Dihedral Angles in Magnesia and Alumina: Distributions from Surface Thermal Grooves

Dihedral angles, Ψ s , from surface thermal grooves were measured using a metal reference line technique for polycrystalline MgO, undoped Al 2 O 3 , and MgO‐doped Al 2 O 3 . The values of Ψ s span the following ranges: 89° to 116° for MgO at 1520 K, 76° to 166° for undoped Al 2 O 3 at 1870 K, and 90° to 139° for MgO‐doped Al 2 O 3 at 1870 K. For all three systems, the median Ψ s values are 105° to 113°, implying that the median γ gb /γ s is 1.1 to 1.2, in contrast to metals where γ gb /γ s ranges typically from 1/4 to 1/2. The widths in Ψ s distributions were different for the three materials with the width increasing in the following order: MgO, MgO‐doped Al 2 O 3 , undoped Al 2 O 3 . For all three materials, the grainboundary grooves and their corresponding Ψ s 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 Ψ s , with the maximum inclination angles of ∼13°, in quantitative agreement with theory.