Metal slip direction

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 ... 

In semicrystalline polymers such as polyethylene, yielding involves significant disruption of the crystal structure. Slip occurs between the crystal lamellae, which slide by each other, and within the individual lamellae by a process comparable to glide in metallic crystals. The slip within the individual lamellae is the dominant process, and leads to molecular orientation, since the slip direction within the crystal is along the axis of the polymer molecule. As plastic flow continues, the slip direction rotates toward the tensile axis. Ultimately, the slip direction (molecular axis) coincides with the tensile axis, and the polymer is then oriented and resists further flow. The two slip processes continue to occur during plastic flow, but the lamellae and spherullites increasingly lose their identity and a new fibrillar structure is formed (see Figure 5.69). 

The speed at which the metal moves through the rollers must change in order to keep the volume rate of flow constant through the roll gap. Hence, as the thickness decreases, the velocity increases. However, the surface speed of the roller is constant, so there is relative sliding between the roller and the metal. The direction of the relative velocity changes at a point along the contact area called the neutral or no-slip point (point N in Figure 7.10). At the neutral point, the roller and metal have the same... 

According to the test, five specimens (each 50 mm long, 100 mm wide, and 1 mm thick) are conditioned at ambient temperature as specified in the test, and placed in a forced draft oven equipped with a biaxial rotator. Specimens should be attached to the rotator by metal slips lined with fluoropolymer film and should not directly contact with the metal clips or metal parts of the oven. The frequency of rotation about the horizontal and vertical axes of the rotator should be 1-3 min . The time to failure is determined by regular visual examination of the specimens as the number of days after which the specimen shows localized crazing, crumbling, or discoloration, or a combination thereof. According to the standard procedure, the oven temperature shall be 150°C (302°F). 

Nishiguchi S, Kato H, Fnjita H, Oka M, Kim HM, Kokubo T, Nakamura T (2001) Titanium metals form direct bonding to bone after alkali and heat treatments. Biomaterials 22 2525-2533 Nordstrom EG, Karlsson KH (1990) Slip-cast apatite ceramics. Ceram Bull 69 824-827 Ogiso M (1998) Reassessment of long-term use of dense HA as dental implant case report. J Biomed Mater Res 43 318-320... 

The preferred slip plane in ionic crystals with the halite (NaCl) structure, such as NaCl or LiF, is 110, and the slip direction used is (110). This slip system is sketched in Figure 10.17. For the more metallic halite structure solids such as titanium carbide (TiC), the slip system is similar to that in face-centred cubic metals, 1 1 1 (110). 

Although polymer crystal structures are known, and some slip mechanisms (slip plane and slip direction) determined, these are less important than for metals. Firstly, the amorphous phase plays an important part in the mechanical properties. Secondly, polymer yield strengths are not determined by obstacles to dislocation movement. However, it is possible to fabricate highly anisotropic forms of semi-crystalline polymers, so crystal characterization and orientation are important. 

The slip direction is usually the direction having the smallest spacing between atoms or ions of the same type (the highest linear density). In metals, the slip plane is often the closest packed plane (the highest planar density). In ceramics, we consider planar density, but there is often... 

Metal Slip plane - Slip direction Comments Ref. 

Fig. 6.14. Slip systems in face-centred cubic metals. The slip planes are the body diagonals the slip directions lie on the plane diagonals or on the edges of the octahedron in figure (c), respectively...

A very simple yield criterion for anisotropic materials is the critical resolved shear stress of Schmid [14]. This is concerned with crystal slip. The law states that yield occurs when the resolved shear stress in the slip direction in the slip plane reaches a critical value. Although this law is extensively used in metal plasticity, it is of restricted application in polymers. 

Metal Structure Slip plane Slip direction N. of slip systems... 

The ductility of a metal depends on the number and type of slip systems. The slip systems for the various lattice types are shown in Table 9.2. Note that in the 111 family there are four different planes (111), (111), (111)/ and (111). (Other permutations of the indices are planes parallel to these four distinct planes.) The slip directions for the (111) must lie in the plane, hence their dot product with the vector normal to the plane must be zero. Thus the three slip directions in the (111) plane will be [110], [101], and [Oil]. (Remember this only works only for cubic systems.) Again, other permutations of these indices are directions parallel to these three directions. 

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. 

With continued extension of a single crystal, both the number of slip lines and the slip step width increase. For FCC and BCC metals, shp may eventually begin along a second slip system, the system that is next most favorably oriented with the tensile axis. Furthermore, for HCP crystals having few slip systems, if the stress axis for the most favorable slip system is either perpendicular to the slip direction (A = 90°) or parallel to the slip plane = 90°), the critical resolved shear stress is zero. For these extreme orientations, the crystal typically fractmes rather than deforms plastically. 

For polycrystalline metals, slip occurs within each grain along those slip systems that are most favorably oriented with the applied stress. Furthermore, during deformation, grains change shape and extend in those directions in which there is gross plastic deformation. 

Consider a metal single crystal oriented such O that the normal to the shp plane and the slip direction are at angles of 60° and 35°, respectively, with the tensile axis. If the critical resolved shear stress is 6.2 MPa (900 psi), will an applied stress of 12 MPa (1750 psi) cause the single crystal to yield If not, what stress will be neeessary ... 

The plastic deformation of crystalline solids proceeds by a process of slip and/or twinning on certain crystal planes and in certain crystal directions. In metals the slip planes (denoted by ) are usually those having the highest atomic density and they are the most widely spaced. The slip directions (denoted by < >) in the plane are those having the highest linear atomic density. A particular combination of slip plane and slip direction is referred to as a slip system. 

Slip of a dislocation on a different slip plane from the original slip plane is called cross-slip. Cross-slip may then occur for screw dislocations or screw segments of curved mixed dislocations. In order for cross-slip to occur, a dislocation must not be extended on the original slip plane or a constriction must form in an extended dislocation. The latter is a process which occurs with some difficulty and thus extensive cross-slip is only expected in crystals which have narrow dislocations such as aluminium and most body-centered cubic metals. Body-centered cubic metals such as iron have wavy slip lines because cross-slip on a number of different slip planes is possible 110, 112, and 123. The slip direction is always <111>. 

A usual metal crystal contains about 10 dislocation lines/cm. Moving all of these out of the crystal under the influence of an applied stress does not account for the observed shear strain. Suppose our crystal is a cube 1 cm on a side and it is oriented in such a way that a shear stress is applied on the slip plane in a possible slip direction. Suppose one-third of the 10 dislocation lines have this slip direction and are oriented favorably for slip (this is clearly an overestimate), then taking b equal to 3 x 10 cm gives a total shear strain of 1 %. Shear strains much greater than this are observed, sometimes exceeding 100% in single crystals. Clearly, dislocation multiplication processes... 

Slip casting of metal powders closely follows ceramic slip casting techniques (see Ceramics). SHp, which is a viscous Hquid containing finely divided metal particles in a stable suspension, is poured into a plaster-of-Paris mold of the shape desired. As the Hquid is absorbed by the mold, the metal particles are carried to the wall and deposited there. This occurs equally in all directions and equally for metal particles of all sizes which gives a uniformly thick layer of powder deposited at the mold wall. 

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