M. Rieth, Forschungszentrum Karlsruhe, Eggenstein-Leopoldshafen (Germany)
Tungsten and selected W-alloys are considered to be the primary candidates for armour and structural materials of ITER and of even more ambitious DEMO divertor designs. But even dispersion strengthened W alloys can recrystallize during DEMO relevant exposure times in the temperature range where the structural material of advanced divertors must operate (up to about 1300 °C). A critical issue is the recrystallization of W-alloys and its interaction with aging time as well as elastic-plastic deformation during mechanical loading. To evaluate their applicability for the use as structural material, a special creep testing facility has been developed and implemented successfully. It allows for performing fully instrumented creep and creep rupture tests on selected reference W-alloys at typical temperatures of the divertor structural material. Pure tungsten and W-La2O3 (WL10) rods were used for specimen fabrication. Within the test program special emphasis has been laid on the effect of any interaction between recrystallization and long term creep properties. In addition aging tests and microstructural analysis of the materials as-delivered have been performed. These have shown that pure tungsten rod material shows starting recrystallization at 1100 °C after 200 hours whereas the La2O3 dispersion strengthened tungsten material remains stable. Further, if not recrystallized, tungsten shows a very high plasticity at 1100 °C that leads to necking of more than 90 %. At the same condition WL10 shows less ductility (necking of only 20 %) but increased creep strength by about 10-20 %. At 1300 °C, creep strength of pure tungsten drops dramatically due to severe recrystallization. First pores develop at grain triple points, and then cracks propagate along grain boundaries, which finally lead to inter-granular fractures.