Effect of Heat Treatment of Molten 6Bi(2)O(3)center dot SiO2 on the Phase Composition and Microstructure of Its Solidification Products

We have studied the effect of heat treatment (isothermal holding in the temperature zone C, initial cooling temperature, and cooling rate) of a melt with the composition 85.7 mol % Bi2O3 + 14.3 mol % SiO2 (6 : 1) on the phase composition and microstructure of the forming crystals and found conditions for crystallization of a delta-Bi2O3-based metastable solid solution (delta*) at high melt cooling rates. The results demonstrate that low melt cooling rates lead to crystallization of the delta*-phase and an alpha-Bi2O3-based solid solution, followed by partial or complete eutectoid decomposition of dendritic delta*-phase crystals into a mixture of metastable phases, containing beta*-Bi2O3 and eta-Bi2SiO5 as well. Raising the melt cooling rate limits both the formation of secondary phases and eutectoid decomposition. The results we obtained make it possible to control the formation of polycrystalline Bi12SiO20 and delta-Bi2O3 upon 6Bi(2)O(3)center dot SiO2 melt solidification.