Slip basal

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. 

In the study of the nucleation and propagation of dislocations (Figure 10.11) from scratches in a geometry designed to produce slip on non-basal planes, edge dislocations were found to glide on non-basal planes, but screw dislocations were completely immobile except in the basal plane. Detailed measurements of the dislocation velocities using stress pulse techniques showed that... 

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. 

As will be explained later, it is considered that the surface of such a film normally consists of a thin layer of fully-ordered crystalline material with the basal planes oriented parallel to the plane of the substrate surface. Conformal contact between two such films will then be similar to the contact between two adjacent lamellae within a crystal. As a first approximation it might therefore be assumed that interfacial slip will resemble intracrystalline slip. However each surface may be degraded by the presence of contaminants, surface defects, and deviations from planarity, and it cannot be assumed that interfacial friction will be completely governed by the same considerations as intracrystalline friction. 

A study carried out by Masao Uemura and colleagues was specifically designed to distinguish between the occurrence of cleavage, shear and interfacial slip (which they referred to as intercrystalline slip.) They concluded that cleavage took place when surface material was not fully oriented parallel to the basal planes, and that the friction coefficient was then of the order of 0.1. When shear was taking place the coefficient of friction was about 0.06, whereas when interfacial slip between fully basal-plane oriented surfaces was occurring the coefficient of friction was as low as 0.025. 

Figure 5 represents a typical evolution of the dislocation pattern during the deformation. The simulation was performed in a 20 mm diameter crystal, with 2 initial basal planes activated (one system in each plane) at the beginning of the deformation. It clearly appears that the double cross-slip mechanism propagates the plasticity in many other basal planes. One can also notice the asymmetry in the plane expansion due to the dislocation interactions. 

Figure 5 Thickening of slip planes due to the double cross-slip of basal dislocations.

The double cross-slip mechanism can then be considered as the most probable deformation process, complementary to the basal slip. Indeed, dislocation climb can hardly be invoked in this torsion loading conditions since most of the dislocations are of screw type. 

Usually, creep deformation of ice single crystals is associated to a steady-state creep regime, with a stress exponent equal to 2 when basal glide is activated . In the torsion experiments performed, the steady-state creep was not reached, but one would expect it to be achieved for larger strain when the immobilisation of the basal dislocations in the pile-ups is balanced by the dislocation multiplication induced by the double cross-slip mechanism. 

At high temperatures (above about 1,200°C) alumina can deform by dislocation motion. The important paper by Merritt Kronberg [26], see also [1], p. 32, and [27], showed the details of dislocation motion in alumina. Basal slip on the close-packed oxygen planes is most common in alumina, with additional slip systems on prism planes. 

M.L. Kronberg, Plastic deformation of single crystals of sapphire Basal slip and twinning, Aeta Met. 5, 507-529 (1957). 

Fig. 8.9. A typical creep curve for an ice single crystal slipping on basal planes (after Higashi et al. 1965).

So far we have considered only slip parallel to the basal plane in ice and, though this is a very strongly preferred deformation mode, it is not the only one which can occur. Experiments to... 

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