SLIP AND TWINNING IN SAPPHIRE (ALPHA-AL2O3)

The plastic deformation of sapphire (alpha-Al2O3) has been studied under hydrostatic confining pressure at temperatures below the ambient pressure brittle-to-ductile transition temperature. Samples oriented for prism plane slip (Type I samples) were deformed via dislocation slip at temperatures as low as 200 degrees C. Samples oriented for basal slip (Type II samples) could be plastically deformed at temperatures as low as 400 degrees C but showed more complicated deformation behavior, inasmuch as the sample orientation also allowed for the activation of basal twinning and two of the three rhombohedral twin systems. The temperature dependence of the critical resolved shear stress tau(crss), In tau(crss) = In tau(O) - B.T for basal slip was significantly greater than that for prism plane slip (B-basal > B-prism plane ), causing the latter system to be the easy slip system below approximate to 600 degrees C (basal slip is the easy slip system at elevated temperatures). Type II samples deformed primarily by basal twinning in preference to both rhombohedral twinning and basal slip. The different temperature dependence of tau(crss) for basal and prism plane slip is attributed to details of the dislocation core structure; prism plane dislocations, having a large Burgers vector (\bpp\ = 0.822 nm), can dissociate into three collinear partials (\bp\ = 0.274 nm) separated by relatively low-energy stacking faults, whereas the comparable dissociation of basal dislocations(\b(B)\ = 0.476 nm) produces two noncollinear partials separated by a relatively high energy stacking fault. Thus, dissociation of basal dislocations is most likely restricted to the dislocation core, which is manifested in a higher Peierls stress at low temperatures for basal slip compared to prism plane slip.