Secondary slip plane

Since MgO has a structure similar to that of NaCl crystal, its slip systems are also of the type 110 (110). There are six such planes in a cubic structure. In a cubic structure, four of the six 110 slip-plane projections onto the (001) slip planes leave traces along the (100) direction and lie at 45° to the surface these are known as 45° slip planes . Two have traces along the (110) direction, lie at 90° to the surface and are referred to as 90° slip planes . The shortest Burgers vector for a perfect dislocation in the NaCl structure is a/2 (110). This is the operative slip direction. There is only one (110) slip direction in each 110 slip plane. Therefore, there can be only one kind of mobile dislocation in each slip plane. The secondary slip systems are of the type 001 (110). Since the primary and secondary slip planes have a common Burgers vector, this is the cross-slip plane. No two primary slip planes have the same Burgers vector. 

Although the external shear stress on this so-called secondary slip plane or cross slip plane is smaller than on the primary one, moving along this path can be easier than trying to overcome the obstacle by cutting or the Orowan mechanism. This is the case if the effective shear stress t (see section 6.2.9) on the secondary slip plane is larger than on the primary one due to the absence of the obstacle force. Because screw dislocations can use this additional mechanism, they are frequently able to overcome obstacles more easily than edge dislocations. 

The electroviscous effect present with solid particles suspended in ionic liquids, to increase the viscosity over that of the bulk liquid. The primary effect caused by the shear field distorting the electrical double layer surrounding the solid particles in suspension. The secondary effect results from the overlap of the electrical double layers of neighboring particles. The tertiary effect arises from changes in size and shape of the particles caused by the shear field. The primary electroviscous effect has been the subject of much study and has been shown to depend on (a) the size of the Debye length of the electrical double layer compared to the size of the suspended particle (b) the potential at the slipping plane between the particle and the bulk fluid (c) the Peclet number, i.e., diffusive to hydrodynamic forces (d) the Hartmarm number, i.e. electrical to hydrodynamic forces and (e) variations in the Stern layer around the particle (Garcia-Salinas et al. 2000). 

The rock-salt structure is shown in Fig. 6.13. In crystals having this structure, the smallest spacing between ions of the same type is along <110> and the most widely spaced planes with these closely packed directions are the 100 planes. Experimental observations confirm the slip direction as <110> but the slip planes are usually found to be the 110 planes. The systems for which slip is easiest are termed the primary slip systems and, thus, for rock-salt structures they are usually 110 < li0>. Slip may occur with greater difficulty on other systems and these are termed secondary slip systems. Slip does not occur on 100 planes because of the electrostatic interaction that occurs between the ions in this process. This is depicted in Fig. 6.14, in which the initial (a, c) and mid-shear (b, d) positions of the ions are shown. For slip on 100 planes (a to b), the distance between like ions is increased and between opposite ions, it is decreased. For slip... 

Mechanical (101) [101] twins have been identified in experimentally deformed hornblende single crystals, as well as dislocations on the (100)[001] slip system [333,334]. In hornblendes from naturally deformed rocks dislocations on (hkO) planes were documented, mainly [001] screws [335-338]. A systematic investigation of dynamically recrystallized hornblende from a high-temperature shear zone discovered microstructures typical of dislocation creep, with subgrain boundaries and free dislocations [313]. The primary slip system is (100)[001] consistent with experimental results. Secondary, slip systems are (010)[100] and 110)5<110>. There is evidence for cross-slip of [0 01] screws producing heUcal microstructures [Fig. 13(b)]. Amphibole structures are intermediate between pyroxenes and sheet silicates and indeed chain multiplicity faults have been described [339] and transitional structures may be facilitated by movement of partial dislocations [340]. 

Even though the bcc system has many more slip systems than the fee, fee metals tend to be more ductile than bcc metals because their slip planes have a larger area density fraction. These could be considered as primary slip systems. Recall from Chapter 4 that the area density fraction of the fee (111) plane was 0.907 while the area density fraction of the bcc (110) plane was only 0.833. The higher index bcc slip systems have even smaller area densities, as may be seen in Figure 9.3 and can be considered as secondary slip systems. The lower area density slip systems in the bcc structure do not operate as effectively at low temperatures, which can cause a ductile-to-brittle transition as the temperature is lowered. 

---