Sapphire basal plane slip

Figure 9.4 Plots of log(CRSS) versus temperature for basal and prism plane slip in sapphire showing both experimental data and curves fitted from Eq. (1) [22].

The value of Q is higher for basal slip (1.9 eV) than prism plane slip (1.5 eV) in sapphire, as expected from the stronger temperature dependence of the CRSS for the former. 

The availability of sizable single crystals has led to a significant literature on the deformation of sapphire of various orientations, and at various temperatures. As already noted, the first such study was by Wachtman and Maxwell in 1954 [6], who activated (0001) 1/3 (1120) basal slip at 900 °C via creep deformation. Since that time, it has become clear that basal slip is the preferred slip system at high temperatures, but that prism plane slip, 1120 (1100), can also be activated and becomes the preferred slip system at temperatures below 600°C. Additional slip systems, say on the pyramidal plane 1012 1/3 (1011), have very high CRSSs and are thus difficult to activate. Both, basal and rhombohedral deformation twinning systems, are also important in AI2O3 (these are discussed later in the chapter). 

Stress-strain curves in sapphire exhibit a three-stage hardening both for basal and prism plane slip [97, 201]. The mechanisms for each are quite different from each other, and also from those described above. 

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