[en] The aim of the present work is the development of fabrication processes for NiTi shape memory alloys by powder metallurgical means. The starting materials used were prealloyed powders as well as elemental powder mixtures. Three techniques seem to be very promising for shaping of NiTi compacts. Hot Isostatic Pressing (HIP) has been examined for the production of dense semi-finished components. A promising technique for the production of dense and porous coatings with an increased wear resistance is Vacuum Plasma Spraying (VPS). Metal Injection Moulding (MIM) is especially suitable for near-net shape fabrication of small components with a complex geometry considering that large numbers of units have to be produced for compensating high tool and process costs. Subsequently, thermal treatments are required to establish defined shape memory properties. The reproducibility and stability of the shape memory effect are main aspects thinking about a production of NiTi components in an industrial scale. (author)

[en] The severe particle, radiation and neutron environment in a future fusion power plant requires the development of advanced plasma-facing materials. At the same time, the highest level of safety needs to be ensured. The so-called loss-of-coolant accident combined with air ingress in the vacuum vessel represents a severe safety challenge. In the absence of a coolant the temperature of the tungsten first wall may reach 1200 °C. At such a temperature, the neutron-activated radioactive tungsten forms volatile oxide which can be mobilized into atmosphere. Smart tungsten alloys are being developed to address this safety issue. Smart alloys should combine an acceptable plasma performance with the suppressed oxidation during an accident. New thin film tungsten–chromium–yttrium smart alloys feature an impressive 10⁵ fold suppression of oxidation compared to that of pure tungsten at temperatures of up to 1000 °C. Oxidation behavior at temperatures up to 1200 °C, and reactivity of alloys in humid atmosphere along with a manufacturing of reactor-relevant bulk samples, impose an additional challenge in smart alloy development. First exposures of smart alloys in steady-state deuterium plasma were made. Smart tungsten–chroimium–titanium alloys demonstrated a sputtering resistance which is similar to that of pure tungsten. Expected preferential sputtering of alloying elements by plasma ions was confirmed experimentally. The subsequent isothermal oxidation of exposed samples did not reveal any influence of plasma exposure on the passivation of alloys. (paper)

[en] Smart tungsten-based alloys are under development as plasma-facing components for a future fusion power plant. Smart alloys are planned to adjust their properties depending on environmental conditions: acting as a sputter-resistant plasma-facing material during plasma operation and suppressing the sublimation of radioactive tungsten oxide in case of an accident on the power plant. New smart alloys containing yttrium are presently in the focus of research. Thin film smart alloys are featuring an remarkable 10⁵-fold suppression of mass increase due to an oxidation as compared to that of pure tungsten at 1000 °C. Newly developed bulk smart tungsten alloys feature even better oxidation resistance compared to that of thin films. First plasma test of smart alloys under DEMO-relevant conditions revealed the same mass removal as for pure tungsten due to sputtering by plasma ions. Exposed smart alloy samples demonstrate the superior oxidation performance as compared to tungsten–chromium–titanium systems developed earlier. (paper)

[en] Tungsten-chromium-yttrium (WCrY) smart alloys are foreseen as first wall materials for future fusion devices such as DEMO. While suppressing W oxidation during accidental conditions, they should behave like pure W during plasma operation due to preferential sputtering of the lighter alloying elements Cr and Y causing W enrichment at the surface. This paper reports on the results of the simultaneous exposure of WCrY and pure W reference samples to mixed D + 1%Ar+5 %He plasma in the linear plasma device PSI-2. Further, a comparison with exposures to pure D and D + 1 %Ar plasma is made. At incident ion energies of 120 eV, exposure to pure D plasma results in a W-enriched alloy surface due to the preferential sputtering of Cr and Y, while the addition of Ar leads to enhanced erosion for W and WCrY and reduces the W enrichment in smart alloys. With the addition of He to the plasma, erosion of WCrY is enhanced compared to that of pure W. To investigate the plasma impact on the oxidation behaviour, plasma-exposed and reference samples were oxidised in controlled dry oxygen-containing atmosphere at $1000{}^{\circ <!--\; ???\; -->}\mathrm{C}$. The sample geometry has a great impact on the oxidation behaviour. Yet, it can be shown that the good oxidation-suppressing properties of WCrY smart alloys are preserved during plasma exposure. (topical issue article)

[en] The sintering behavior of nanocrystalline ZnO was investigated at only 250 °C. Densification was achieved by the combined effect of uniaxial pressure and the addition of water both in a Field Assisted Sintering Technology/Spark Plasma Sintering apparatus and a hand press with a heater holder. The final pure ZnO materials present high densities (>90% theoretical density) with nano-grain sizes. By measuring the shrinkage rate as a function of applied stress it was possible to identify the stress exponent related to the densification process. A value larger than one points to non-linear relationship going beyond single solid-state diffusion or liquid phase sintering. Only a low amount of water (1.7 wt%) was needed since the process is dictated by the adsorption on the surface of the ZnO particles. Part of the adsorbed water dissociates into H⁺ and OH⁻ ions, which diffuse into the ZnO crystal structure, generating grain boundaries/interfaces with high defect chemistry. As characterized by Kelvin Probe Force Microscopy, and supported by impedance spectroscopy, this highly defective grain boundary area presents much higher surface energy than the bulk. This highly defective grain boundary area with high potential reduces the activation energy of the atomic diffusion, leading to sinter the compound at low temperature.

[en] Highlights: • Optimization of field assisted sintering technology for W–Cr–Y alloys. • Studies on the oxidation resistance of bulk W–Cr–Y alloys at 1273 K for up to 3 weeks. • Sublimation rate of $1\times {10}^{-6}$ mg cm${}^{-2}$ s${}^{-1}$. measured, estimation of radiological hazards. • Suppression of sublimation for 2 days at 1273 K in synthetic air. - Abstract: Tungsten (W) currently is the main candidate as plasma-facing armour material for the first wall of future fusion reactors, like DEMO. Advantages of W include a high melting point, high thermal conductivity, low tritium retention, and low erosion yield. However, in case of an accident, air ingress into the vacuum vessel can occur and the temperature of the first wall can reach 1200 K to 1450 K due to nuclear decay heat. In the absence of cooling, the temperature will remain in that range for several weeks. At these temperatures the radioactive tungsten oxide volatilizes. Therefore, ‘smart’ W alloys are developed that aim to preserve the properties of W during plasma operation coupled with suppressed tungsten oxide formation in case of an accident. This study focusses on oxidation studies at 1273 K of samples produced by mechanical alloying followed by field assisted sintering. In a first step the sintering is optimized for tungsten (W) – chromium (Cr) -yttrium (Y) alloys. It is shown that the best oxidation resistance is achieved with submicron grain sizes. This yields a closed, protective oxide layer. In a second step the influence of the grinding process during sample preparation is analysed. It is shown that scratches initiate failure of the protective oxide. In a third step the oxidation and sublimation is measured for weeks – for the first time the sublimation is directly measured in order to determine the potential hazard in comparison to pure W. It is shown that the oxidation is suppressed in comparison to pure W. However, sublimation at a rate of $1\times {10}^{-6}\mathrm{mg}{\text{cm}}^{-2}{\mathrm{s}}^{-1}$ starts after a few days. Nevertheless, the progess in smart alloys is evident: sublimation is delayed by about two days and complete mechanical destruction of the first wall is avoided.

[en] In future fusion reactors, tungsten is the prime candidate material for the plasma facing components. Nevertheless, tungsten is prone to develop cracks due to its intrinsic brittleness—a major concern under the extreme conditions of fusion environment. To overcome this drawback, tungsten fiber reinforced tungsten (W_f/W) composites are being developed. These composite materials rely on an extrinsic toughing principle, similar to those in ceramic matrix composite, using internal energy dissipation mechanisms, such as crack bridging and fiber pull-out, during crack propagation. This can help W_f/W to facilitate a pseudo-ductile behavior and allows an elevated damage resilience compared to pure W. For pseudo-ductility mechanisms to occur, the interface between the fiber and matrix is crucial. Recent developments in the area of powder-metallurgical W_f/W are presented. Two consolidation methods are compared. Field assisted sintering technology and hot isostatic pressing are chosen to manufacture the W_f/W composites. Initial mechanical tests and microstructural analyses are performed on the W_f/W composites with a 30% fiber volume fraction. The samples produced by both processes can give pseudo-ductile behavior at room temperature. (paper)

[en] For the first wall of a fusion reactor unique challenges on materials in extreme environments require advanced mechanical and thermal properties. Tungsten (W) is the main candidate material for the first wall of a fusion reactor as it is resilient against erosion, has the highest melting point of any metal and shows rather benign behavior under neutron irradiation. However, the intrinsic brittleness of tungsten is a concern in respect with the fusion environment with high transient heat loads and neutron irradiation. Neutron induced effects e.g. transmutation add to embrittlement and are crucial to material performance. To overcome this drawback, tungsten fiber reinforced tungsten (Wf/W) composites are being developed relying on an extrinsic toughing principle. Accordingly, even in the brittle regime this material allows for a certain tolerance towards cracking and damage in general in comparison to conventional tungsten.

[en] WCrY Smart Alloys are developed as first wall material of future fusion devices such as DEMO. They aim at behaving like pure W during plasma operation due to depletion of the alloying elements Cr and Y. The Cr concentration gradients induced by preferential plasma sputtering cause Cr-diffusion. The exposure of WCrY and W samples to pure D plasma, with a plasma ion energy of $220\text{eV}$, is simulated using the dynamic version of SDTrimSP. Cr-diffusion is included into the model. Simulation results are compared with experimental results. At sample temperatures of more than 600^∘C and sputtering by D plus residual oxygen in the plasma ion flux, the Cr-transport to the surface leads to enhanced erosion for WCrY samples. A diffusion coefficient for Cr in WCrY of the order of $1\cdot {10}^{-17}{\text{m}}^2/\text{s}$ is determined. The suitability of WCrY as first wall armour and the influence of further effects, considering especially Cr-diffusion, is discussed.

[en] Highlights: • First plasma exposure of advanced WCrY Smart Alloys in PSI-2. • 220 eV ion energy: Cr-diffusion is significant due to preferential sputtering. • Influence of diffusion on erosion is modelled using SDTrimSP. • 120 eV ion energy: W enrichment results in similar erosion yields for WCrY and W. • Acceptable plasma performance without significant impact on oxidation. - Abstract: In this paper the impact of steady state pure D plasma on WCrY smart alloys at ion energies of 120 and 220 eV is reported. For this purpose a comparison with simultaneously exposed pure W samples is drawn. Different analysis techniques employed for pre- and post-plasma sample analysis hint at a significant depletion of Cr and enrichment of W for lower ion energies. Preferential sputtering leads to enhanced volumetric loss at 220 eV. Analysis of redeposited material indicated local redeposition of Cr. Modelling the ion irradiation with SDTrimSP is used to further interpret experimental results. Depending on the sample temperature during plasma exposure and the magnitude of the ion flux, diffusion of Cr towards the surface is a determining factor for erosion of smart alloys for higher ion energies.