaviors can be observed when dislocations interact with motionless point defects: thermally activated behaviors and athermal behaviors. In this paper, it is shown that these two observed behaviors can be consistently explained by a "brownian picture" of the motion of dislocations interacting with PD distributed at different distances from the dislocation glide plane.
Physical Review Materials
Body-centred cubic metals and alloys irradiated by energetic particles form highly mobile prismatic dislocation loops with a/2 111-type Burgers vectors. We show how to simulate diffusion of prismatic loops using the discrete dislocation dynamics approach that treats elastic forces acting between the loops and stochastic forces associated with ambient thermal fluctuations. We find that interplay between stochastic thermal forces and internal degrees of freedom of loops, in particular the reorientation of the loop habit planes, strongly influences the observed loop dynamics. The loops exhibit three fundamental types of reactions: coalescence, repulsion, and confinement by elastic forces. The confinement reactions are highly sensitive to the internal degrees of freedom of the loops. Depending on the orientation of the loop habit planes, the barrier to enter an elastically confined bound state is lowered substantially, whereas the lifetime of the bound state increases by many orders of magnitude.