V.I. Dubinko

Dislocations mediate hydrogen retention in tungsten

In this letter, a comprehensive mechanism for the nucleation and growth of bubbles on dislocations under plasma exposure of tungsten is proposed. The mechanism reconciles long-standing experimental observations of hydrogen isotopes retention, essentially defined by material microstructure, and so far not fully explained. Hence, this work provides an important link to unify materials modelling with experimental assessment of W and W-based alloys as candidates for plasma facing components.

New insight on bubble-void transition effects in irradiated materials

Abstract An account of elastic interaction between cavities and point defects is shown to result in new critical quantities for bubble-void transition effects in irradiated cubic crystals. In contrast to previous theories, the present one gives not only critical quantities which determine the onset of bias-driven void swelling but the maximum stationary number density and the corresponding mean radius of voids as well as the duration of the bimodal regime. The void density and swelling rate are shown to be independent from the gas level. In the region of low temperatures/high dose rates, the void density appears to be independent from irradiation parameters as well. The relationships among material constants are found at which the stabilization of gas bubbles occurs via the dislocation loop punching mechanism resulting in a drastic change in the cavity behaviour under irradiation such as the saturation (or even suppression) of void swelling and void lattice formation. The theoretical results are compared with experimental data and further experimental tests are proposed.

Dislocation mechanism of deuterium retention in tungsten under plasma implantation.

We have developed a new theoretical model for deuterium (D) retention in tungsten-based alloys on the basis of its being trapped at dislocations and transported to the surface via the dislocation network with parameters determined by ab initio calculations. The model is used to explain experimentally observed trends of D retention under sub-threshold implantation, which does not produce stable lattice defects to act as traps for D in conventional models. Saturation of D retention with implantation dose and effects due to alloying of tungsten with, e.g. tantalum, are evaluated, and comparison of the model predictions with experimental observations under high-flux plasma implantation conditions is presented.

Theory of the late stage of radiolysis of alkali halides

Abstract Recent results on heavily irradiated natural and synthetic NaCl crystals give evidence for the formation of large vacancy voids, which were not addressed by the conventional Jain–Lidiard model of radiation damage in alkali halides. This model was constructed to describe metal colloids and dislocation loops formed in alkali halides during earlier stages of irradiation. We present a theory based on a new mechanism of dislocation climb, which involves the production of VF centers (self-trapped hole neighboring a cation vacancy) as a result of the absorption of excess H centers. Voids are shown to arise due to the reaction between F and VF centers at the surface of halogen bubbles. Critical parameters associated with the bubble-to-void transition are evaluated. Voids can grow to sizes exceeding the mean distance between colloids and bubbles, eventually absorbing them, and, hence, igniting a back reaction between the halogen gas and metal. The amount of radiation damage in alkali halides should be evaluated with account of void formation, which strongly affects the radiation stability of material.

A new mechanism for radiation damage processes in alkali halides

We present a theory of radiation damage formation in alkali halides based on a new mechanism of dislocation climb, which involves the production of VF centers (self-trapped hole neighboring a cation vacancy) as a result of the absorption of H centers of dislocation lines. We consider the evolution of all experimentally observed extended defects: metal colloids, gas bubbles, and vacancy voids. Voids are shown to arise and grow large due to the reaction between F and VF centers at the surface of halogen bubbles. Voids can ignite a back reaction between the radiolytic products resulting in decomposition of the irradiated material.

Irradiation hardening of reactor pressure vessel steels due to the dislocation loop evolution

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.

Theory of irradiation swelling in materials with elastic and diffusional anisotropy

New critical quantities for bubble-void transition effects in irradiated materials are derived with account of elastic interaction between cavities, dislocations and point defects in the approximations of a weak elastic and diffusional anisotropy. The elastic anisotropy is shown to result in quadratic (in the gas pressure to shear modulus ratio) corrections to the cavity bias which can substantially increase the critical quantities for highly overpressurized gas bubbles. On the other hand, the diffusional anisotropy difference between point defects (essential in non-cubic metals) can either increase or decrease the critical quantities depending on the distribution of dislocations over crystallographic directions. In contrast to previous theories, the present one gives not only the onset of bias-driven void swelling but the corresponding mean cavity parameters as well. The maximum void density and the corresponding swelling rate in the post-transient regime are argued to be determined only by the mean dislocation density and material constants. The relationships among material constants are found by which the stabilization of gas bubbles occurs via the dislocation loop punching mechanism even at elevated temperatures.

Impact of glissile interstitial loop production in cascades on void ordering and swelling saturation under irradiation

Abstract According to the dislocation model of void ordering and swelling saturation by the present author, these phenomena arise due to the absorption by voids of perfect glissile dislocation loops produced by irradiation. The formation and glide of small interstitial loops has been also confirmed by recent molecular dynamics (MD) studies of displacement cascades. The cascade mechanism of the loop production is shown to explain an absence of visible dislocation loops in some experiments on void lattices, which is a very important argument in favor of the present theory. However, according to the MD simulations, the glide of such loops seems not to depend on the stacking fault energy of the host lattice, contrary to the predictions of the elastic continuum theory. The latter shows that high stacking energy (as in most bcc metals and in fcc Ni and Al) favors the unfaulting of small loops, which seems to be in agreement with experimentally observed void lattice formation in these metals as compared to the resistance of the low stacking energy metals (such as Cu, Ag, Au and most steels) to void lattice formation. This discrepancy between continuum theory and MD simulations shows the need for further studies of displacement cascades, in particular, in more complex systems modeling the effects of impurities on the nature of interstitial clusters. An outstanding problem is to find impurities that can facilitate the unfaulting process and, hence, void ordering and swelling saturation in those fcc metals which are currently supposed to be void lattice resistant.

Rate Theory of Acceleration of Defect Annealing Driven by Discrete Breathers

Novel mechanisms of defect annealing in solids are discussed, which are based on the large amplitude anharmonic lattice vibrations, a.k.a. intrinsic localized modes or discrete breathers (DBs). A model for amplification of defect annealing rate in Ge by low energy plasma-generated DBs is proposed, in which, based on recent atomistic modelling, it is assumed that DBs can excite atoms around defects rather strongly, giving them energy \(\gg k_{\textit{BT}}\) for \(\sim \)100 oscillation periods. This is shown to result in the amplification of the annealing rates proportional to the DB flux, i.e. to the flux of ions (or energetic atoms) impinging at the Ge surface from inductively coupled plasma (ICP).

Experimental Observation of Intrinsic Localized Modes in Germanium

Deep level transient spectroscopy shows that defects created by alpha irradiation of germanium are annealed by low energy plasma ions up to a depth of several thousand lattice units. The plasma ions have energies of 2–8 eV and therefore can deliver energies of the order of a few eV to the germanium atoms. The most abundant defect is identified as the E-center, a complex of the dopant antimony and a vacancy with an annealing energy of 1.3 eV as determined by our measurements. The inductively coupled plasma has a very low density and a very low flux of ions. This implies that the ion impacts are almost isolated both in time and at the surface of the semiconductor. We conclude that energy of the order of an eV is able to travel a large distance in germanium in a localized way and is delivered to the defects effectively. The most likely candidates are vibrational nonlinear wave packets known as intrinsic localized modes, which exist for a limited range of energies. This property is coherent with the fact that more energetic ions are less efficient at producing the annealing effect.