Bend grain boundary phase

Figure 10.24. Molecules with a bent molecular core are expected to show a high flexoelectric coefficient. Thus, an electric field-induced bend deformation may be suitable to generate a lattice of edge dislocations, thereby leading to a bend grain boundary phase.

Maybe, bend grain boundary (BGB) phases can be induced, indeed (Section 10.9). 

Another class of frustrated phase results from the frustration between bend or twist deformations in smectic phases (Section 5.6) and the tendency to form a layered structure. Twisted grain boundary phases are frustrated smectic phases and both SmA and SmC versions have been observed. The phases are denoted TGBA and TGBC respectively and are formed by chiral mesogens. The phases are macroscopically chiral and result from arrays of screw dislocations (i.e. defects in lattice order) which lead to a twist in the director between grains of layers, i.e. to a helical rotation of layers. 

A hardness of 9-11 GPa is reported for polycrystalline MoSi (Newman, 1998). Addition of 2 wt% of C removes the silica grain boundary phase on MoSi particles, thus improving the Vickers hardness of the monolithic material (Maloy, 1991). In addition, carbon interacts with MoSij to form SiC. Concerning TaSi, it possesses higher hardness, 16 GPa (Schultes, 2006), which in principle could be further increased with C additions. Strength data are available only for MoSij, about 650 MPa from room temperature up to 1000°C for 4-point bending strength, above this temperature, its ductile behaviour prevents from its measurement. 

Microcracks are also known to occur in the elevated temperature creep of alumina with little or no preexisting glass phase. For example, the development of microcracks during creep fracture of two hot-pressed aluminas, which were free of grain boundary glass films, was studied by Wilkinson et al.52 who employed tensile and four-point bend specimens. They found that the concentrations and morphology of the cavities and microcracks were strongly... 

Figure 22. L.h.s. Four basic space charge situations involving ionic conductors (here silver ion conductor) a) contact with an isolator, b) contact with a second ion conductor, c) grain boundary, d) contact with afluid phase. R.h.s. Bending of energy levels and concentration profiles in space charge zones ( = 0 refers to the interfacial edge).

The occurrence of all those interfaces which have been mentioned depends upon the conditions of preparation of the solid - as, for example, upon the method of solidification or of deposition from the vapour phase, or upon rolling, drawing, bending, etc. - and upon the subsequent annealing process by means of which transformation, recrystallization, or relaxation (i. e. the formation of low-angle grain boundaries at the expense of randomly distributed dislocations) can proceed [7]. 

Fig. 14. Schematic model of the grain boundary fragmentation mechanism a - an undeformed dendrite, b - after bending, c - the reorganization of the lattice bending to give grain boundaries, d - for ycz > 2 ys-L the grain boundaries have been "wetted" by the liquid phase (Doherty et al., 1984).

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