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Fracture Behavior
of Engineered Fiber/Matrix Interfaces in Fibrous Tungsten-Tungsten Composites

J. Du, T. Höschen, M. Rasinski, J.H. You, A. Brendel, F. Koch, G. Matern

Tungsten is a favored candidate for plasma-facing material due to its refractory nature and low erosion loss. However its inherent brittleness prohibits structural application, especially at lower temperatures. Neutron irradiation will even worsen the situation. Therefore an effective toughening mechanism is required to increase the toughness to an acceptable level. In this work we introduce a novel metallurgical concept to maximize the energy absorption of tungsten during crack propagation. Our approach is based on a fiber-reinforced composite combined with engineered interfaces. Both the fiber and matrix are tungsten. An interface between the fiber and matrix consists of a thin layer made of either single material or bi-material multi-layers. Since the thickness of interfacial film is about 1 micron, large amount of fibers do not affect the chemical composition.
From a mechanical viewpoint this composite is essentially heterogeneous, because a tungsten wire usually has significantly larger ductility and strength compared to its bulk counterpart. Hence the fiber reinforcement leads to enhancement of toughness and strength. On the other hand, the numerous internal interfaces can contribute additionally to fracture resistance. When a running matrix crack meets a fiber, crack may deflect along the interface. Simultaneously energy dissipation processes will be activated such as interface debonding or plastic flow of a soft layer. Such interfacial energy dissipation mechanisms play an important role in the fracture toughness of the composite. 
In this study, we present the first experimental results obtained from extensive fiber push-out tests. Interfacial fracture behavior under mode 2 load was investigated. The tests were conducted on single-filament composite specimens. Various kinds of interfaces were prepared ranging from ceramic multi-layers to ductile copper/tungsten multi-layers. The fibers were coated using magnetron sputtering. The outer tungsten matrix mantle was deposited by CVD process. Due to the chemical uniformity, no residual stress could develop. All interfacial structures remained intact and stable even after high temperature heat treatment. The electron microscopy of the damaged interfaces cut by FIB revealed the clear evidences of considerable energy dissipation events. This finding was also well supported by the load-displacement responses. Theoretical interpretation is given based on finite element simulation.

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