Yielding and Flow of c‐Axis Sapphire Filaments

Abstract

Yielding and flow in uniaxial, constant‐strain‐rate tensile tests of single‐crystal sapphire filaments oriented with the tensile axis parallel to the c axis were studied between 1760° and 1875°C. The existence of a strain‐aging effect leading to non‐reproducible upper yield points was established. The flow stress exhibited a power‐law dependence on strain rate, with stress exponents ranging from 8.50 at 1775° to 12.4 at 1875°C. The apparent activation enthalpies calculated from constant‐temperature experiments reflect a dependence of flow stress on temperature which includes both substructure and deformation‐mechanism temperature dependencies. The preliminary activation enthalpies calculated from differential temperature and differential strain‐rate (constant‐structure) experiments are comparable to the value for defect diffusion in sapphire, 80 kcal/mol. From direct evidence, the {101} planes are identified as the dominant slip planes with the 1/3 〈101〉 Burgers vectors inferred from the testing geometry.