Crystallization of MgO‐Al ₂ O ₃ ‐SiO ₂ ‐ZrO ₂ Glasses

Abstract

The crystallization of MgO‐Al 2 O 3 ‐SiO 2 ‐ZrO 2 glasses at 1000°C was studied. Isothermal heat treatments of a cordierite‐based glass (2MgO.2Al 2 O 3 .5SiO 2 = Mg 2 Al 4 Si 5 O 18 ) with 7 wt% ZrO 2 produced surface crystallization of α‐cordierite and tetragonal ZrO 2 ( t ‐ZrO 2 ). These phases advanced into the glass by cocrystallization of t ‐ZrO 2 rods in an α‐cordierite matrix with a well‐defined orientation relation. The t ‐ZrO 2 rods were unstable with respect to diffusional breakup (a Rayleigh instability) and decomposed into rows of aligned ellipsoidal and spheroidal particles. The t ‐ZrO 2 was very resistant to transformation to monoclinic symmetry. With a similar glass containing 15 wt% ZrO 2 , surface crystallization of α‐cordierite and t ‐ZrO 2 was accompanied by internal crystallization of t ‐ZrO 2 dendrites. Transformation of the dendrites to mono‐clinic symmetry was observed under some conditions.