Effect of rapid cooling on the microstructure and mechanical properties of commercial 85 wt.% Al 2O 3

Abstract

Tiles of structural 85 wt.% Al 2O 3 were subjected to post-fabrication heat treatment. Microstructural investigation indicated that the material contained two interconnected phases: a skeleton of crystalline Al 2O 3 and about 30 vol.% of an Al 2O 3-rich alumino-silicate glass. Rapid cooling of tiles from high temperatures resulted in an increase in transverse rupture strength σ trs and of fracture toughness K Ic for three different directions of loading and crack propagation. A pronounced increase in K Ic was also observed for cracks propagating in the mid-plane of the tile normal to the cooling direction, while macroscopic tensile thermal stresses should lead to a decrease in σ trs and K Ic for this case. X-ray analysis showed that rapid cooling from temperatures above 1300°C resulted in dissolution of the anorthite and cordierite precipitates that were observed in the glassy phase in slowly cooled samples. Annealing of rapidly cooled samples at temperatures up to 1000°C with subsequent slow cooling completely removed the effect of rapid cooling on the mechanical properties, even though no precipitation of anorthite or cordierite was observed. The effect of heat treatment on the volume fraction of the glassy phase, the grain size and the density was also investigated.