A unique combination of advantageous properties makes tungsten a candidate for plasma-facing components in future fusion reactors. However, its inherent brittleness and thus the lack of damage tolerance is a critical concern. In particular, embrittlement during operation due to recrystallization or neutron irradiation is up to now an unsolved problem. In tungsten fibre-reinforced tungsten composites (Wf/W) the gain in strength and ductility by intense cold working in the fibres is combined with extrinsic toughening mechanisms. The Wf/W composite concept features a possibility of local energy dissipation and therefore an increased toughness. This is effective in the as-produced and in the embrittled case, as extrinsic toughening also works for fully brittle composite components.
A novel manufacturing method for Wf/W has been developed by combining the deposition process of the well-known chemical coating process for tungsten with the transport process of chemical infiltration techniques used for the fabrication of composites [1]. The resulting process, the chemical vapour infiltration of tungsten (W-CVI) allows the fabrication of Wf/W composites at temperatures below 700° C and without mechanical load. The W-CVI method was qualified and optimized by extensive deposition experiments in which variations of fibre arrangement, temperature and gas flow were studied. This was accompanied by the development of an analytical process description taking into account mass transport (macro-kinetics) and deposition reaction (micro-kinetics). In conclusion, using W-CVI on a fibrous preform allows the fabrication of Wf/W with a density up to 95%.
The effectiveness of extrinsic toughening in Wf/W was proven by means of sophisticated mechanical tests. Tests on miniaturized samples in combination with in-situ observation by high energy synchrotron tomography were used to identify toughening mechanisms. The contribution of the fibre plasticity to the toughening was studied by means of tension tests. A newly designed method for 4-point-bending in combination with synchrotron tomography allowed the validation of active toughening mechanisms for embrittled samples. Bending tests on larger samples in combination with in-situ observation in an electron microscope were used to show the effectiveness of the toughening. A significant increase of the toughness (by a factor of 2) was proven. The experimental results gave input to an analytical estimation which shows the high potential of Wf/W in the context of toughening [2].
- J. Riesch, T. Höschen, A. Galatanu, J.-H. You, Proceedings of ICCM18 (2011)
- J. Riesch, PhD thesis, TU München (2012)