Slip and Twinning in Sapphire (α‐Al₂O₃)

Abstract

The plastic deformation of sapphire (a-AI,O,) 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°C. Samples oriented for basal slip (Type I1 samples) could be plastically deformed at temperatures as low as 400°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 ( I~~J ,for basal slip was significantly greater than that for prism plane slip (Bbasal > BpriPmplane), causing the latter system to be the easy slip system below -600°C (basal slip is the easy slip system at elevated temperatures). Type I1 samples deformed primarily by basal twinning in preference to both rhombohedral twinning and basal slip. The different temperature dependence of T , , for basal and prism plane slip is attributed to details of the dislocation core structure; prism plane dislocations, having a large Burgers vector ( lbppl = 0.822 nm), can dissociate into three collinear partials ([bpi = 0.274 nm) separated by relatively low-energy stacking faults, whereas the comparable dissociation of basal dislocations (lbsl = 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.

Disconnection arrays in a rhombohedral twin in α-alumina

“…are dissociated by climb at high temperature [23]. This is also the case in low angle GBs in alumina [24], as well as in strontium titanate [25].…”

Superplastic deformation of directionally solidified nanofibrillar Al2O3–Y3Al5O12–ZrO2 eutectics

“…In the case of AI2O3, plastic deformation mainly occurs by basal slip and, to a lesser extent and higher temperature, by prism plane slip. 27 The Schmid factor for both basal and prismatic slip was zero, however, because the rods were textured and the c-axis of sapphire was parallel to the longitudinal stresses induced by bending. Although pyramidal and rhombohedral slip have also been reported in AI2O3, the stresses required are too high to be active in the ternary eutectic rods.…”

Oxide Ceramics with High Density Dislocations and Their Properties

“…9,12) As the primary slip systems of sapphire, the basal slip ((0001), b ¼ 1=3h11 2 20i) and the prismatic plane slip (f11 2 20g, b ¼ h1 1 100i) are known, but the basal slip system is dominantly activated at high temperature above 700 C. 21,22) In order to form the dislocations by introducing only the basal slip, deformation was performed along the compression axis with the 45 tilted direction to both the basal plane and the slip direction so that the Schmidt factor is 0.5. This is the condition where other slip systems do not occur easily from the fact that sapphire has a rhombohedral structure.…”