Plastic deformation slip system

Single-crystal and polycrystalline transition metal carbides have been investigated with respect to creep, microhardness, plasticity, and slip systems. The fee carbides show slip upon mechanical load within the (111) plane in the 110 direction. The ductile-to-brittle transformation temperature of TiC is about 800 °C and is dependent on the grain size. The yield stress of TiC obeys a Hall- Petch type relation, that is, the yield stress is inversely proportional to the square root of the grain size. TiC and ZrC show plastic deformation at surprisingly low temperatures around 1000 °C. 

At room temperature, NiAl deforms almost exclusively by (100) dislocations [4, 9, 10] and the availability of only 3 independent slip systems is thought to be responsible for the limited ductility of polycrystalline NiAl. Only when single crystals are compressed along the (100) direction ( hard orientation), secondary (111) dislocations can be activated [3, 5]. Their mobility appears to be limited by the screw orientation [5] and yield stresses as high as 2 GPa are reported below 50K [5]. However, (110) dislocations are responsible for the increased plasticity in hard oriented crystals above 600K [3, 7]. The competition between (111) and (110) dislocations as secondary slip systems therefore appears to be one of the key issues to explain the observed deformation behaviour of NiAl. 

In static friction, the change of state from rest to motion is caused by the same mechanism as the stick-slip transition. The creation of static friction is in fact a matter of choice of system state for a more stable and favorable energy condition, and thus does not have to be interpreted in terms of plastic deformation and shear of materials at adhesive junctions. 

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... 

The transition metal carbides do have a notable drawback relative to engineering applications low ductility at room temperature. Below 1070 K, these materials fail in a brittle manner, while above this temperature they become ductile and deform plastically on multiple slip systems much like fee (face-centered-cubic) metals. This transition from brittle to ductile behavior is analogous to that of bee (body-centered-cubic) metals such as iron, and arises from the combination of the bee metals strongly temperature-dependent yield stress (oy) and relatively temperature-insensitive fracture stress.1 Brittle fracture is promoted below the ductile-to-brittle transition temperature because the stress required to fracture is lower than that required to move dislocations, oy. The opposite is true, however, above the transition temperature. 

This is an important point. A sublattice phase with the FCC structure should not, generally speaking, be considered CCP with regards to slip. The atoms or ions on one sublattice may very well be in a CCP-hke arrangement, but they can be kept apart by large atoms or ions residing on the other sublattice (the interstitial sites). Slip is easiest along tmly close-packed layers of identically sized spheres that are in contact and, preferably, without obstacles such as interstitials. Thus, another reason for low ductility in intermetallics and ceramics is the lack of a sufficient number of active slip systems to allow plastic deformation. 

In general, metals can be worked extensively, either at room temperature or at high temperatures. This is so mainly because of the availability of a large of number slip systems for plastic deformation. This allows us to use metal drawing techniques to obtain filamentary metals. Metallic fibers are, generally, not spun from a molten state, although this can be done in some cases (see Section 5.2). When metals are cold worked (i.e. below the recrystallization temperature), they... 

As a consequence of the available slip systems, the strength and ductility are highly anisotropic with a hard <100> direction and soft <110> and <111 > directions (Darolia et al., 1992 b Glatzel etal. 1993b Takasugi etal., 1993a). NiAl with the hard orientation shows practically no ductility - in spite of indications of local plastic deformation (Vehoff, 1992) - below the brittle-to-ductile transition temperature (BDTT) which is of the order of 350°C - corresponding to 0.33 (r =... 

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