A survey was made of the high-temperature oxidation of yttrium metal and its effect on hardness. Corrosion of the metal was very slow at 450°C, but increased rapidly with temperature, becoming very severe at 925°C. Oxidation was found to increase the hardness of yttrium metal due to the diffusion of oxygen into the metal lattice.
Carlson, O. N.; Schmidt, Frederick A.; and Wells, R. L., "A study of the high-temperature air oxidation of yttrium metal" (1960). Ames Laboratory Technical Reports. 16.