Der Spröd-duktil-Übergang in ultrafeinkörnigem Wolfram

Exzellente mechanische Eigenschaften bei hohen Materialtemperaturen und die höchste Schmelz-temperatur aller Metalle erheben Wolfram (W) zum Material der Wahl für Komponenten, die höchste Wärmelasten zu widerstehen haben. Eine hohe Spröd-duktil-Übergangstemperatur und das hier-durch bedingte spröde Materialverhalten bei Raumtemperatur (RT) behindern jedoch die Ausle-gung, den sicheren Umgang und Betrieb von Komponenten aus W. Aktuelle Studien weisen darauf hin, dass diese Limitierungen durch eine hochgradige plastische Verformung von W überwunden werden können. Solch hochgradig umgeformten Materialien besitzen eine ultrafeinkörnige (UFG) Mikrostruktur und zeigen selbst bei RT eine nennenswerte Brucheinschnürung im Zugversuch bzw. stabiles Risswachstum in Experimenten bruchmechanischer Natur. Die materialphysikalischen Hin-tergründe der Duktilisierung von W durch die UFG Mikrostruktur konnten bisher nicht abschließend geklärt werden. Im Fokus der Diskussionen stehen aktuell: (i) Was ist der ratenlimitierende Prozess der Rissspitzenplastizität und somit der Mechanismus, der den Spröd-duktil Übergang (BDT) in UFG W kontrolliert? (ii) Welchen Beitrag leistet die UFG Mikrostruktur zu der beobachteten Ver-schiebung der BDT-Temperatur? Im Rahmen dieser Arbeit wurde eine Garnitur aus fünf UFG W-Materialien mittels hochgradigem Warm- und Kaltwalzen produziert. Unter Zuhilfenahme eines sequenziellen Produktionsprozesses konnten, bei unveränderter chemischer Zusammensetzung, die Umformgrade der Materialien ge-staffelt realisiert werden. Der Einfluss der plastischen Verformung auf die BDT-Temperaturen wur-de anhand von bruchmechanischen Versuchen bestimmt und die Übergangstemperatur von UFG W hinsichtlich einer möglichen Ratenabhängigkeit überprüft. Diese Arbeit stellt nach besten Wissen die erste experimentelle Untersuchung dar, in welcher eine Ratenabhängigkeit der BDT-Temperatur in UFG W nachgewiesen werden konnte. Damit geht ein-her, dass in dieser Ausarbeitung erstmalig ein Versuch unternommen werden konnte anhand von BDT-Arrhenius-Aktivierungsenergien den ratenkontrollierenden Prozess des BDT in UFG W zu iden-tifizieren. Die Ergebnisse belegen, dass die Kinkenpaarbildung, selbst bei einem BDT weit unter-halb von RT, den ratenlimitierenden Prozess der Rissspitzenplastizität darstellt. Unter quasi-statischer Belastung kontrolliert damit die Kinkenpaarbildung in W über viele mikrostrukturelle Grö-ßenordnungen hinweg den BDT; beginnend mit Einkristallen, über grob- und feinkörnige Zustände hinunter bis zu UFG Mikrostrukturen. Hinsichtlich der mit einer plastischen Verformung einherge-henden Reduktion der BDT-Temperatur rücken Einflussanalysen die Korngrenzen in den Mittelpunkt des Interesses. Eine in dieser Arbeit entwickelte Formulierung beruhend auf den mittleren Abstän-den der Groß- und Kleinwinkelgrenzen (i) entlang der Rissfront und (ii) parallel zum Normalenvektor der nominellen Rissebene befähigt zu erfolgreichen Prognosen über die verformungsinduzierte Reduktion der Übergangstemperatur. Im Kontext aktueller Simulationen zum Einfluss der mittleren Distanz von Versetzungsquellen und der freien Weglänge von Versetzungen stützen die Befunde dieser Ausarbeitung die Hypothese einer entlang der Rissfront assistierten Emission von Verset-zungen als Quelle der für W beobachteten verformungsinduzierten Reduktion der BDT-Temperatur. Als Quintessenz dieser mehr als 500 bruchmechanischen Versuchen umfassenden Studie zum BDT in UFG W (zuzüglich der mikrostrukturellen Charakterisierung) wird geschlussfolgert, dass der ge-ringe Abstand von Korngrenzen in UFG Materialien mit einer hohen Dichte an Punkten der Verset-zungsnukleation entlang der Rissfront korrespondiert und hierdurch eine effektive Abschirmung der Rissspitze erzielt wird

Der Spröd-duktil-Übergang in ultrafeinkörnigem Wolfram

Hohe Spröd-duktil-Übergangstemperaturen schließen den Einsatz von konventionell gefertigtem Wolfram (W) als Strukturwerkstoff aus. Ultrafeinkörnige (UFG) W-Werkstoffe zeigen jedoch den Spröd-duktil-Übergang bereits bei sehr tiefen Temperaturen und weisen folglich Raumtemperaturduktilität auf. Die korrelative Mikrostrukturanalyse offenbarte einen starken Zusammenhang zwischen der Übergangstemperatur und dem mittleren Korngrenzenabstand entlang der Rissfront

The brittle-to-ductile transition in cold-rolled tungsten sheets: the rate-limiting mechanism of plasticity controlling the BDT in ultrafine-grained tungsten

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

The brittle-to-ductile transition in cold-rolled tungsten sheets: Contributions of grain and subgrain boundaries to the enhanced ductility after pre-deformation

One of the key demands on tungsten (W) as designated plasma-facing material (PFM) is the capability to fulfill a structural function. Since W has refused ductilization strategies by alloying alone, the production of W materials with enhanced ductility has come into focus considering tailored microstructures. This work addresses the rolling-induced microstructural modifications of warm- and cold-deformed W sheets and is supplemented by a comprehensive fracture mechanical study as a fundament for correlations between the spatial distribution of boundaries and brittle-to-ductile transition (BDT) temperature. Here we show that an extended Hall–Petch-like relationship is well suited to describe the rolling-induced reduction in BDT temperature and moreover has the potential to reflect the anisotropic nature of the transition temperature in severely rolled W sheets. Using the data of warm- and cold-rolled W sheets and also of strongly recovered W, best description of the BDT temperature was achieved by using as microstructural variables (i) the mean spacing between boundaries which intersect with the crack front and (ii) the mean boundary spacing along the normal of the crack plane. Taking into account the similarity to recent simulative-derived relationships, our findings support the theory suggesting the stimulated dislocation nucleation at boundaries as the decisive factor for more effective shielding of the crack tip in UFG materials and, in consequence, significantly reduced BDT temperatures. Besides, this work gives strong indications that the reduction of the BDT temperature in UFG W is not related to coincidence site lattice (CSL) boundaries

The brittle-to-ductile transition in cold-rolled tungsten sheets: On the loss of room-temperature ductility after annealing and the phenomenon of 45° embrittlement

The high brittle-to-ductile transition (BDT) temperature of conventionally produced tungsten (W), challenges the design of W-based structural components. Recent studies have demonstrated the potential of cold rolling to produce W sheets, which are ductile at room temperature and exhibit a BDT temperature of 208 K. In order to assess the thermal stability of these materials, we conducted isothermal heat treatments (at 1300 K, for annealing durations between 0.1 h and 210 h) combined with studies on the evolution of mechanical properties and microstructure of a severely deformed undoped W sheet. With this work, we demonstrate the need for a stabilized microstructure before utilization of cold-rolled W in high-temperature applications can take place successfully. After annealing at 1300 K for 6 h, the material properties changed remarkably: The BDT temperature increases from 208 K to 473 K and the sharp BDT of the as-rolled condition transforms into a wide transition regime spanning over more than 200 K. This means in fact, an endangered structural integrity at room temperature. We also address the so-called phenomenon of 45° embrittlement of W sheets. Here we show that cleavage fracture in strongly textured W sheets always takes place with an inclination angle of 45° to the rolling direction, independent of the studied material condition, whether as-rolled or annealed. An in-depth study of the microstructure indicates a correlation between an increased BDT temperature caused by annealing and microstructural coarsening presumably by extended recovery. We conclude that 45° embrittlement needs to be comprehended as a combined effect of an increased spacing between grain boundaries along the crack front, leading to an increased BDT, and a high orientation density of the rotated cube component or texture components close to that, which determine the preferred crack propagation of 45° to the rolling direction

Elucidating the microstructure of tungsten composite materials produced by powder injection molding

A detailed microstructural analysis is one key factor for establishing structure–property relationships, which themselves are essential for manufacturing any device or part thereof. In particular, this paper focuses on the microstructural analysis of tungsten composite materials produced by powder injection molding (PIM). Our combined scanning electron microscopy (SEM) and transmission electron microscopy (TEM) approach revealed that W/TiC and W/Y2O3 composites are promising candidates for e.g. plasma facing components in future fusion reactors. The grains size distribution of all present phases was a log-normal one. TiC and Y2O3 precipitates in contrast to HfC ones limited the grain growth of the tungsten matrix during sintering about three times more efficient. The precipitate grain size was for all samples in the range of 1.7 µm–3.5 µm. Chemical interaction was only observed for TiC-based composites in the form of W diffusion into the TiC precipitate forming a mixed (Ti, W) carbide retaining the face-centered cubic (fcc) based crystal structure of pure TiC. The tungsten content in Y2O3 and HfC precipitates was found to be negligible. La2O3 was only observed in TEM attached to (Ti, W)C particles in the form of about 100 nm sized precipitates. As result, the Y2O3 and TiC containing samples are considered as promising materials for further detailed mechanical and microstructural investigations

Effect of neutron irradiation on ductility of tungsten foils developed for tungsten-copper laminates

Severe plastic deformation of tungsten (W) is known to be an efficient way to reduce its inherently high ductile-to-brittle transition temperature (DBTT), what is essential for its use in components of a fusion reactor. Thin rolled W foils possess superior mechanical behaviour at room temperature (RT), as demonstrated in previous works. It was then proposed to expand the beneficial mechanical properties of the foil to bulk by fabricating tungsten-copper (W-Cu) laminate composites, which can be used for structural applications. Neutron irradiation in HFIR resulted in embrittlement of the laminate already after 0.016 dpa, with the W foil determining the composite behaviour. In this work, for the first time, we investigate the effect of neutron irradiation on individual W foil, and determine the resulting DBTT shift with the help of cantilever bend tests, using bulk W and the W-Cu composite as reference. The W foil and the bulk samples were irradiated to 0.15 dpa at 400 °C in the BR-2 reactor in Mol (Belgium). We also hypothesise that diffusion of Cu atoms into W could modify the response to irradiation in these materials. We substantiate it with complementary density functional theory (DFT) ab initio calculations to analyse the Cu-vacancy and Cu-self-interstitial interaction, which helps to elucidate co-alignment of the fluxes of point defects and Cu solutes in W matrix. Irradiated foil was found to retain its ductility at RT. No significant irradiation hardening or DBTT shift were detected in the irradiated W foil compared to the bulk W. The different irradiation effect on embrittlement in individual foils and in the laminate may be attributed to the irradiation-assisted diffusion of Cu solutes in W foil, which could form intermetallic phases and affect the accumulation of lattice defects

Cold Spray metal powder deposition with 9 %Cr-steel applied for the HCPB First Wall fabrication: Proof of concept and options for ODS steel processing

At the KIT a hybrid manufacturing concept for nuclear fusion First Walls is developed combining aspects of conventional and Additive Manufacturing (AM) technologies. The state of the art for ITER does not cover all specifications of a DEMO relevant First Wall. Thus, additional R&D-work has been initiated in terms of manufacturing. The AM technology basis used in the presented process combination is Cold Spray metal powder deposition applied in alternation with machining including the feature of filling grooves temporarily with a water-soluble granulate for creation of closed channels and cavities. Thus, the technology provides the option to manufacture shells with a thin gas tight membrane on top of previously machined structures. This membrane is used as pressure seal and makes the joining of shells by Hot Isostatic Pressing (HIP) into one monolithic body possible. This paper describes the manufacturing process and recalls differences and common aspects with regard to conventional concepts of First Wall manufacturing. The achievement of Technology Readiness Level TRL 3 by mechanical qualification and comparison of the results to other HIP joint experiments is also demonstrated. Finally, an outlook is given concerning integration options of the technology into manufacturing of shells with cooling channel structures using Oxide Dispersion Strengthened (ODS) materials