Crystallization of mullite-zirconia amorphous materials prepared by rapid-quenching

Abstract

Amorphous materials have been prepared by rapid quenching, -105°C/s, of the melt of the mixed oxides, 20wt% ZrO2-80wt% (3.4Al2O3⋅2SiO2) using an arc-imaging furnace and a twin-roller. This paper deals with the crystallization and the products of these amorphous materials obtained by repeating under various conditions. The crystallization of this amorphous material took place at -990°C precipitating t-ZrO2 and mullite phases after showing glass-transition temperature at 919°C on heating by 10°C/min. The mullite had markedly larger lattice parameters than that of stoichiometric mullite (3Al2O3⋅2SiO2) indicating higher (>83wt%) Al2O3 contents in the early stage of crystallization, then showed smaller lattice parameters on prolonged reheating. The TEM observation of the sample re-heated at 1200°C for 12h revealed that the product consists of mullite grains of 2-5μm in which fine (10-20nm) t-ZrO2 crystals are contained. The characteristic arrangements of the fine ZrO2 crystals in the mullite grains suggest that these t-ZrO2 crystals are precipitated by exsolution on the crystallization of the mullite from the amorphous matrix. Reeating of the amorphous materials at 880°C reduced the glass-transition and crystallization temperatures and increased the lattice parameters of mullite. This suggests that the heat treatment at 880°C caused a phase separation in amorphous samples. The phase separation would yield the precipitation of Al2O3-rich mullite in the early stage of crystallization with the exsolution of t-ZrO2 micro-crystals.