Slip system

During dislocation movement, parts of the crystal slip relative to each other. The slip direction and the amount of slip are determined by the Burgers vector b. For an edge dislocation, the slip direction is also the direction of the dislocation movement, for a screw dislocation, these directions are perpendicular. The plane separating the two slipped crystal parts is called the slip plane, and the combination of slip direction and slip plane is called a slip system. 

For an edge or a mixed dislocation, h and t are not parallel and the slip plane is uniquely determined by the dislocation itself  

For a screw dislocation, this is different because b and t are parallel. The direction of movement is thus not determined uniquely. The dislocation can move on different planes and can thus overcome obstacles by cross slip (cf. figure 6.9). During cross slip, it changes from the slip plane with the largest 

The face-centred cubic crystal is close-packed (see section 1.2.2). Planes of type 111 and directions of the type (110) are close-packed and thus form the slip systems (figure 6.14). If we consider planes as identical differing only 

The body-centred cubic crystal is not close-packed. The slip systems with the closest packed directions and planes in this lattice are of the type 110 (111) (figure 6.15). With two slip directions per plane and six different slip planes, twelve slip systems result. As summarised in table 6.2, slip is also possible on other crystallographic planes that are only slightly more difficult to activate [55]. 

Oxide Slip system Burgers vector (nm) Other slip systems 

Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. HI, Mechanical Behavior, p. 75. Copyright 1965 by John Wiley Sons, New York.) 

Dislocations do not move with the same degree of ease on all erystallographie planes of atoms and in all crystallographic directions. Typically, there is a preferred plane, and in that plane there are specific directions along which dislocation motion occurs. This plane is called the slip plane it follows that the direction of movement is called the slip direction. This combination of the slip plane and the slip direction is termed the slip system. The slip system depends on the crystal structure of the metal and is such that the atomic distortion that accompanies the motion of a dislocation is a minimum. For a particular crystal structure, the slip plane is the plane that has the densest atomic packing—that is, has the greatest planar density. The slip direction corresponds to the direction in this plane that is most closely packed with atoms—that is, has the highest linear density. Planar and linear atomic densities were discussed in Section 3.11. 

Slip Systems for Face-Centered Cubic, Body-Centered Cubic, and Hexagonal Close-Packed Metals 

Metals Slip Plane Slip Direction Number of Slip Systems 

The possible slip systems for BCC and HCP crystal structures are listed in Table 7.1. For each of these structures, slip is possible on more than one family of planes (e.g., llO, 21l, and 321 for BCC). For metals having these two crystal structures, some slip systems are often operable only at elevated temperatmes. 

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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. 

For the deformation of NiAl in a soft orientation our calculations give by far the lowest Peierls barriers for the (100) 011 glide system. This glide system is also found in many experimental observations and generally accepted as the primary slip system in NiAl [18], Compared to previous atomistic modelling [6], we obtain Peierls stresses which are markedly lower. The calculated Peierls stresses (see table 1) are in the range of 40-150 MPa which is clearly at the lower end of the experimental low temperature deformation data [18]. This may either be attributed to an insufficiency of the interaction model used here or one may speculate that the low temperature deformation of NiAl is not limited by the Peierls stresses but by the interaction of the dislocations with other obstacles (possibly point defects and impurities). 

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... 

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

As mentioned previonsly, a nnmber of slip systems can operate simnltaneonsly, bnt there will be one system that has the orientation which affords it the largest resolved shear stress of all the slip systems in operation. This system will be the one that has the maximum geometric factor, (cos< f)cosl. )inax. since the applied stress is the same for all slip systems. So, the maximum resolved shear stress, T max. is given by ... 

Figure 5.16 Resolved shear stress as a function of dislocation density for copper. Data are for polycrystalline copper O single-crystal copper with one slip system operative 0 single-crystal copper with two slip systems operative and A single-crystal copper with six slip systems operative. From K. M. Rails, T. H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission John Wiley Sons, Inc.

At elevated temperatures, the thermal recovery processes described in Section 5.1.2.3 can occur concurrently with deformation, and both strength and strain hardening are consequently reduced. The latter effect results in decreasing the difference between yield and tensile strengths until at sufficiently high temperatures, they are essentially equal. At lower temperatures, temperature has a marked influence on deformation in crystalline materials. Temperature can affect the number of active slip systems in some... 

Table 5.5 Slip Systems in Some Ceramic Crystals...

Crystal Slip System Number of Independent Systems Comments... 

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