Slip systems basal

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. 

Of the 12 slip systems possessed by the CCP stmcture, five are independent, which satisfies the von Mises criterion. For this reason, and because of the multitude of active slip systems in polycrystalline CCP metals, they are the most ductile. Hexagonal close-packed metals contain just one close-packed layer, the (0 0 0 1) basal plane, and three distinct close-packed directions in this plane [I I 2 0], [2 I I 0], [I 2 I 0] as shown in Figure lO.Vh. Thus, there are only three easy glide primary slip systems in HCP metals, and only two of these are independent. Hence, HCP metals tend to have low... 

Figure 6.2 (a) The hexagonal dose-packed structure of a-alumina. (b) Two important slip systems, basal (0001) [0001] and prismatic (0110)[2110], in a hexagonal structure. 

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. 

Slip in hexagonal metal crystals occurs mainly parallel to the basal plane of the unit cell, normal to the c axis. The slip systems can be described as 000 1 (1 1 20), of which there are three. Body-centred cubic metals have slip described by 1 1 0 (I 1 1), giving 12 combinations in aU. Other slip systems also occur in metals, but those described operate at lowest energies. 

The slip system for a-alumina (corundum) is given in Table 17.1. The primary slip plane is the basal plane, (0001) the slip direction is <1120>. The arrangement of atoms on the slip plane was shown in Figure 12.12. For... 

However, in sharp contradistinction, the mechanical properties of the MAX phases cannot be more different than those of their binary cousins. The mechanical properties of the MAX phases are dominated by the fact that basal-plane dislocations multiply and are mobile at temperatures as low as 77 K and higher. The presence of basal slip is thus crucial to understanding their response to stress. This is true despite the fact that the number of independent slip systems is less than the five needed for ductility. In typical ceramics at room temperature, the number of independent slip systems is essentially zero. The MAX phases, thus occupy an interesting middle ground, in which in constraineddeformationmodes,highly oriented microstructures, and/or at higher temperatures they are pseudo-ductile. In unconstrained deformation, and especially in tension at lower temperatures, they behave in a brittle fashion. 

As noted above, basal plane - and only basal plane - dislocations are responsible for how the MAX phases respond to stress. There are no credible reports that twins and/or nonbasal dislocations participate in any meaningftil way in their deformation. It follows that at all times the number of slip systems active is less than the five are needed for polycrystalline ductility. As will become apparent shortly, most of the present understanding on the deformation of the MAX phases is based on early work carried out on Ti3SiC2, which is the most extensively MAX phase studied and best understood to date. However, there is little doubt - as confirmed by more recent studies - that what applies to Ti3SiC2 also applies to other MAX phases. 

The MAX phases, ice and graphite, and other layered minerals such as mica, are plastically anisotropic. This plastic anisotropy, combined with the fact that they lack the five independent slip systems needed for ductility, quickly lead to a very uneven states of stress when a polycrystalline sample is loaded [129]. The glide of basal plane dislocations takes place only in favorably oriented or soft grains, which rapidly transfer the load to hard grains - that is, those not favorably oriented to the applied stress. Needless to say, this leads to high internal stresses. 

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). 

The plastic deformation of the sapphire occurred due to basal and prismatic slip during loading above T. Basal slip was found in A- and B-oriented specimens and prismatic slip in C-oriented specimens. The resolved stresses at yield (according to the author) are comparable to those measured by other researchers under compression in the appropriate 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. 

As the name implies, the hexagonal close-packed crystal has the highest possible packing density. Its stacking sequence (cf. figure 1.9) differs from that of the face-centred cubic lattice. Only the 0001 -basal planes are close-packed. They contain the three (1120) close-packed directions, resulting in only three independent slip systems (figure 6.16). 

Note the possible significance of the slip direction here, i.e., <1120) for both 0001 and lOTO systems for 0001 slip, dislocations will be moving parallel to the indented surface when indenting the basal plane, and for (1010) slip only screw dislocations will be emerging onto the indented plane. On the (lOTo) indented surface, edge dislocations will emerge for 1010 slip but only screw dislocations for basal slip. 

Results from a more specific examination of hardness anisotropy using the Knoop indenter have been given in Section 3.5.1, which shows that the direction of maximum hardness in the basal, (00.1) plane depends on the cation in the mirror plane active slip systems are (00.1)(1120) for Na" -j8- AI2O3 and (00.1 ) 10l0> for Ag, K, and T1 AI2O3, which suggests... 

Experimental data suggest that the displacive a-p transition may not strongly influence the dominant slip systems. This is not unexpected because the main basal and prismatic slip systems have hexagonal symmetry. However, textures are... 

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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