GRAIN-BOUNDARY PHASES AND WAKE ZONE CHARACTERIZATION IN MONOLITHIC ALUMINA

The postfracture tensile (PFT) technique has been used to characterize the wake zones of two alumina microstructures over a temperature rang e of 20 degrees to 1280 degrees C. Differing primarily in purity and g rain size distribution, the behaviors of a commercial 99.7% alumina an d a nominally pure alumina, Lucalox (General Electric, Cleveland, OH), were evaluated, Previously, the authors have demonstrated the dominan t role of thermal expansion anisotropy in central of the toughening pr ocesses through 600 degrees C. In this paper we relate microstructural aspects to those toughening mechanisms causing the behavioral change near 800 degrees C, typical of commercial aluminas, This temperature c oincides closely with the softening point of a glassy phase reported f or some commercial aluminas, Since Lucalox exhibits the same behavior, but contains no added glassy phases, two possibilities remain: (1) tr ace quantities of a grain boundary phase are sufficient to promote the observed behavior, or (2) changes in toughening mechanisms result fro m more subtle microstructural transitions, Based upon studies of strai n rate and time-dependent behavior, we propose that the topographic ch anges of the fracture surface near this temperature may explain the in crease in toughening behavior at high temperatures.