Metal slip plane

Metals Slip Plane Slip Number of Direction Slip Systems ... 

Metal Slip plane - Slip direction Comments Ref. 

The differing malleabilities of metals can be traced to their crystal structures. The crystal structure of a metal typically has slip planes, which are planes of atoms that under stress may slip or slide relative to one another. The slip planes of a ccp structure are the close-packed planes, and careful inspection of a unit cell shows that there are eight sets of slip planes in different directions. As a result, metals with cubic close-packed structures, such as copper, are malleable they can be easily bent, flattened, or pounded into shape. In contrast, a hexagonal close-packed structure has only one set of slip planes, and metals with hexagonal close packing, such as zinc or cadmium, tend to be relatively brittle. 

Beside dislocation density, dislocation orientation is the primary factor in determining the critical shear stress required for plastic deformation. Dislocations do not move with the same degree of ease in all crystallographic directions or in all crystallographic planes. There is usually a preferred direction for slip dislocation movement. The combination of slip direction and slip plane is called the slip system, and it depends on the crystal structure of the metal. The slip plane is usually that plane having the most dense atomic packing (cf. Section 1.1.1.2). In face-centered cubic structures, this plane is the (111) plane, and the slip direction is the [110] direction. Each slip plane may contain more than one possible slip direction, so several slip systems may exist for a particular crystal structure. Eor FCC, there are a total of 12 possible slip systems four different (111) planes and three independent [110] directions for each plane. The... 

Despite the similarities in brittle and ductile behavior to ceramics and metals, respectively, the elastic and permanent deformation mechanisms in polymers are quite different, owing to the difference in structure and size scale of the entities undergoing movement. Whereas plastic deformation (or lack thereof) could be described in terms of dislocations and slip planes in metals and ceramics, the polymer chains that must be deformed are of a much larger size scale. Before discussing polymer mechanical properties in this context, however, we must first describe a phenomenon that is somewhat unique to polymers—one that imparts some astounding properties to these materials. That property is viscoelasticity, and it can be described in terms of fundamental processes that we have already introduced. 

DISLOCATION. In crystallography, a type of lattice imperfection whose existence in metals is postulated in order to account for the phenomenon uf crystal growth and of slip, particularly for the low value of shear stress required lo initiate slip. One section of the crystal adjacent to the slip plane is assumed to contain one mure atomic plane that the section on the opposite side of the slip plane. Motion of the dislocation results in displacement of one of the sections with respect to another. 

When new atoms are introduced into a metallic crystalline structure, the new atoms produce additional forces on surrounding atoms and cause some structural distortion due to their different size. The displaced atoms cannot slide along crystalline slip planes as easily as they did before. 

R. G. Raicheff, A. Damjanovic, and J. O M. Bockris,. 1. Chem. Phys. 49 926 (1968). The effect of stressing metals upon the rate of appearance of slip planes of different indices. 

In both types of alloys, the added element distorts the lattice but does not destroy it. Metals have slip planes, which under stress slide by one another. The hardness and strength of metals is related to the ease with which these planes glide by one another. The non-uniform lattice created by alloying makes it more difficult for the planes of atoms to slide across each other. Thus, more force must be applied to deform or fracture the alloy. Think of it this way It is easier for the slip planes to slide by one another if the surfaces of the planes are uniform and smooth (as in pure metals) rather than... 

Slip planes Plastic deformation (or yielding) of a solid metal occurs when parallel lattice planes slip past each other. Those planes are called slip planes. 

The energy balance considerations in Griffith s original concept were later refined by Orowan and Irwin to include the effects of plasticity and elasticity for applicability to metals (Orowan, 1952 Irwin, 1957). Metals fail by ductile fracture, where the crack growth occurs in the direction of the primary slip system. When the slip plane is inclined to the crack, atoms across the slip plane slide past one another, relieving the stress, which results in a zigzag crack path. This is illustrated in Figure 10.14. 

Large number of slip planes accounts for malleability of ccp metallic crystals (may be bent, flattened, or pounded into different shapes). Examples Coinage metals Cu, Ag, and Au Metallic crystals with the hep structure tend to be brittle (only one slip plane). Examples Zn, Cd... 

In contrast, for plastically deformed surfaces, friction anisotropy appears to be primarily attributable to the movement of atomic slip planes within the bulk of the metal, and not to commensurability at the sliding interface. Early experiments using diamond surfaces showed that friction anisotropy disappeared at low loads where no plastic deformation was evidenced. 

Both the above simulations considered identical tips and substrates. Failure moved away from the interface for geometric reasons, and the orientation of the interface relative to easy slip planes was important. In the more general case of two different materials, the interfacial interactions may be stronger than those within one of the materials. If the tip is the weaker material, it will be likely to yield internally regardless of the crystallographic orientation. This behavior has been observed in experiments between clean metal surfaces where a thin tip is scraped across a flat substrate [31]. When the thin tip is softer than the substrate, failure is localized in the tip, and it leaves material behind as it advances. However, the simulations considered in this section treated the artificial case of a commensurate interface. It is not obvious that the shear strength of an interface between two incommensurate surfaces should be sufficient to cause such yield, nor is it obvious how the dislocation model of Hurtado and Kim applies to such surfaces. 

The octahedral shear stress criterion has some appeal for materials that deform by dislocation motion In which the slip planes are randomly oriented. Dislocation motion Is dependent on the resolved shear stress In the plane of the dislocation and In Its direction of motion ( ). The stress required to initiate this motion is called the critical resolved shear stress. The octahedral shear stress might be viewed as the "root mean square" shear stress and hence an "average" of the shear stresses on these randomly oriented planes. It seems reasonable, therefore, to assume that slip would initiate when this stress reaches a critical value at least for polycrystal1ine metals. The role of dislocations on plastic deformation in polymers (even semicrystalline ones) has not been established. Nevertheless, slip is known to occur during polymer yielding and suggests the use of either the maximum shear stress or the octahedral shear stress criterion. The predictions of these two criteria are very close and never differ by more than 15%. The maximum shear stress criterion is always the more conservative of the two. 

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