Influence of processing route and yttria additions on the oxidation behavior of tungsten

The oxidation resistance in dry air of pure tungsten and tungsten reinforced with a dispersion of 0.6 wt.% Y2O3 nanoparticles has been evaluated between 873 and 1073 K, temperature range that divertor in fusion power plants should endure during long-term times in the case of loss of coolant accident and/or air ingress in the vessel. Both materials were prepared by a powder metallurgy route involving hot isostatic pressing of ball milled tungsten powders and tungsten with dispersed Y2O3 nanoparticles. The results have been compared with those of pure tungsten processed by conventional techniques. Thermogravimetric tests at 873 K revealed that the processing route as well as yttria addition considerably affected the oxidation resistance of pure tungsten. Mass gain of W-0.6Y2O3 at 873 K was five and two times lower than that of pure tungsten prepared by conventional processing techniques and by powder metallurgy, respectively. This different behavior was related to changes in the structure and composition of the oxide scale. Above 873 K, the kinetics were significantly accelerated for all materials due to the development of a non-protective oxide scale from the earliest oxidation stages, although the kinetics of Y2O3-containing material were still the slowest, specially at 973 K. It was analyzed how yttria additions modify the oxidation mechanism of tungsten.