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

Abstract Beside its armor function, the designated divertor plasma-facing material tungsten (W) is also foreseen to fulfill a structural function. For fail-safe operation, strategies to overcome the brittle nature at room temperature of W are highly demanded. Since in an environment with high neutron fluences W-alloys with sufficient Rhenium concentrations have to be ruled out, the generation of tailored microstructures has come in focus. In order to assess deformation-induced modifications in the microstructure, we conducted electron backscatter diffraction (EBSD) and electron channeling contrast imaging experiments (ECCI) on warm- and cold rolled W sheets combined with studies on the related shift in brittle-to-ductile transition (BDT) temperature. With this work, we demonstrate the existence of empirical relationships between the evolution in transition temperature and the changing density of boundaries with increasing strain conducted by rolling, whereby best correlations were derived considering both, low-angle and high-angle boundaries. A Hall–Petch-like relationship based on the mean boundary spacing along the crack front and a modified version also reflecting the high aspect ratios of the grains are well-suited to describe the evolution in transition temperature and its anisotropic nature. Our findings support the theory suggesting the assisted dislocation nucleation at boundaries as decisive factor for more effective shielding of the crack tip in UFG materials leading to reduced BDT temperature. In addition, this work gives strong indications that the rolling-induced reduction of the BDT temperature in W is not related to coincidence site lattice (CSL) boundaries.