DIHEDRAL ANGLES IN MAGNESIA AND ALUMINA - DISTRIBUTIONS FROM SURFACE THERMAL GROOVES

Dihedral angles, Ψs, from surface thermal grooves were measured using a metal reference line technique for polycrystalline MgO, undoped Al2O3, and MgO‐doped Al2O3. The values of Ψs span the following ranges: 89° to 116° for MgO at 1520 K, 76° to 166° for undoped Al2O3 at 1870 K, and 90° to 139° for MgO‐doped Al2O3 at 1870 K. For all three systems, the median Ψs values are 105° to 113°, implying that the median γgb/γs is 1.1 to 1.2, in contrast to metals where γgb/γs ranges typically from 1/4 to 1/2. The widths in Ψs distributions were different for the three materials with the width increasing in the following order: MgO, MgO‐doped Al2O3, undoped Al2O3. For all three materials, the grainboundary grooves and their corresponding Ψs were not symmetrical with respect to the surface normal. The asymmetry for MgO was due to the pinning of the grain boundaries by the surface thermal grooves. The range of inclination angles of the grain boundary to the surface was a function of Ψs, with the maximum inclination angles of ∼13°, in quantitative agreement with theory. Copyright © 1990, Wiley Blackwell. All rights reserved