Glide basal

At low temperatures, A1203 is hard and brittle, but it can be plastically deformed at high temperatures. The primary glide plane is the basal (0001) plane, and the Burgers displacement at low temperatures is 5.84 A. When the Al atoms become mobile at high temperatures this shortens to about 2.76 A. 

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... 

The initial dislocation density of the sample was estimated to be less than 10 m ". The orientation of the samples was chosen in order to align the torsion axis as close as possible with the c-axis ( 1°). The maximum resolved shear stress is then applied on the basal planes. The plastic deformation is accommodated by the glide of screw dislocations on the... 

Begining of the simulation (left), after deformation (right). Basal dislocations glide horizontally (horizontal lines), dislocations in the prismatic plane vertically (white vertical lines). The center of the cylinder is represented. 

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. 

Another apparent anomaly in the plastic behaviour of ice is the absence of any preferred glide direction in the basal plane. Metals glide preferentially in the most closely packed direction of the most closely packed plane and we might expect similar preference for glide in the directions in ice, since the necessary dislocations exist. Instead of his, the glide is nearly isotropic in the... 

Muguruma, J. Higashi, A. (19636). Non-basal glide bands in ice crystals. Nature, Land. 198, 573. [196]... 

Figure 6.18 Dislocation structure in sapphire deformed 4% by basal glide at 1400°C, consisting of A) glide dislocations, B) edge dipoles, C) faulted dipoles and D, E) dislocation loops. Basal foil, 650 kV. (Micrograph from B. J. Pletka and T. E. Mitchell, Case Western Reserve University, reproduced courtesy of The American Ceramic Society, Westerville, OH.)...

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. 

This reaction can occur by glide in the basal plane only for screw dislocations, and is thought to be the mechanism for the formation of a dislocation network in crystals undergoing deformation by prism plane slip [97]. Alternatively, the dislocation can lower its energy by dissociating into three collinear partials according to the reaction (see also Table 9.3) ... 

Figure 9.15 Dislocations in sapphire deformed on the basal plane to 3.6% shear strain at 1400°C. Examples of glide dislocations (C), regular dipoles (D), faulted dipoles (F),...

Figure 9.24 Stress-strain curves of sapphire deformed in basal glide at various temperatures [201].

In hexagonal metals, which glide preferentially in the basal plane (cf. section 6.2.4), the rotation of the lattice by twinning can lead to a more favourable orientation of the crystal and thus increase the deformability by subsequent dislocation movement. 

Slip Modes in the a-Phase. Various modes of slip can occur in a-Ti or in the a-phase of titanium alloys (see Table 4). In general, slip can occur on prismatic, pyramidal and basal planes by the movement of , [c] and -type dislocations. Since the <1120> slip directions are common to aU three planes, the -type dislocations can ghde on prism, pyramid, and basal planes. The -type slip can take place on prismatic and pyramidal planes. The [c]-type glide is restricted to only prismatic planes and generally does not occur. 

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