VARIATION OF GLASSY PHASE IN Al//2O//3-SiO//2 CERAMIC SHELL MOULDS.

It was shown in previous studies that an effective way to increase the high-temperature creep resistance of shell molds is to make the glassy phase crystallize. To control the glassy phase in shell molds, we must understand how various factors influence it variation. Variation of the amount and composition, and transformation of glassy phase in shell molds made from low-impurity fused refractories which have compositions varying from mullite composition to corundum composition are investigated. The traditional hf leaching techniques used for determining the amount of glassy phase has been modified and the error induced in the method by neglecting the amount of mullite dissolved in hydrofluoric acid has been eliminated. The stepwise regression method is used to process experimental data on the amount and composition of glassy phase, and regression equations are obtained that are more suitable than other methods for estimating the amount and composition of glassy phase in such multicomponent, non-equilibrium systems as the shells. Results indicate that the two main modes of transformation from glassy phase into crystalline phase in Al//2O//3-SiO//2 shell molds during firing are: (1) formation of mullite through chemical reaction; (2) crystallization of mullite from the glassy phase that comes from the refractories. The results appear to be helpful in controlling glassy phase in Al//2O//3-SiO//2 shell molds.