Grain boundaries bicrystals

The Stonybrook group under Dudley has studied the behaviour of ice bicrystals and has shown that under certain conditions, grain boundaries can act as somces of dislocations. Grain boundary facets have been shown to act as dislocation nucleation sites and grain boundaries themselves have been observed to act as barriers to dislocation motion. 

D. A. Molodov, J. Swiderski, G. Gottstein, W. Lojkowski and L. S. Shvindlerman, Effect of pressure on grain boundary migration in aluminium bicrystals, Acta metall. mater. 42 3397 (1994). 

D. A. Molodov, U. Czubayko, G. Gottstein and L. S. Shvindlerman, Mobility of <111> tilt grain boundaries in the vicinity of the special misorientation S7 in bicrystals of pure aluminium, Scripta metull. mater. 32 529 (1995). 

Y. Komem, P. Petroff, and R.W. Balluffi. Direct observation of grain boundary dislocation climb in ion-irradiated gold bicrystals. Phil. Mag., 26 239-252, 1972. 

Figure 13.15 Bicrystal specimen with a planar grain boundary initially located a...

Figure 35. Impedance spectra of the cell 02, Pt I SrTi031 SrTi031 Pt, 02 as function of d.c. bias. Electrodes are parallel to the bicrystal boundary (Z5 tilt grain boundary, iron content 2 x 1018 cnT3). Both bulk and boundary resistances are predominantly electronic resistances.222 Reprinted from I. Denk, J. Claus and J. Maier, J. Electrochem. Soc., 144 (1997) 3526-3536. Copyright 1997 with permission from The Electrochemical Society.

The analogous conclusions are arrived at for the ionic conductivity in the case of cell 5. The blocking technique can be extended in order to de-convolute ionic and electronic conductivities for a grain boundary of a bicrystal (see Figure 43)236 or even in composites.240 In both the cases, the relations are substantially more complicated because of local inhomogeneities. Some of the pitfalls of the Wagner-Hebb technique have been described by Riess.241... 

Figure 43. Temperature dependence of total and partial conductivities of (a) bulk and (b) grain boundary111 in a 0.016 wt% Fe-doped SrTi03 bicrystal at about 2.0 Pa oxygen. The lower electronic and the much lower ionic grain boundary values are due to Mott-Schottky layer (see Part I, Section V).233 Reprinted from X. Guo, J. Fleig and J. Maier, J. Electrochem. Soc., 148 (2001) J50-J53. Copyright 2001 with permission from The Electrochemical Society, Inc.

A. Luque et al Molecular dynamics simulation of crack tip blunting in opposing directions along a symmetrical tilt grain boundary of copper bicrystal. Fatigue Fract. Eng. Matls. Struct. 30, 1008-1015 (2007)... 

Figure 8.37. Lattice-fringe image showing a high-angle grain boundary in a bicrystal of wet synthetic calcite (From Hay and Evans 1988.)...

Figure 4.2 Two possible models for the interface between nanocrystalline grains.(a) A disordered interface [2] (b) A normal grain boundary, a bounda in a ZnO bicrystal [17].

Fig. 9.38. Schematic of increasingly complex grain boundary distributions within materials (adapted from Randle (1994)) (a) bicrystal geometry with a single grain boundary, (b) bamboo-like distribution of grain boundaries in a one-dimensional array, (c) polycrystalline film, and (d) three-dimensional polycrystal.

Our intent in this section was not to give an exhaustive account of the nature of grain boundaries in real polycrystals. Rather, it was meant as a gentle reminder that despite the importance of bicrystals, they are but a first step in understanding the deeper question of the nature of grain boundaries in real microstructures. 

Simple grain boundaries in metals have been exhaustively considered from the standpoint of empirical descriptions of the total energy. The logic of much of this work is founded on the contention that prior to understanding the susceptibility of boundaries to segregation and fracture or their role as short-circuit paths for diffusion, it is necessary to first understand the structures themselves. The majority of this work has emphasized bicrystals. 

One of the most important research issues related to theory and application of high Tc superconductors is the understanding of the influence of grain boundaries (GBs) on critical current density. Dimos et al. [4.1] reported that the critical current density through a GB fabricated on a bicrystal substrate... 

Many TEM studies show that the GB films grown by physical vapor deposition (PVD) on bicrystal substrates are wavy and the typical facet size is about 50 nm [4.35-4.37]. Therefore, the dependence of critical current density on misorientation angle is difficult to study with these films. Recently, liquid-phase epitaxy (LPE) was successfully used to obtain large single-facet grain boundaries. Fig. 4.20 shows a plan-view image of the GB of a YBCO film grown on a 24° MgO bicrystal. It clearly shows that the GB is a symmetrical... 

The bicrystal single grain boundary Josephson junction is fabricated by depositing an epitaxial c-axis oriented cuprate film on a substrate bicrystal... 

Here dJg(y) is the beam-induced change of the supercurrent density at the coordinate point y. A typical LTSEM voltage image obtained in this way is shown in Fig. 5.11 for a bicrystal YBa2Cu307 5 grain boundary junction of 23 pm width. The image is recorded at 83 K and displays the pair current density distribution of the 4-5 vortex state. (It is this state which is also shown... 

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