Tungsten alloys currently represent prospective candidates to replace tungsten in the first wall applications in future fusion facilities. Main purpose of the alloying of tungsten is to gain additional properties such as ability of self-passivation under accidental conditions. The self-passivating alloys are designed to minimize possible accident consequences related mainly to a LOCA (Loss of Coolant Accident) event with simultaneous air ingress into the reactor vessel. A LOCA would lead to a temperature rise of the in-vessel components up to 1200℃ (depending on the reactor design) due to the nuclear afterheat. However, pure tungsten at temperatures above 500℃, in the presence of air or water reacts to produce volatile tungsten trioxide which would be the cause of mobilization and potential release of radioactivity. Unlike pure tungsten, self-passivating tungsten alloys contain additions with the ability to form compact oxide scale. As soon as the oxide scale is formed, activated tungsten cannot be released to the surroundings. The main oxide-forming elements added to tungsten is chromium. It was found that the alloy performs correctly only in the saturated state, i.e. exceeding equilibrium solubility limit of Cr. This means that under normal operation condition and accidental condition the alloys is thermodynamically unstable. According to the W-Cr phase diagram, the single solid BCC solution experiences tendency for phase separation within the miscibility gap to reach thermodynamic unequilibrium. Within the miscibility gap two BCC phases are predicted to coexists, i.e. Cr rich BCC phase and W rich BCC phase.
There is very little known about W-Cr system. Phase content of a W-Cr alloy was evaluated last in few papers from ‘60s and ‘70s. Back then tungsten-chromium alloys had no large-scale application; however, in the future W-Cr alloy is foreseen to cover hundreds of square meters of fusion reactor vessel. For example, International Thermonuclear Experimental Reactor that is currently under construction in Cadarache (France) has surface of 620 m2. Expected lifetime of one plasma facing component is 5 years. Within this time period it is expected that the armor material of the component, where a self-passivating tungsten alloy is foreseen, will not experience a significant change in microstructure, phase content, mechanical and oxidation behavior. In the last two years, questions on thermal stability of W-Cr alloys has re-occurred in literature and conferences. The preliminary results show that the decomposition kinetics can be fast around the temperature of 1000℃, i.e. around the accidental conditions. In such a case, changes in the phase content and degradation of oxidation performance is inevitable. Further, the decomposition rate decreases with decreasing temperature, thus it is very difficult to experimentally verify the lifetime of the component if the changes will be apparent after year-long heat treatment. This might be further complicated by the decomposition behavior that might lead to very fine precipitates difficult to observe in conventional scanning electron microscopes. All this changes will significantly influence the oxidation performance of the alloy. Thus, it is of a prime importance to gain more knowledge about tungsten-alloys especially regarding thermal stability and stabilizing the saturated solid solution. The ab initio computational modelling methods have the potential to bring broader understanding of the W-Cr alloys system and help to answer following questions: (1) what are the mechanisms of W-Cr solid solution stabilization? (2) What is the formula describing the decomposition kinetics at lower temperatures? (3) Is different stabilization necessary for spinodal region and nucleation and growth region of the miscibility gap?
This work is supported by the GAČR standard grant No. 20-18392S Tailoring thermal stability of W-Cr based alloys for fusion applications (CZ: Modifikace teplotní stability slitin na bázi W-Cr pro aplikaci ve fúzních reaktorech).