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

Numerical analysis of TDS spectra under high and low flux plasma exposure conditions

A recently developed numerical model, based on the dislocation-driven nucleation of gas bubbles, is used to analyse experimental results on deuterium retention in tungsten under ITER relevant plasma exposure conditions. Focus is placed on understanding the relation between exposure temperature and flux on primary features of thermal desorption spectra: peak positions and intensities of the desorption flux. The model allows one to relate the peak positions with the size of plasma induced deuterium bubbles and envisage exposure conditions (temperature and flux) for their formation. Based on the performed analysis, dedicated experimental conditions to validate the model are proposed

Effect of statistically stored dislocations in tungsten on the irradiation induced nano-hardening analyzed by different methods

Tungsten self-ion irradiation was performed at 800 °C up to 0.01-1 dpa on two different W grades with essentially different dislocation density. Nanoindentation was applied to characterize the radiation hardening in two W grades with different microstructure. Different methods to analyze the indentation curves were applied to extract the bulk equivalent radiation hardening. It was shown that depending on the applied method, different outcomes may occur. The most satisfactory procedure was established and a consistent set of parameters was found. The bulk equivalent radiation hardening was found to saturate above 0.1 dpa. The characteristic distance between irradiation induced defects acting as dislocation pinning points was found to decrease up to 0.1 dpa, and then saturate/increase with irradiation dose. No essential difference in radiation hardening was observed between the studied W grades with essentially different initial dislocation density

Effect of high flux plasma exposure on the micro-structural and mechanical properties of ITER specification tungsten

We have performed a combined study using transmission electron microscopy (TEM), nuclear reaction analysis (NRA) and nano-indentation (NI) techniques to reveal the impact of high flux plasma exposure on the properties of a sub-surface region of the commercially available pure tungsten fabricated following the ITER specification. TEM examination revealed the formation of a dense dislocation network and dislocation tangles, resulting in a strong increase in the dislocation density by at least one order of magnitude as compared to the bulk density. The plasma-induced dislocation microstructure vanishes within a depth of about 10-15 mu m from the top of the exposed surface. Surface hardness after the plasma exposure was characterized by NI and was found to increase significantly in the sub-surface region of 1.5-3 mu m. That was attributed to the resistance of the plasma-induced dislocation networks and deuterium-induced defects, whose presence within a depth of similar to 1 mu m was unambiguously detected by the NRA measurements as well. (C) 2016 Elsevier B.V. All rights reserved

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

Modelling deuterium release from tungsten after high flux high temperature deuterium plasma exposure

Tungsten is a primary candidate for plasma facing materials for future fusion devices. An important safety concern in the design of plasma facing components is the retention of hydrogen isotopes. Available experimental data is vast and scattered, and a consistent physical model of retention of hydrogen isotopes in tungsten is still missing. In this work we propose a model of non-equilibrium hydrogen isotopes trapping under fusion relevant plasma exposure conditions. The model is coupled to a diffusion-trapping simulation tool and is used to interpret recent experiments involving high plasma flux exposures. From the computational analysis performed, it is concluded that high flux high temperature exposures (T = 1000 K, flux = 10(24) D/m(2)/s and fluence of 10(26) D/m(2)) result in generation of sub-surface damage and bulk diffusion, so that the retention is driven by both sub-surface plasma-induced defects (bubbles) and trapping at natural defects. On the basis of the non-equilibrium trapping model we have estimated the amount of H stored in the sub-surface region to be similar to 10 (5) at (1), while the bulk retention is about 4 x 10 (7) at (1), calculated by assuming the sub-surface layer thickness of about 10 mu m and adjusting the trap concentration to comply with the experimental results for the integral retention. (C) 2016 Elsevier B.V. All rights reserved

Sub-surface microstructure of single and polycrystalline tungsten after high flux plasma exposure studied by TEM

We have performed high flux plasma exposure of tungsten and subsequent microstructural characterization using transmission electron microscopy (TEM) techniques. The aim was to reveal the nanometric features in the sub-surface region as well as to compare the microstructural evolution in tungsten single crystal and ITER-relevant specification. In both types of samples, TEM examination revealed the formation of a dense dislocation network and dislocation tangles. The estimated dislocation density in the sub-surface region was of the order of 10(14) m(-2) and it gradually decreased with a depth position of the examined sample. Besides individual dislocation lines, networks and tangles, the interstitial dislocation loops have been observed in all examined samples only after the exposure. Contrary to that, examination of the pristine single crystal W and backside of the plasma-exposed samples did not reveal the presence of dislocation loops and tangles. This clearly proves that high flux plasma exposure induces severe plastic deformation in the sub-surface region irrespective of the presence of initial dislocations and sub-grains, and the formation of dislocation tangles, networks and interstitial loops is a co-product of thermal stress and intensive plasma particles uptake. (C) 2016 Elsevier B.V. All rights reserved

Tensile properties of baseline and advanced tungsten grades for fusion applications

This work aims to establish a mechanical reference database of tungsten materials that are currently under assessment of their susceptibility to neutron irradiation. To obtain the mechanical properties, we performed a set of parametric tests using mini-tensile sample geometry and fracture surface analysis. Six different types of tungsten-based materials were assessed: two commercial grades produced according to ITER specifications in Europe and China - i.e., Plansee (IGP) and AT&M (CEFTR), and four perspective lab-scale grades. These are grades reinforced with particles of TiC, Y2O3, and ZrC (W1TiC, W2YO, and W0.5ZC, respectively) as well as fine grain structure W (FG). Tests were performed in the temperature range 150–600°C, selected specifically to reveal the ductile to brittle transition temperature and mechanisms of full plastic deformation. Most of the materials showed onset of the ductile behavior at 300 °C, except FG and IGP (in transverse orientation) grades. High yield strength and ultimate tensile strength were recorded for CEFTR, W0.5ZC, and W1TiC at the maximum investigated temperature (600°C), which can be considered as promising for performance in the high-temperature regime. The lowest threshold temperature for ductility was determined to be 200°C registered for the W0.5ZC grade, CEFTR (in longitudinal orientation) grades, and IGP (in longitudinal orientation) grades, hence demonstrating its high potential for divertor applications