MASSIVE TRANSFORMATION IN THE Y2O3-BI2O3 SYSTEM

Cubic solid solutions in the Y2O3-Bi2O3 system with approximately 25% Y2O3 undergo a transformation to a rhombohedral phase when annealed at temperatures less-than-or-equal-to 700-degrees-C. This transformation is composition-invariant and is thermally activated, and the product phase can propagate across matrix grain boundaries, indicating that there is no special crygtallographic orientation relationship between the product and the parent phases. Based on these observations, it is proposed that cubic --> rhombohedral phase transformation in the Y2O3-Bi2O3 system is a massive transformation. Samples of composition 25 % Y2O3-75% Bi2O3 with and without aliovalent dopants were annealed at temperatures less-than-or-equal-to 700-degrees-C for up to 10 000 h. ZrO2 as a dopant suppressed while CaO and SrO as dopants enhanced the kinetics of phase transformation. The rate of cubic/rhombohedral interface migration (growth rate or interface velocity) was also similarly affected by the additions of dopants; ZrO2 suppressed while CaO enhanced the growth rate. Diffusion studies further showed that ZrO2 suppressed while CaO enhanced cation interdiffusion coefficient. These observations are rationalized on the premise that cation interstitials are more mobile compared to cation vacancies in cubic bismuth oxide. The maximum growth rate measured was approximately 10(-10) m/s, which is orders of magnitude smaller than typical growth rates measured in metallic alloys. This difference is explained in terms of substantially lower diffusion coefficients in these oxide systems compared to metallic alloys.