Andrii Dubinko

Synergy of plastic deformation and gas retention in tungsten

Taking the example of tungsten, we demonstrate that high-flux plasma exposure of recrystallized and plastically deformed samples leads to principal differences in the gas trapping and associated surface modification. Surface of the exposed pre-deformed samples exhibits ruptured µ m-sized blisters, a signature of bubbles nucleated close to the surface on the plastically induced dislocation network. Contrary to the recrystallized samples, no stage attributable to gas bubbles appeared in the desorption spectrum of the deformed samples demonstrating the strong impact of dislocations on hydrogen retention.

Interaction of discrete breathers with primary lattice defects in bcc Fe

The interaction of discrete breathers with the primary lattice defects in transition metals such as vacancy, dislocation, and surface is analyzed on the example of bcc iron employing atomistic simulations. Scattering of discrete breathers on the lattice defects induces localized atomic excitations, with intensity and relaxation time depending on the defect structure and breather kinetic energy. The dissipation of the intrinsic breather energy due to the scattering is computed and analyzed. It is concluded that the breather-to-defect energy transfer may stipulate the activation of the lattice defects causing unexpected athermal effects such as enhanced mass transfer or electroplasticity, already experimentally reported but so far not fully understood at the atomic-scale level.

Effect of plastic deformation on deuterium retention and release in tungsten

The effect of severe plastic deformation on the deuterium retention in tungsten exposed to high-flux low-energy plasma (flux ∼ 1024 D/m2/s, energy ∼ 50 eV, and fluence up to 3 × 1026 D/m2) at the plasma generator Pilot-PSI was studied by thermal desorption spectroscopy and scanning electron microscopy. The desorption spectra in both reference and plastically deformed samples were deconvolved into three contributions attributed to the detrapping from dislocations, deuterium-vacancy clusters, and pores, respectively. The plastically induced deformation, resulting in high dislocation density, does not change the positions of the three peaks, but alters their amplitudes as compared to the reference material. The appearance of blisters detected by scanning electron microscopy and the desorption peak attributed to the release from pores (i.e., deuterium bubbles) were suppressed in the plastically deformed samples but only up to a certain fluence. Beyond 5 × 1025 D/m2, the release from the bubbles in the deformed material is essentially higher than in the reference material. Based on the presented results, we suggest that a dense dislocation network increases the incubation dose needed for the appearance of blisters, associated with deuterium bubbles, by offering numerous nucleation sites for deuterium clusters eventually transforming into deuterium-vacancy clusters by punching out jogs on dislocation lines.

Microstructural modifications in tungsten induced by high flux plasma exposure: TEM examination

We have performed microstructural characterization using transmission electron microscopy (TEM) techniques to reveal nanometric features in the sub-surface region of tungsten samples exposed to high flux, low energy deuterium plasma. TEM examination revealed formation of a dense dislocation network and dislocation tangles, overall resulting in a strong increase in the dislocation density by at least one order of magnitude as compared to the initial one. Plasma-induced dislocation microstructure vanishes beyond a depth of about 10 mu m from the top of the exposed surface where the dislocation density and its morphology becomes comparable to the reference microstructure. Interstitial edge dislocation loops with Burgers vector a(0)/2 and a(0) were regularly observed within 6 mu m of the sub-surface region of the exposed samples, but absent in the reference material. The presence of these loops points to a co-existence of nanometric D bubbles, growing by loop punching mechanism, and sub-micron deuterium flakes, resulting in the formation of surface blisters, also observed here by scanning electron microscopy.

Study of the Microstructure Induced by High-Flux Plasma via Transmission Electron Microscopy

The annealed and heavily deformed states of the tungsten microstructure are studied using transmission electron microscopy after irradiation by high-flux plasma. Exposure to plasma substantially increases the dislocation density in the surface layers of both samples, namely, by more than an order of magnitude as compared to the initial value. At a distance of more than 10–15 μm from the surface, the material microstructure is comparable with that observed in the bulk of the sample not exposed to plasma. The given observation indicates that high-flux plasma produces deep and localized plastic deformation in the subsurface layer regardless of the initial hardening and dislocation density.

Thermal Desorption Spectroscopy of Deformed and Undeformed Tungsten after Exposure to a High-Intensity Plasma Flow

As a result of the exposure of tungsten to a high-intensity plasma flow, it is established that the exposure of recrystallized and plastically deformed samples leads to fundamentally different mechanisms of confinement of plasma particles and associated deformation of the surface. The surface of the exposed deformed samples contains micrometer-sized ruptured blisters: an indication of the formation of subsurface bubbles on a grid of dislocations forming during deformation. Desorption spectra of both types of sample are decomposed into three peaks, corresponding to the detachment of plasma–gas particles from dislocations, deuterium-vacancy clusters, and pores. Plastic deformation, which leads to an increase in the dislocation density, does not change the position of the three peaks in the desorption spectra but increases their amplitude in comparison with the recrystallized material.