Opportunities and limitations for ion beams in radiation effects studies: Bridging critical gaps between charged particle and neutron irradiations

Effect of defect imbalance on void swelling distributions produced in pure iron irradiated with 3.5 MeV self-ions

Density functional theory models developed over the past decade provide unique information about the structure of nanoscale defects produced by irradiation and about the nature of short-range interaction between radia- tion defects, clustering of defects, and their migration pathways. These ab initio models, involving no experimental input parameters, appear to be as quantitatively accurate and informative as the most advanced experimental techniques developed for the observation of radiation damage phenomena. Density functional theory models have effectively created a new paradigm for the scientific investigation and assessment of radiation damage effects, offering new insight into the origin of temperature- and dose-dependent response of materials to irradiation, a problem of pivotal significance for applications.

Density Functional Theory Models for Radiation Damage

Phase-field simulation of irradiated metals: Part I: Void kinetics

We develop a generalized Langevin spin dynamics (GLSD) algorithm where both the longitudinal and transverse (rotational) degrees of freedom of atomic magnetic moments are treated as dynamic variables. This removes the fundamental limitation associated with the use of stochastic Landau-Lifshitz (sLL) equations, in which the magnitude of magnetic moments is assumed constant. A generalized Langevin spin equation of motion is shown to be equivalent to the sLL equation if the dynamics of an atomic moment vector is constrained to the surface of a sphere. A fluctuation-dissipation relation for GLSD and an expression for the dynamic spin temperature are derived using the Fokker-Planck equation. Numerical simulations, performed using ferromagnetic iron as an example, illustrate the fundamental difference between the twoand three-dimensional dynamic evolution of interacting moments, where the three-dimensional GLSD includes the treatment of both transverse and longitudinal magnetic excitations.

Longitudinal magnetic fluctuations in Langevin spin dynamics

In molecular dynamics simulations, temperature is evaluated, via the equipartition principle, by computing the mean kinetic energy of atoms. There is no similar recipe yet for evaluating temperature of a dynamic system of interacting spins. By solving semiclassical Langevin spin-dynamics equations, and applying the fluctuation-dissipation theorem, we derive an equation for the temperature of a spin ensemble, expressed in terms of dynamic spin variables. The fact that definitions for the kinetic and spin temperatures are fully consistent is illustrated using large-scale spin dynamics and spin-lattice dynamics simulations.

http://ira.lib.polyu.edu.hk/bitstream/10397/5383/1/Ma_Temperature_Dynamic_Spin.pdf

Temperature for a dynamic spin ensemble

The evolution of a void ensemble in the presence of one-dimensionally migrating self-interstitials is considered, consistently taking into account the nucleation of voids via the stochastic accumulation of vacancies. Including the stochastic fluctuations of the fluxes of mobile defects caused by the random nature of diffusion jumps and cascade initiation, the evolution of the void ensemble is treated using the Fokker-Planck equation approach. A system instability signaling a nonequilibrium phase transition is found to occur when the mean free path of the one-dimensionally moving self-interstitials becomes comparable with the average distance between the voids at a sufficiently high void-number density. Due to the exponential dependence of the void nucleation probability on the net vacancy flux, the nucleation of voids is much more favored at the void lattice positions. Simultaneously, voids initially nucleated at positions where neighboring voids are nonaligned will also shrink away. These two processes leave the aligned voids to form a regular lattice. The shrinkage of nonaligned voids is not a usual thermodynamic effect, but is a kinetic effect caused entirely by the stochastic fluctuations in point-defect fluxes received by the voids. It is shown that the shrinkage of the nonaligned voids, and thus the formation of the void lattice, occurs only if the effective fraction of one-dimensional interstitials is small, less than about 1%. The formation of the void lattice in this way can be accomplished at a void swelling of below 1%, in agreement with experimental observation. The dominance of void nucleation at void-lattice positions practically nullifies the effect of void coalescence induced by the one-dimensional selfinterstitial transport.

/pdf/void-lattice-formation-as-a-nonequilibrium-phase-transition-118xq66wem.pdf

Void lattice formation as a nonequilibrium phase transition

Department of Mechanical Engineering, The Hong-Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong~Received 25 July 2002; revised manuscript received 6 November 2002; published 5 March 2003!We found that by assuming that the density of dislocations in an irradiated material varies as a function ofthe distance to grain boundaries and that mobile interstitial defect clusters perform three-dimensionaldiffu-sional motion it is possible to achieve signiﬁcantly better agreement with experimental observations of proﬁlesof heterogeneous void swelling than in the model where defects diffuse purely one-dimensionally. This ap-proach explains the origin of several distinct features characterising the effect of heterogeneous void swelling,including the variation of the shape of swelling proﬁles as a function of irradiation dose, the formation of peaksof swelling and void denuded zones, and the occurrence of anomalously large voids in the regions adjacent tograin boundaries.DOI: 10.1103/PhysRevB.67.094103 PACS number~s!: 61.72.Mm, 61.80.Az, 61.82.Bg, 28.52.FaI. INTRODUCTION

Heterogeneous void swelling near grain boundaries in irradiated materials

We investigate the dynamics of nucleation and the growth of voids in an irradiated material in the presence of a spatially heterogeneous dislocation microstructure. We find that, due to the sensitivity of the void nucleation rate to the local vacancy supersaturation, voids nucleate and grow almost exclusively in the regions where the density of dislocations is low. Numerical simulations show that the relatively high void growth rates observed experimentally in the regions of low dislocation density, leading to segregated evolution of dislocations and voids, can be naturally described by solutions of a spatially heterogeneous reaction-diffusion model that takes continuous nucleation of voids into account but does not assume the occurrence of long-range one-dimensional transport of clusters of self-interstitial atoms through the material.

/pdf/segregation-of-voids-in-a-spatially-heterogeneous-46uj7iyha4.pdf

A. A. Semenov

Papers

Temperature for a dynamic spin ensemble

Void lattice formation as a nonequilibrium phase transition

Heterogeneous void swelling near grain boundaries in irradiated materials

Segregation of voids in a spatially heterogeneous dislocation microstructure