Pyramidal slip system

Further below, time-dependent deformation (creep) iiutiated by climb will be extensively discussed. In this section, an example of dislocation climb is illustrated. Figure 3.70 shows dislocation climb in an AI2O3-YAG specimen. Here, climb was assisted by thermal activation. Such a dislocation network, resulting from the reaction of dislocations from the basal and pyramidal slip systems, involves dislocation climb. It is a diffusion-controlled deformation mode characterizing creep deformation and, in this particular case, the activation energy determined is Q = 670 kJ/mol. 

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. 

Early studies on BeO single crystals by Bentle and Miller [51] identified four slip systems basal slip, (0001)(1120) prismatic slip, 1100 (1120) and 1100 [0001] and pyramidal slip, Il22 (1123). Not surprisingly, significant plastic anisotropy was found. At 1000 °C, the yield stresses for these systems were as follows 35 MPa for basal slip 49 and llOMPa for prismatic slip along (1120) and [0001], respectively and >250 MPa for pyramidal slip. More recent studies on creep deformation in BeO... 

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). 

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. 

The basal plane in the hep metals Zn and Cd is the most densely packed plane and slip generally occurs in this plane and in the <1120> direction. Prismatic and pyramidal systems, in addition to the basal system, have been shown to operate at room and elevated temperatures, depending on the mode of deformation. The c/a ratio plays an important role in determining the ease with which other slip systems... 

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