Prism 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). 

The yield stress for both basal and prism plane slip as a function of temperature have been determined over a vide temperature range (Figure 9.4). The Castaing Law - In(CRSS) decreasing linearly with temperature - has been explained by Mitchell et al. [22] in terms of conventional kink pair nucleation and kink diffusion (as discussed in Section 9.2). 

Prism-plane slip occurs by the motion of (lOlO) dislocations on the 1210 plane, rather than 1 /3 (1210) dislocations in the lOlO plane, in spite of the unusually large magnitude of the Burgers vector of (lOlO) dislocations. Such dislocations... 

This reaction can occur by glide in the basal plane only for screw dislocations, and is thought to be the mechanism for the formation of a dislocation network in crystals undergoing deformation by prism plane slip [97]. Alternatively, the dislocation can lower its energy by dissociating into three collinear partials according to the reaction (see also Table 9.3) ... 

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. 

The temperature dependency of the critical resolved shear stresses (CRSS) for a single a AI2O3 crystal was estimated by Lagerlof et al. experimentally. They showed that the relation between temperatures and CRSS of both basal slip and prism plane slip in a single a — AI2O3 crystal could be described by a simple logarithmic law over a wide range of temperature ... 

At high temperatures (above about 1,200°C) alumina can deform by dislocation motion. The important paper by Merritt Kronberg [26], see also [1], p. 32, and [27], showed the details of dislocation motion in alumina. Basal slip on the close-packed oxygen planes is most common in alumina, with additional slip systems on prism planes. 

Three independent slip systems are not sufficient for arbitrary deformations. For the hexagonal crystal, this is easily understood because shear deformation out of the common slip plane of the three systems is impossible. Therefore, other, more difficult, slip systems must be activated. Because real metals never show the ideal hexagonal structure, but possess either a stretched or a compressed unit cell (varying ratio <=/ ), it depends on the chemical element which other systems are activated. Table 6.3 gives a synopsis of the most important slip systems. The slip systems with the horizontal slip plane are called basal slip systems. If the slip planes are on the prism faces of the unit cell, they are called prismatic slip systems. The other slip systems are called pyramidal slip systems. 

T13AI has an ordered DOig structirre that contains three independent slip systems that account for dislocation motion on the hasal 0001, prism 1010, and pyramidal 0221 planes ( f 1, 2). Prism shp requires only a single dislocation without creating a near-neighbor antiphase boundaiy, and additional shp requires movement of two dislocations (superdislocations) (Ref 3). In addition, two independent shp systems involving (c + a) shp occur to satisfy the Von Mises criterion for viniform deformation. 

Slip Modes in the a-Phase. Various modes of slip can occur in a-Ti or in the a-phase of titanium alloys (see Table 4). In general, slip can occur on prismatic, pyramidal and basal planes by the movement of , [c] and -type dislocations. Since the <1120> slip directions are common to aU three planes, the -type dislocations can ghde on prism, pyramid, and basal planes. The -type slip can take place on prismatic and pyramidal planes. The [c]-type glide is restricted to only prismatic planes and generally does not occur. 

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