Measurements of dislocation velocity in sapphire (alpha-alumina) single crystals

The dislocation sub-structure around indents made on the basal plane in sapphire single crystals at elevated temperatures have been investigated using selective etching, optical microscopy and transmission electron microscopy. Dissociation of screw < 1&d1;100 > dislocation into three collinear partials are observed. Details of the core structure of the screw dislocations are reported.; The basal plane dislocation velocity was also measured on four point bend samples.; After the incubation time, the presence of mobile basal dislocations causes plastic deformation of the bend sample. The velocity of such dislocations was estimated from the bending displacement rate of the four point bend sample.; The basal slip dislocation velocity was faster than that of the prism plane slip dislocations even though the applied shear stress was lower. This is consistent with the fact that basal slip is the easy slip system at elevated temperature. The yield drop and incubation time in sapphire undergoing basal slip can be explained by the recombination of climb-dissociated partials back onto the glide plane prior to slip. A simple model using this assumption to explain the macroscopic behavior was developed and compared with the experimental observations. The theoretical model was consistent with the experimental observations.; A study of the brittle-to-ductile transition (BDT) behavior of sapphire single crystals was performed to compare the activation parameter for dislocation motion and the brittle-to-ductile transition model suggested by St. John. [1] Assuming that the BDT behavior is related to the velocity of prism plane slip dislocation, the activation energy of prism plane slip was estimated to be around 3.5 eV (in the range of 2.3--6.9 eV). This value is compatible with the result of the dislocation velocity using the etch pit techniques on bend samples. (Abstract shortened by UMI.); [1] C. St. John, "The Brittle-Ductile Transition in Pre-Cleaved Silicon Single Crystals", 32, 1193--1212 (1975)...