Brittle-to-ductile transition in tungsten single and polycrystals: Microsturcture and failure mechanisms

• Applicant: Professor Dr. Alexander Hartmaier
• Subject Area: Synthesis and Properties of Functional Materials
• Term: from 2006 to 2009
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 5455539

The microstructure plays a decisive role for the failure mode and the resulting fracture toughness of all brittle materials and particularly for brittle metals. Grain boundaries are easy fracture paths and serve as obstacles and sources for dislocations at the same time. In a similar way a pre-existing dislocation microstructure will have a pronounced influence on crack-tip plasticity and fracture toughness. A detailed understanding of all these micro-mechanisms contributing to deformation and fracture is therefore required to better control materials response with respect to fracture and the BDT. Because of the availability of detailed studies of single crystalline tungsten this material is chosen as out model material. In the proposed research project the brittle and ductile failure mechanisms of tungsten will be investigated in a combined experimental and modeling effort. The projects starts from a solid understanding of fracture in tungsten single crystals that has been developed in the literature in the last decades, and to which the applicants contributed significantly. The new research initiative to investigate polycrystalline materials and materials with a pre-existing dislocation microstr-ucture is a Konsequent step towards making the knowledge gained on single crystals applicable in a technological framework. At the same time there is a number of unsolved questions of great fundamental interest. For example: How are cracks initiated in the absence of brittle inclusions; Which role do grain boundaries or pre-existing dislocation forests play in this process; Does the initiation process decide whether predominantly interor transgranular fracture occurs? The main objektive of this project is the development of qualitative understanding and a quantitative description of the deformation and damage mechanisms in tungsten (single and) polycrystals. Theoretical and experimental investigations on atomic, mesoscopic and macroscopic length and time scales are required for an understanding of the influence of microstructure on the transition from brittle to ductile failure. The project relies on a combination of multiscale modeling and experimental work, because only the mutual guidance both approaches provide in designing new experiments and simulations holds the promise for a successful research project. Moreover experimental verification of simulation results and validation of critical input pararneters for empirical models are essential for the progress in modeling. The development of a physically motivated quantitative description of fracture toughness and the brittle-to-ductile transition in tungsten polycrystals is a goal that is attraktive from fundamental as well as from applied aspects. Based on the state of knowledge in the literature and the experience of our team this goal seems now to be within reach.

• DFG Programme: Research Grants
• Participating Persons: Professor Dr. Peter Gumbsch; Professorin Dr. Sabine Maria Weygand