Grain boundary high-angle

For coarse-grained metals, dislocation movement and twinning are well known primary deformation mechanisms. Ultrafine, equiaxed grains with high-angle grain boundaries impede the motion of dislocations and... 

In materials with high anisotropy in the grain boundary energy and, thus, with faceted grain boundaries, the angles between boundaries may not be uniquely defined by an interfacial tension condition because of torque on facets. ... 

An alternative, but to some extent complementaty approach to the structure of grain boundaries notes that as the tilt angle between the crystals forming the grain boundary increases, planes of lower atomic concentrations, the high index planes, such as (221), (331) and (115) in the face-centred strucmre, become parallel to the grain boundary. There is therefore a decrease in the number of metal-metal bonds at the boundary as the tilt angle increases. 

If the weak bonding model is valid for high angle grain boundaries (>20°) it follows that diffusion in amorphous preparations of a given material should also show this measure of enhanced diffusion, when compared widr die crystalline material. 

Figure 12 An interface in 2212 showing atomically sharp high angle grain boundary with grain to grain connectivity. The diffraction pattern from the grains is inset. Grain B is 2212 phase and grain C is strontium-copper oxide.

A high-angle tilt grain boundary results when 6 > 15°. For 6 < 10°, a low-angle tilt grain boundary results, and Eq. (1.37) can be simplified to tmO 6 = b/h. In a similar manner, a twist grain boundary is a set of screw dislocations. 

Figure B.2 Symmetric large-angle (113)(TlO] tilt boundary in A1 viewed along the [110] tilt axis by high-resolution electron microscopy. The tilt angle is 50.48°. The inset shows a simulated image [4], Reprinted, by permission, from K.L. Merkle, L.J. Thompson, and F. Phillipp, Thermally activated step motion observed by high-resolution electron microscopy at a (113) symmetric tilt grain-boundary in aluminum," Philosophical Magazine Letters, vol. 82. pp. 589-597. Copyright (c) 2002 by Taylor and Francis Ltd., http //www.tandf.co.uk/journals.

H. Fang et al Atomistic simulations of Bauschinger effects of metals with high angle and low angle grain boundaries. Comp. Meth. App. Mech. Eng. 193, 1789-1802 (2004)... 

The theoretical estimate of Slack has predicted k = 1.7 W/(cm K) for room temperature thermal conductivity of GaN [12], The thermal conductivity of GaN layers grown on sapphire substrates by the HVPE method [23] was measured by Sichel and Pankove using the heat flow method [24], The room temperature thermal conductivity was k = 1.3 W/(cm K). Sichel and Pankove attributed the smaller value to high impurity content, at least 1018 cm 3, and the presence of small angle grain boundaries. 

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