Elucidating the dual role of grain boundaries as dislocation sources and obstacles and its impact on toughness and brittle-to-ductile transition

In this paper, we resolve the role of grain boundaries on toughness and the brittle-to-ductile transition. On the one hand, grain boundaries are obstacles for dislocation glide. On the other hand, the intersection points of grain boundaries with the crack front are assumed to be preferred dislocation nucleation sites. Here, we will show that the single contributions of grain boundaries (obstacles vs. source) on toughness and the brittle-to-ductile transition are contradicting, and we will weight the single contributions by performing carefully designed numerical experiments by means of two-dimensional discrete dislocation dynamics modelling. In our parameter studies, we vary the following parameters: (i) the mean free path for dislocation glide, δ, combined with (ii) the (obstacle) force of the grain boundary, ϕ, and (iii) the dislocation source spacing along the crack front, λ. Our results show that for materials or microstructures for which the mean distance of the intersection points of grain boundaries with the crack front is the relevant measure for λ, a decrease of grain size results in an increase of toughness. The positive impact of grain boundaries outweighs the negative consequences of dislocation blocking. Furthermore, our results explain the evolving anisotropy of toughness in cold-worked metals and give further insight into the question of why the grain-size-dependent fracture toughness passes through a minimum (and the brittle-to-ductile transition temperature passes through a maximum) at an intermediate grain size. Finally, a relation of the grain-size-dependence of fracture toughness in the form of K(dδ, dλ) = KIC + kdδ0.5/dλ is deduced

Duktilisierung von Wolfram : Synthese, Analyse und Charakterisierung von Wolframlaminaten aus Wolframfolie

Der in dieser Arbeit untersuchte Ansatz ist die Duktilisierung von Wolfram durch Synthese eines Wolframlaminats. Da Wolframfolie duktil ist und sich selbst bei Raumtemperatur plastisch verformen lässt, könnte es durch Übereinanderschichten und geeignetes Verbinden mehrerer Folienlagen gelingen, die duktilen Eigenschaften der Wolframfolie auf ein Massivbauteil zu übertragen

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