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

• Published in: Journal of materials science Vol. 55; no. 26; pp. 12314 - 12337
• Main Authors: Bonnekoh, Carsten, Reiser, Jens, Hartmaier, Alexander, Bonk, Simon, Hoffmann, Andreas, Rieth, Michael
• Format: Journal Article
• Language: English
• Published: New York Springer US 01.09.2020; Springer; Springer Nature B.V
• Subjects: Activation energy; ambient temperature; Analysis; Brittleness; Characterization and Evaluation of Materials; Chemistry and Materials Science; Classical Mechanics; cold; Cold rolling; Constraining; Crack tips; Crystallography and Scattering Methods; Dislocations; Ductile fracture; Ductile-brittle transition; Fracture toughness; Gibbs free energy; Materials Science; Metals & Corrosion; Microstructure; Nucleation; Plastic deformation; Plastic properties; plasticity; Polymer Sciences; Room temperature; Sheets; Single crystals; Solid Mechanics; Temperature; Tungsten; Ultrafines
• ISSN: 0022-2461; 1573-4803; 1573-4803
• DOI: 10.1007/s10853-020-04801-5

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.