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This paper is the second part of our series on ultrafine-grained (UFG) tungsten produced by cold rolling. The aim of this paper is to determine tensile properties, with a focus on uniform elongation and flow stress, and to identify the mechanism of plastic deformation in cold rolled tungsten plates and foils. For this purpose, five plates have been rolled out of a single sintered compact with increasing degrees of deformation, which enables the analysis of mechanical properties with changing microstructure without influence from the chemical composition. Uniaxial tensile tests have been performed in the range of room temperature to 800 °C. The tensile curves show a clear trend of increased strength for finer microstructures and simultaneously evolving room temperature ductility in the low temperature regime for cold rolled plates. The thinnest foil (100 μm) behaves uniquely by forming a plateau in the stress-strain curve, which hints at a change in the deformation mechanism. Analyzing the temperature and grain size dependence of the flow stress reveals that the bulk plasticity is still controlled by screw dislocations and further suggests a change in the dislocation-grain boundary interactions from blocking at low homologous temperatures to absorbing at elevated temperatures. A fracture analysis of cold rolled tungsten foils confirms a plastic deformation prior to fracture and reveals a steady transition from brittle towards lamellar fracture for the highly deformed tungsten foils.

Surface functionalization by topographic micro- and nano-structures in order to achieve unique properties, like super-hydrophobicity or ultrahigh light absorption, is a common strategy in nature. In this paper, direct laser interference patterning (DLIP) is presented as a promising tool allowing for the generation of such surface patterns on technical surfaces in order to mimic these biological surfaces and effects. Friction optimization and antibacterial effects by DLIP are exemplarily described. Topographic surface patterns on the micro- and nano-scale demonstrated a significant reduction in the coefficient of friction and bacterial adhesion. It was shown that in both cases, the control of the contact area between surfaces or between surface and bacteria is of utmost importance.

Conventionally produced tungsten (W) sheets are brittle at room temperature. In contrast to that, severe deformation by cold rolling transforms W into a material exhibiting room-temperature ductility with a brittle-to-ductile transition (BDT) temperature far below room temperature. For such ultrafine-grained (UFG) and dislocation-rich materials, the mechanism controlling the BDT is still the subject of ongoing debates. In order to identify the mechanism controlling the BDT in room-temperature ductile W sheets with UFG microstructure, we conducted campaigns of fracture toughness tests accompanied by a thermodynamic analysis deducing Arrhenius BDT activation energies. Here, we show that plastic deformation induced by rolling reduces the BDT temperature and also the BDT activation energy. A comparison of BDT activation energies with the trend of Gibbs energy of kink-pair formation revealed a strong correlation between both quantities. This demonstrates that out of the three basic processes, nucleation, glide, and annihilation, crack tip plasticity in UFG W is still controlled by the glide of dislocations. The glide is dictated by the mobility of the screw segments and therefore by the underlying process of kink-pair formation. Reflecting this result, a change of the rate-limiting mechanism for plasticity of UFG W seems unlikely, even at deformation temperatures well below room temperature. As a result, kink-pair formation controls the BDT in W over a wide range of microstructural length scales, from single crystals and coarse-grained specimens down to UFG microstructures.

By uncoupling the manufacturability from the design process, additive manufacturing of the baseline material EUROFER97 can open significant design freedom for divertor and breeding blankets in fusion technology. As additive manufactured components are known to possess unique microstructures compared to EUROFER97 from standard technologies, the aim of this paper is to investigate additive manufactured EUROFER97 components and the influence of post processing steps on their microstructure and mechanical properties from a materials science point of view. This paper covers the technological fabrication process of EUROFER97 by selective laser melting (SLM), including the production of pre-alloyed EUROFER97 powder, an SLM-parameter study and the design and production of custom-build thin walled test components by SLM. In the initial state after fabrication, SLM-EUROFER97 components exhibit a bimodal, anisotropic microstructure with large ferritic grains. The fraction of ferritic grains increases with decreasing wall thickness. A heat treatment including austenitization, quenching and tempering, allows to achieve a fully martensitic, uniform microstructure for all wall thicknesses. Therefore, there is no influence of wall thickness on mechanical properties of EUROFER97 produced by SLM to be expected, as long as the SLM-part is submitted to an appropriate heat treatment. Further, the comparison of the initial state after fabrication and after post processing reveals the necessity of both hot isostatic pressing and heat treatment to improve the performance. While all material conditions lead to sufficient tensile properties, the Charpy impact properties of SLM-EUROFER97 are inferior in comparison to conventionally produced EUROFER97. A heat treatment alone only improves the ductile-to-brittle transition temperature, whereas hot isostatic pressing reduced the residual porosity of the SLM parts and a subsequent heat treatment improved the ductile-to-brittle transition temperature as well as the upper shelf energy.

This paper is the first part of our series on ultrafine-grained (UFG) tungsten produced by cold rolling. The aim of our project is to investigate the correlation between microstructure, mechanical properties and deformation mechanisms in UFG tungsten, a material interesting for future high-temperature applications. For this purpose, a batch of tungsten sheets with different thicknesses has been produced by subsequent cold rolling out of a single sintered compact. The aim of this paper is to characterise the microstructure of the as-received tungsten sheets. Quantitative grain size analysis by EBSD confirms a grain refinement by cold rolling well down into the UFG-regime, reaching 240 nm in the S-direction for the 100 μm foil. The grain refinement comes with an increase in the number of HAGBs, while LAGBs show no correlation with the degree of deformation introduced by cold rolling. All plates are distinctively textured, the main texture being the {001}〈110〉-orientation (rotated cube texture). Vickers microhardness measurements confirm a steady increase in hardness with advanced rolling up to 687 kg/mm 2 for the 100 μm foil. This indicates that the one-dimensional grain refinement to UFG by cold rolling is sufficient to achieve properties comparable to other UFG tungsten specimens. Therefore, it can be concluded that the batch of samples introduced in this paper will be fit to investigate the evolution of mechanical properties and deformation mechanisms in correlation with the microstructure.

Here we show that cold rolling decreased the brittle-to-ductile transitions (BDT) temperature of tungsten (W). Furthermore, we show that the BDT temperature correlates with the grain size (the smaller the grain size, the lower the BDT temperature) following a Hall–Petch-like equation. This relation between the grain size and the BDT temperature is well known from ferrous materials and is generally accepted in the steel community. Our ductilisation approach is the modification of the microstructure through cold rolling. In this work, we assess three different microstructures obtained from (i) hot-rolled, (ii) cold-rolled, and (iii) hot-rolled and annealed (1 h/2000 °C, annealed in H2) tungsten plates. From these plates, Charpy impact test samples with dimensions of 1 × 3 × 27 mm3, without notch, were cut and tested in the L-S and T-S directions. The results show the following BDT temperatures: 675 °C/948 K (L-S, “annealed”), 375 °C/648 K (L-S, “hot-rolled”) and 125 °C/398 K (L-S, “cold-rolled”). The microstructure of the plates is analysed by means of SEM (EBSD: grain size, subgrains, texture, KAM), FIB (channelling contrast) and TEM analyses (bright field imaging). The question of how cold rolling decreases the BDT temperature is discussed against the background of (i) microcracking, crack branching, and crack bridging effects; (ii) texture effects; (iii) the role of dislocations; and (iv) the impact of impurities, micropores, and sinter pores. Our results suggest that the availability of dislocation sources (dislocation boundaries, grain boundaries; in particular, IDBs and HAGBs) is the most important parameter responsible for the increase of the cleavage resistance stress, σF, or the decrease of the BDT temperature, respectively.

Here we show that cold-rolling is a method to achieve room-temperature ductility in commercial purity, monolithic tungsten (W). Furthermore, we show that a decrease in rolling temperature concomitantly increases the strength and ductility of tungsten. So cold-rolling is a way to overcome the strength–ductility trade-off. In this work, we assess three different cold-rolled microstructures obtained from rolling at (i) 1000 °C (1273 K), (ii) 800 °C (1073 K), and (iii) 600 °C (873 K). Benchmark experiments were performed on a sintered ingot as well as on a hot-rolled plate. From these plates tensile test specimens were cut by spark erosion and tested at room temperature. The results show an increase of total uniform elongation, A ut , ranging from 1.38% (cold-rolled at 1000 °C (1273 K), and 800 °C (1073 K)) up to 1.47% (cold-rolled at 600 °C (873 K)) and an increase of the total elongation to fracture, A t , ranging from approximately 3% (cold-rolled at 1000 °C (1273 K), and 800 °C (1073 K)) up to 4.19% (cold-rolled at 600 °C (873 K)) with decreasing rolling temperature. The microstructure of the plates is analysed by means of scanning electron microscopy (SEM) (grain size, subgrains, crystallographic texture) and transmission electron microscopy (TEM) (bright field imaging, scanning TEM). Furthermore, strain-rate jump tests have been performed at 400 °C (673 K) to determine the strain-rate sensitivity, m , (sintered ingot m = 0.088, cold-rolled at 600 °C (873 K) m = 0.011) and the activation volume, V , (hot-rolled W plate V = 191 b 3 , cold-rolled at 600 °C (873 K) V = 111 b 3 ) of the tungsten sheets. The question of why cold-rolling increases both strength and ductility is discussed against the background of cold-rolling-induced lattice defects. We speculate that the increase of ductility is caused by the ordered glide of screw dislocations, that move with low deformation incompatibility along the high-angle grain boundary (HAGB) channels (confined plastic slip).

For high-temperature environments, as in future fusion reactors, the use of tungsten materials has been sincerely discussed in the last decade. Although severe cold-rolling of tungsten leads to significant improvements in mechanical properties, the fine-grained microstructure of such tungsten material has to be stabilized. For that, the use of potassium-doping (K-doping) in tungsten sheets is investigated in our ongoing study. In this work, we compare mechanical properties of warm- and cold-rolled sheets of pure tungsten and K-doped tungsten (for five different degree of deformation respectively) by means of fracture toughness tests and tensile tests.Fracture toughness and brittle-to-ductile transition temperatures (TBDT) are assessed, showing a slightly lower transition temperature for the cold-rolled K-doped sheets (lower than −100 °C for the 50 µm thick foil). The better performance of the K-doped sheet is related to its finer grain size. The thickest K-doped sheet shows a much higher TBDT than its pure tungsten counterpart. This is presumably caused by the presence of several tens of micrometre thick bands, containing only low angle boundaries, in the microstructure of the K-doped sheet.Tensile tests reveal an outstanding yield strength of 2860 MPa and an ultimate tensile strength of 2970 MPa for the thinnest K-doped sheet with similar, but slightly lower values for the pure tungsten counterpart. Both thinnest sheets show a drastic increase in ultimate tensile strength in correlation to their mean grain size, much higher than expected by a Hall-Petch relation. This deviation has been observed for the microhardness as well and is assumed to be caused by an extraordinary increase in the density of dislocations.Our results indicate that no disadvantages in tensile strength and brittle-to-ductile transition are to be expected compared to pure tungsten, when K-doped tungsten is used to inhibit recrystallization in high-temperature environments.