Primary glide plane

The structure of Ni3Al is the Ll2 (Cu3Au) structure (Figure 8.5). It is fee with the corners occupied by A1 atoms, and the face-centers by Ni atoms. The primary glide planes are (111) and the glide directipns are (110). Therefore, the shears in the cores of dislocations in these crystals are broken into four parts as illustrated in Figures 8.6, 8.7, and 8.8. Each unit dislocation in the structure is split into four partial dislocations. 

Figure 9.2 is schematic diagram of the crystal structure of most of the alkali halides, letting the black circles represent the positive metal ions (Li, Na, K, Rb, and Cs), and the gray circles represent the negative halide ions (F, Cl, Br, and I).The ions lie on two interpenetrating face-centered-cubic lattices. Of the 20 alkali halides, 17 have the NaCl crystal structure of Figure 9.1. The other three (CsCl, CsBr, and Csl) have the cesium chloride structure where the ions lie on two interpenetrating body-centered-cubic lattices (Figure 9.3). The plastic deformation on the primary glide planes for the two structures is quite different.

It may be apparent from studying the perovskite structure that it is likely to exhibit quite anisotropic plastic (hardness) behavior, and it does. The primary glide plane is (110) and the glide direction is (1-10). 

At low temperatures, A1203 is hard and brittle, but it can be plastically deformed at high temperatures. The primary glide plane is the basal (0001) plane, and the Burgers displacement at low temperatures is 5.84 A. When the Al atoms become mobile at high temperatures this shortens to about 2.76 A. 

Table 6.1 Comparison of primary glide planes in crystals having the rock-salt...

Crystal Primary glide plane Polarizability (10 /m ) Lattice constant (nm)... 

TABLE 17.2 Comparison of Primary Glide Planes in Crystals Having a Rocksalt Structure... 

Primary glide occurs on the (111) planes. Shear of a carbon layer over a metal layer (or vice versa), when the core of a dislocation moves, severely disturbs the symmetry, thereby locally dissociating the compound. Therefore, the barrier to dislocation motion is the heat of formation, AHf (Gilman, 1970). The shear work is the applied shear stress, x times the molecular (bond) volume, V or xV. Thus, the shear stress is proportional to AHf/V, and the hardness number is expected to be proportional to the shear stress. Figure 10.2 shows that this is indeed the case for the six prototype carbides. 

Fig. 11.36. Schematic of the forest hardening process. Dislocation gliding in the primary slip plane forms junctions as a result of encounters with dislocations piercing that plane.

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... 

It is interesting to note that the plane and direction of the glide mechanism correspond to the primary slip system for NaCl [302], Indeed,... 

TEM observations of thin foils parallel to the primary (001) slip plane of the deformed samples show typical dislocation substructures for YCSZ in easy glide. The microstructure consists mostly of straight-edge dislocations and a few dislocation loops, as may be seen in Fig. 4.57. By means of the line-intercept technique, the average dislocation density was estimated to about 10 m . ... 

---