Basal plane slip

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... 

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. 

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... 

Slip in hexagonal metal crystals occurs mainly parallel to the basal plane of the unit cell, normal to the c axis. The slip systems can be described as 000 1 (1 1 20), of which there are three. Body-centred cubic metals have slip described by 1 1 0 (I 1 1), giving 12 combinations in aU. Other slip systems also occur in metals, but those described operate at lowest energies. 

The slip system for a-alumina (corundum) is given in Table 17.1. The primary slip plane is the basal plane, (0001) the slip direction is <1120>. The arrangement of atoms on the slip plane was shown in Figure 12.12. For... 

Erbium. The easy direction in Er is the c-axis, and below the second-order Neel transition (Tn = 85 K), the moments order in a longitudinal sine-wave structure. As the temperature is lowered the sine squares up. Aroimd Tb = 52 K, a basal plane component begins to order, leading to a helical AFM structure. Finally, at 7c = 20K a first-order transition into a steep cone (opening angle 30 ) FM spin structure takes place. Several (first-order) spin-slip transitions occur in the helical AFM phase. 

However, in sharp contradistinction, the mechanical properties of the MAX phases cannot be more different than those of their binary cousins. The mechanical properties of the MAX phases are dominated by the fact that basal-plane dislocations multiply and are mobile at temperatures as low as 77 K and higher. The presence of basal slip is thus crucial to understanding their response to stress. This is true despite the fact that the number of independent slip systems is less than the five needed for ductility. In typical ceramics at room temperature, the number of independent slip systems is essentially zero. The MAX phases, thus occupy an interesting middle ground, in which in constraineddeformationmodes,highly oriented microstructures, and/or at higher temperatures they are pseudo-ductile. In unconstrained deformation, and especially in tension at lower temperatures, they behave in a brittle fashion. 

As noted above, basal plane - and only basal plane - dislocations are responsible for how the MAX phases respond to stress. There are no credible reports that twins and/or nonbasal dislocations participate in any meaningftil way in their deformation. It follows that at all times the number of slip systems active is less than the five are needed for polycrystalline ductility. As will become apparent shortly, most of the present understanding on the deformation of the MAX phases is based on early work carried out on Ti3SiC2, which is the most extensively MAX phase studied and best understood to date. However, there is little doubt - as confirmed by more recent studies - that what applies to Ti3SiC2 also applies to other MAX phases. 

The MAX phases, ice and graphite, and other layered minerals such as mica, are plastically anisotropic. This plastic anisotropy, combined with the fact that they lack the five independent slip systems needed for ductility, quickly lead to a very uneven states of stress when a polycrystalline sample is loaded [129]. The glide of basal plane dislocations takes place only in favorably oriented or soft grains, which rapidly transfer the load to hard grains - that is, those not favorably oriented to the applied stress. Needless to say, this leads to high internal stresses. 

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