$\alpha$-Al$_2$O$_3$ sapphire and rubies deformed by dual basal slip at intermediate temperatures (900C-1300$^\circ$C) -- II. dissociation and stacking faults

Abstract

Sapphire and rubies (undoped and Cr-doped $\alpha$-Al$_2$O$_3$ single crystals) have been deformed in compression at temperatures lower than those previously used in studies of dislocations in the basal slip plane (see part I). Above 1400$^\circ$C, several features associating stacking faults out of the basal planes and partial dislocations (dissociation, faulted dipoles) have been observed in previous transmission electron microscope investigations. The formation of these features involves climb controlled by atomic diffusion. Properties of climb dissociated dislocations are discussed in relation with dislocation dynamics. TEM examination of dislocation structures at lower deformation temperatures (1000-1100$^\circ$C) shows that similar features are formed but that they often imply cross-slip. A new mechanism for the formation of faulted dipole by glide is presented and an explanation for the 30{\deg} Peierls valley orientation is proposed. The presence of chromium has a small influence on stacking fault energies on planes perpendicular to the basal plane