Stability of oxide phases in W–Cr–Y SMART alloys

Abstract

The addition of Cr and Y into tungsten can dramatically increase the oxidation resistance of the first wall of a future fusion reactor, thereby reducing the risk of formation of volatile WO${}_3$ and the release of radioactive material. Experimental observations suggest that in these SMART alloys, yttrium facilitates the formation of a self-passivating layer of Cr₂O₃ at the metal surface, however, how exactly the Y does this remains unclear. Therefore, this work explores the phase stability of compounds consisting of W–Y–Cr–O and solution energies for the different components in tungsten using density functional theory. The simulations suggest that there is a substantial thermodynamic driving force for the formation of Y${}_2$O${}_3$, especially from yttrium and oxygen solvated in bulk tungsten. These observations suggest that the role of the yttrium may be to remove the oxygen that may inhibit Cr diffusion to the surface from the tungsten grains. This observation is in accordance with experimental studies showing that the oxidation resistance in the alloy occurs when the oxygen–yttrium ratio in the alloy is close to the stoichiometric ratio for Y${}_2$O${}_3$.