Thermal shock of tungsten carbide in plasma-facing conditions

Tungsten carbide (WC) has been found to have higher resistance to plasma-induced thermal shock compared to rolled tungsten. The electron beam device JUDITH 1 was used to simulate likely thermal shock conditions induced by edge localised modes and plasma disruptions. Loading conditions of 100–1000 cycles, heat fluxes of 0.19–1.13 GW/m2 and base temperatures of 400–1000 °C were employed on two candidate WC-based materials: a monolithic WC ceramic, and a WC-FeCr composite. Surprisingly, the monolith outperformed the composite under all conditions. This was unexpected, particularly at 400 °C, based on the calculated thermal shock resistance parameters. The result was explained by preferential melting of the metallic FeCr binder. Compared to available data collected under identical conditions on rolled tungsten plate, monolithic WC had lower surface roughness from thermal shock damage, particularly when tested at 400 °C. This shows promise for its use as a plasma facing material. Strategies for further improving performance are discussed

Recrystallization and composition dependent thermal fatigue response of different tungsten grades

Industrial pure tungsten grades, manufactured by using a variety of manufactured techniques, are available worldwide in many different types of semifinished products, i.e. rods, wires, ribbons, and sheets. Thereby, the recrystallization temperature varies depending on the applied degree of deformation but also depending on the materials composition, i.e. the materials purity and in particular the level of certain impurities. In order to compare different available industrial tungsten grades and a newly developed PIM-W grade, on the one hand recrystallization studies at three different temperatures from 1300 to 1800 °C for 1 h were performed using Vickers hardness testing. On the other hand, the thermal shock induced low cycle thermal fatigue response of the material in its different recrystallization stages was done using high heat flux tests at 1000 °C base temperature, applying 1000 shots with 1 ms and 0.38 GW/m2 and post mortem characterization, i.e. profilometry and metallography. The obtained results are related to the chemical composition of the individual tungsten grades obtained from Auger electron spectroscopy analyses on cold fracture surfaces

High pulse number thermal shock testing of tungsten alloys produced by powder injection molding

The investigation of plasma facing materials (PFM) subjected to a large number (≥10,000) of thermal shocks is of interest to determine long term morphological changes which might influence component lifetime in and plasma performance of a fusion reactor. The electron beam facility JUDITH 2 was used to simulate these conditions experimentally. In this study eight different tungsten grades produced by powder injection molding (PIM) were investigated: Two pure tungsten grades, one with 2 wt% Y₂O₃, three with 1, 2 and 3 wt% TiC, and two with 0.5 and 1 wt% TaC. Samples of 10 × 10 × 4 mm³ were brazed to a copper cooling structure and subjected to 10⁵ thermal shocks of 0.5 ms duration and an intensity of L$_{abs}$=0.55 GW/m² (F$_{HF}$=12 MWs½/m2) at a base temperature of T$_{base}$ = 700 °C. The PIM grades showed damages in general comparable with a sintered and forged pure tungsten reference grade (>99.97 wt% W) that complies with the ITER specifications. One exception was the 2 wt% TiC doped material which failed early during the experiment by delamination of a large part of the surface. The Y₂O₃ doped material showed a comparatively good performance with respect to crack width (<15 μm) and roughening (R$_{a}$ = 0.75 μm), but showed melt droplets of ∼3–4 μm diameter, while the 1 wt% TiC doped material showed wide cracks (up to 50 μm) and strong roughening (R$_{a}$ = 2.5 μm). The paper discusses the post-mortem analysis of all grades, comparing them with respect to roughness (from laser profilometry), crack network characteristics and local melt droplet formation or other special morphological features (from SEM images) as well as crack depth (from metallographic cross sections)

Mechanical and microstructural investigations of tungsten and doped tungsten materials produced via powder injection molding

The physical properties of tungsten such as the high melting point of 3420°C, the high strength and thermal conductivity, the low thermal expansion and low erosion rate make this material attractive as a plasma facing material. However, the manufacturing of such tungsten parts by mechanical machining such as milling and turning is extremely costly and time intensive because this material is very hard and brittle. Powder Injection Molding (PIM) as special process allows the mass production of components, the joining of different materials without brazing and the creation of composite and prototype materials, and is an ideal tool for scientific investigations. This contribution describes the characterization and analyses of prototype materials produced via PIM. The investigation of the pure tungsten and oxide or carbide doped tungsten materials comprises the microstructure examination, element allocation, texture analyses, and mechanical testing via four-point bend (4-PB). Furthermore, the different materials were characterized by high heat flux (HHF) tests applying transient thermal loads at different base temperatures to address thermal shock and thermal fatigue performance. Additionally, HHF investigations provide information about the thermo-mechanical behavior to extreme steady state thermal loading and measurements of the thermal conductivity as well as oxidation tests were done. Post mortem analyses are performed quantifying and qualifying the occurring damage with respect to reference tungsten grades by metallographic and microscopical means

The EUROfusion materials property handbook for DEMO in-vessel components—Status and the challenge to improve confidence level for engineering data

The development of a specific materials database and handbook, for engineering design of in-vessel components of EU-DEMO, is an essential requirement for assessing the structural integrity by design. For baseline in-vessel materials, including EURFOER97, CuCrZr, Tungsten as well as dielectric and optical materials, this development has been ongoing for several years within the Engineering Data and Design Integration sub-project of the EUROfusion Materials Work Package. Currently the database is insufficient to ensure reliable engineering design and safety or hazard analysis and mostly does not yet exist in established nuclear codes. In this paper the current status of EU-DEMO database and handbook for key in-vessel materials is provided. This comprises practical steps taken to obtain the raw data, screening procedures and data storage, to ensure quality and provenance. We discuss how this procedure has been utilized to produce materials handbook chapter on EUROFER97 and the critical challenges in data accumulation for CuCrZr and Tungsten, planned mitigations and the implications this has on structural design. Finally, key elements and methodology of our strategy to develop the materials database and handbook for the in-vessel materials are outlined, including concepts to accommodate sparse irradiated materials data and links to EU-DEMO engineering design criteria