Defects in plastically deformed 6H SiC single crystals studied by transmission electron microscopy

Abstract

Transmission electron microscopic studies have been performed on lattice defects in 6H silicon carbide single crystals; the crystals were subjected to microhardness indentation tests between room temperature and 1200°C and then to uniaxial compression at temperatures from 1100 to 1650°C. It was observed that their basal dislocations, the main defects introduced by deformation, were dissociated into Shockley partials separated by very wide stacking faults, the energy of which was evaluated to be 2.5 ± 0·9 mJ m⁻². The lineshapes of the partials varied considerably, depending on the sign and angle (with the dislocation line) of the Burgers vector; this suggests that the mobility of the partials is affected by the geometry of dislocations. The fact that basal dislocations were aligned more or less in crystallographic 〈1120〉 directions indicates the presence of a high Peierls potential. Macroplastic deformation was observed above 1000°C. This critical temperature seems to be determined by the motion of the least mobile partials. The ductile–brittle transition occurred between 800 and 1000°C. This transition is considered to have been induced by the temperature-enhanced mobilization of the slowest partials or by the thermally activated constriction of dissociated pairs, both of which are prerequisites for dislocation multiplication. The hardness of the crystals showed a strong temperature dependence below 800°C. This can be explained by assuming that, although no dislocation multiplication occurs, only very mobile partials experience thermally activated glide. Other implications of the experimental findings are discussed.