Slip hexagonal

Beryllium is a light metal (s.g. 1 -85) with a hexagonal close-packed structure (axial ratio 1 568). The most notable of its mechanical properties is its low ductility at room temperature. Deformation at room temperature is restricted to slip on the basal plane, which takes place only to a very limited extent. Consequently, at room temperature beryllium is by normal standards a brittle metal, exhibiting only about 2 to 4% tensile elongation. Mechanical deformation increases this by the development of preferred orientation, but only in the direction of working and at the expense of ductility in other directions. Ductility also increases very markedly at temperatures above about 300°C with alternative slip on the 1010 prismatic planes. In consequence, all mechanical working of beryllium is carried out at elevated temperatures. It has not yet been resolved whether the brittleness of beryllium is fundamental or results from small amounts of impurities. Beryllium is a very poor solvent for other metals and, to date, it has not been possible to overcome the brittleness problem by alloying. 

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. 

In a three-dimensional lattice, we have observed planes of atoms (or ions) composing the lattice. Up to now, we have assumed that these planes maintain a certain relation to one another. That is. we have shown that there are a set of planes as defined by the hkl values, which in turn depends upon the type of Bravais lattice that is present. However, we find that it is possible for these rows of atoms to "slip" from their equilibrium positions. Hiis gives rise to another type of lattice defect called "line defects". In the following diagram, we present a hexagonal lattice in which a line defect is present ... 

Here, the axis of slip is shown in the hexagonal lattice as being on, or near the surface of the array of atoms. In the cubic lattice, a slip plane is depicted where a line of atoms is missing and the lattice has moved to accommodate this type of lattice defect. 

Dislocation movement in copper is described by a slip plane 111 and a slip direction, the direction of dislocation movement, [110]. Each 111 plane can be depicted as a hexagonal array of copper atoms (Fig. 3.11a). The stacking of these planes is represented by the sequence. .. ABC. .. where the first layer is labeled A, the second layer, which fits into the dimples in layer A is labeled B... 

Fig. 14. Schematic representation of cross-slip in a hexagonal metal ia> slip on CRYSTAL FIELD THEORY. A theory developed in the early 1930s...

Thus a new felting die assembly was required. The successful procedure involved a die formed from brass 0.0625 thick, perforated with 1/16 holes in a hexagonal pattern. This die was covered with a small "sock made of 44 x 44 mesh Saran screen, 0.010 thick. The sock has been sewn in the desired form with very fine cotton thread. The preform was felted onto this sock, which was then slipped from the die, collapsed withdrawn from the interior of the preform... 

Slip occurs along specific crystal planes (slip planes) and in specific directions (slip directions) within a crystal structure. Slip planes are usually the closest-packed planes, and slip directions are the closest-packed directions. Both face-centered-cubic (FCC) and hexagonal-close packed (HCP) structure are close packed structures, and slip always occurs in a close packed direction on a closepacked plane. The body-centered-cubic (BCC) structure is not, however, close packed. In a BCC system, slip may occur on several nearly close packed planes or directions. Slip planes and directions, as well as the number of independent slip systems (the product of the numbers of independent planes and directions), for these three structures are listed in Table 7.2. 

Of the 12 slip systems possessed by the CCP stmcture, five are independent, which satisfies the von Mises criterion. For this reason, and because of the multitude of active slip systems in polycrystalline CCP metals, they are the most ductile. Hexagonal close-packed metals contain just one close-packed layer, the (0 0 0 1) basal plane, and three distinct close-packed directions in this plane [I I 2 0], [2 I I 0], [I 2 I 0] as shown in Figure lO.Vh. Thus, there are only three easy glide primary slip systems in HCP metals, and only two of these are independent. Hence, HCP metals tend to have low... 

Figure 6.2 (a) The hexagonal dose-packed structure of a-alumina. (b) Two important slip systems, basal (0001) [0001] and prismatic (0110)[2110], in a hexagonal structure. 

Hexagonal close-packed structures (hep) have only one set of slip planes. 

Depolarized Light Study. The samples used for DLI were melted between cover slips and rapidly quenched. The cover slips were held 0.01 mm. apart with a glass spacer. This treatment should result in the hexagonal, aLj phase as the room temperature solid. 

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