Wall Forum 2024

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Temperature-dependent grain boundary permeation of hydrogen in tungsten has been investigated using a recently developed hydrogenography technique that utilizes patterned films on the back of the permeation samples. A tungsten oxide layer was first developed as a hydrogen indicator that could withstand greater temperatures than previously used yttrium, and a method for reliably quantifying the results was established. The findings indicated that the hydrogen permeation flux across the grain boundaries is dominating up to 660K, where the first signs of a transition to bulk permeation were seen. [mehr]

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Divertor targets can be subjected to extreme flow conditions such as high heat fluxes up to 40 MW/m², high mass flows, and pressures. Under these extreme heat loads, boiling can occur in the cooling channels of the divertor targets. Computational fluid dynamics (CFD) can be useful in better understanding the thermal behavior of the divertor targets under boiling phenomena. However, the current CFD sub-models developed for boiling flow are highly dependent on the flow conditions. In this study, various models for two-phase subcooled flow boiling are investigated under fusion-relevant conditions. Finally, building on insights gained from the previous numerical analyses, a model combination has been proposed [mehr]

Because of its ductility, low Young’s modulus, high compressive yield strength, good fatigue behavior, and biocompatibility, TiZrNbHfTa is in discussion as implant material in comparison to Ti6Al4V. To improve the wear resistance, as required for use in artificial joints, we deploy single-step and two-step thermal oxidation as a surface hardening method. Single-step thermal oxidation at 600 °C of cold-rolled, single-phase bcc, nanocrystalline TiZrNbHfTa leads to the formation of an adherent, μm-sized, vitreous oxide layer. Underneath this oxide layer, EPMA, XRD, XPS, and NRA-analysis indicate, that the bcc TiZrNbHfTa completely decomposes into another bcc and a hcp-phase during thermal oxidation. Selective internal oxidation of hafnium and zirconium occurs upon oxygen inward diffusion, raising the surface hardness by four times to 1522±64 HV 0.5. However, equal thermal oxidation treatment of coarser grained TiZrNbHfTa leads to catastrophic oxidation. The successful achievement of an adherent oxide layer with internal oxides in an oxygen diffusion zone underneath in case of nanocrystalline TiZrNbHfTa allows to add an additional heat treatment step at 1200 °C under vacuum in order to reduce the internal oxides formed during the single-step process. According to SEM, EDS, XRD, XPS, and APT-analysis, μm-sized oxygen-rich hcp precipitates in a bcc matrix form in the subsurface region. With this precipitates in the subsurface region, a gradual Martens-microhardness decrease from 5 GPa near the surface to 4.2 GPa in the substrate can be achieved. In contrast to the single-step process, the substrate itself is again single-phase bcc due to the heat treatment at 1200 °C. [mehr]

In the present work, H content and defect microstructures in the H supersaturated surface layer (HSSL) formed upon a fluence seriers were performed using ¹⁵N-¹H nuclear reaction analysis (NRA) and transmission electron microscopy (TEM). H concenration up to 16 at.% in HSSL was record-breakingly observed. Detailed experimental insights guide us to the conclusive role of defect sinks (e.g., free surface and vacancy clusters, etc.) in absorbing W self-interstitial atoms and materializing temporary Frenkel pairs (t-FP) as H-decorated vacancy-type defects under low-energy H ion/plasma irradiation. Fundamental effects of mobile hydrogen in irradiated materials and the possibility of SSL formation in W materials under fusion reactor conditions are discussed. [mehr]

Energy applications from gas turbines to nuclear reactors demand increased operation temperatures for improved efficiencies and performance, alongside supporting deep-decarbonization & hydrogen generation. Advances in materials capability are a key enabling technology for the next-generation low-carbon energy applications needed to address the climate emergency. Metals reinforced by ordered intermetallic precipitates form a potent strategy for the development of high temperature strength alongside damage tolerance, which is central to the success of state-of the-art fcc nickel-based superalloys. Such a strategy is equally of interest within bcc systems, such as W, Ti or Fe, to access increased melting point and low cost/density. However, despite their promise, refractory metal ‘bcc-superalloys’ are yet to be commercialised. Here successes, challenges and opportunities for the nascent materials class of bcc-superalloys will be discussed, including titanium- and tungsten-based [1, 2]. Further, strategies for nanostructured alloys and high entropy alloys (HEAs) [3] will be introduced, with a focus on the ability for nano-scale interfaces to act as sinks for irradiation damage. [1] – http://dx.doi.org/10.1016/j.scriptamat.2017.06.038 [2] – https://doi.org/10.1016/j.apmt.2021.101014 [3] – https://doi.org/10.1016/j.actamat.2019.01.006 [mehr]

The talk reviews our positron annihilation studies performed on Eurofer97 steel and its ODS variant, performed in the last about 20 years. Various aspects of radiation environments foreseen for these materials were experimentally simulated via ion implantation and studied by a combination of non-destructive characterization techniques. The primary characterization tools in these studies were two techniques based on positron annihilation, namely positron annihilation lifetime spectroscopy (PALS) and Coincidence Doppler Broadening (CDB) technique. In addition to bulk studies utilizing conventional radioisotope positron sources, ion-implanted specimens were analysed using slow positron beams. This review summarizes the key findings of these techniques concerning the nature of the radiation resistance of EUROFER/ODS steels. [mehr]