High temperature microstructural stability of self-passivating W-Cr-Y alloys for blanket first wall application

Compared to pure tungsten, self-passivating tungsten based alloys for the first wall armor of future fusion reactors shall provide a major safety advantage in case of a loss-of-coolant accident with simultaneous air ingress, due to the formation of a stable protective scale at high temperatures in presence of oxygen preventing the formation of volatile and radioactive WO₃. Recently developed W-Cr-Y alloys produced by mechanical alloying and hot isostatic pressing (HIP) exhibit a strong reduction of oxidation rate compared to pure W and high mechanical strength. A heat treatment after HIP at 1555 °C results in a one-phase material with a high thermal shock resistance. Nevertheless, the microstructure is metastable and its thermal stability under operational conditions has to be assessed. In this work results of thermal stability tests on heat treated W-10Cr-0.5Y alloy subjected to temperatures of 650, 700, 800 and 1000 °C for times ranging from 50 to 1000 h are presented. After 1000 h at 650 °C and 100 h at 700 °C no visible change of the microstructure is detected. After 100 h at 1000 °C a complete decomposition takes place with the formation of a uniform, fine-scale mixture of W- and Cr-rich phases, typical for spinodal decomposition.