W foil: Toughness - Identification of the mechanisms of the evolution of the activation energy of the brittle-to-ductile transition caused by cold rolling

• Applicant: Dr.-Ing. Jens Reiser
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2015 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 274714564

Up to now, tungsten (W) has only been used as a functional material, as its low fracture toughness at room temperature and its high brittle-to-ductile transition temperature (BDTT) exclude W from being used as a structural material. So here the question of how to make W ductile arises. The approach assessed by the author of this proposal is the synthesis of a W laminate made of W foil. Cold-rolled W foil has extraordinary properties in terms of ductility and toughness. Through the synthesis of a W laminate the author succeeded in transferring the properties of the foil to the bulk. Furthermore, the author produced W laminate pipes that are convincing due to their thermo-mechanical properties and are discussed with a view to being used as structural parts for innovative high-temperature energy conversion systems.Based on the work of the author on W laminate materials, several scientific questions arise. Within the framework of this proposal, the mechanism of the evolution of the activation energy of the brittle-to-ductile transition, HBDT, caused by cold rolling will be identified.Results on the BDT of W single crystals from ROBERTS and GUMBSCH are inconsistent and show no clear scientific model. This discrepancy continues for polycrystalline W materials. Here ROBERTS says that grain boundaries have no influence on the BDT, while simulation results from HARTMAIER show that very fine-grained W materials have a reduced rate dependence and thus an increased HBDT. The reason for this behaviour is the confinement of the plastic zone by grain boundaries. This confinement leads to dislocation pile-ups at the grain boundaries and thus reduced mobility. HARTMAIER now assumes that the HBDT might be a kind of dislocation-grain-boundary interaction energy (activation of slip systems in the neighbouring grains). However, according to this model, the fracture toughness would decrease through grain refinement and the BDTT would increase through grain refinement. But this is in conflict with the experimental results of PIPPAN and the author of this proposal.The aim of the project is to identify the mechanisms of the evolution of the activation energy of the brittle-to-ductile transition through cold rolling and to support the understanding and knowledge of the brittle-to-ductile transition of polycrystalline W. In doing this, the mechanisms are identified both in a direct manner through electron microscopy analyses (EBSD, HR-EBSD, KAM, ECCI, TEM) and in an indirect manner through the determination of the activation energy of the brittle-to-ductile transition. Finally, by comparing the results of the electron microscopy analyses with the evolution of the activation energy of the brittle-to-ductile transition, the conflicts in the models of HARTMAIER, ROBERTS, PIPPAN, as well as the author of this proposal, will be solved and a modified and mechanism-based model of the brittle-to-ductile transition of W will be presented.

• DFG Programme: Research Grants