Indirect Dissolution of (Al, Cr) 2 O 3 in CaO—MgO—Al 2 O 3 —SiO 2 (CMAS) Melts

The dissolution of (Al, Cr) 2 O 3 into CaO—MgO—Al 2 O 3 —SiO 2 melts, under static and forced‐convective conditions was investigated at 1550°C in air. With sufficient MgO in the melt, or sufficient Cr 2 O 3 in (Al, Cr) 2 O 3 , a layer consisting of a spinel solid solution, Mg(Al, Cr) 2 O 4 , formed at the (Al, Cr) 2 O 3 /melt interface. The dissolution kinetics of 1.5 and 10 wt% Cr 2 O 3 specimens were determined as a function of immersion time, specimen rotation rate, and magnesia content of the melt. Electron microprobe analysis was used to characterize concentration gradients in the (Al, Cr) 2 O 3 sample, the Mg(Al, Cr) 2 O 4 spinel, or in the melt after immersion of specimens containing 1.5 to 78 mol% Cr 2 O 3 . The dissolution kinetics and microprobe analyses indicated that a steady‐state condition was reached during forced‐convective, indirect (Al, Cr) 2 O 3 dissolution such that spinel layer formation was rate limited by solid‐state diffusion through the spinel layer and/or through the specimen, and spinel layer dissolution was rate limited by liquid‐phase diffusion through a boundary layer in the melt. This is consistent with a model previously developed for the indirect dissolution of sapphire in CMAS melts.