Grain boundaries stress field

Stress in crystalline solids produces small shifts, typically a few wavenumbers, in the Raman lines that sometimes are accompanied by a small amount of line broadening. Measurement of a series of Raman spectra in high-pressure equipment under static or uniaxial pressure allows the line shifts to be calibrated in terms of stress level. This information can be used to characterize built-in stress in thin films, along grain boundaries, and in thermally stressed materials. Microfocus spectra can be obtained from crack tips in ceramic material and by a careful spatial mapping along and across the crack estimates can be obtained of the stress fields around the crack. ... 

In contrast to fluids, crystals have a greater number of control parameters crystal structure, strain and stress, grain boundaries, line defects (dislocations), and the size and shape of crystallites, etc. These are all relevant to kinetics. Treatments that go beyond transport and diffusion in this important field of physical chemistry are scarce. 

These forces are the result of elastic stress fields that. exist near every impurity ion or aggregate and crystal imperfection like a dislocation line or grain boundary. These forces are very strong and are mainly responsible for the creation of second phase impurity aggregates in a host of ionic crystals. If the latent image is considered as a second phase formation of Ag° atoms in the silver halide crystal, then it seems that the elastic forces are those that cause the formation of this Ag aggregate. 

For most metal-reinforced nanocomposites the thermal expansion coefficient of the metal phase will be larger than that of the matrix, reversing the expected stress fields compared to SiC-reinforced alumina. Thus while the tensile radial stresses surrounding occluded particles may induce transgranular cracking, the compressive hoop stresses may inhibit crack propagation if the particles are located at grain boundaries. Macrostresses in sub-micron Ni... 

When two dislocations get close together, much of their stress fields cancel out, especially if they are arrayed in a tilt-type grain boundary, or if they are the two ends of a small loop. Therefore if their strain energy fields were important in dissolution, isolated dislocations would etch more rapidly than those in boundaries. In fact they etch at almost identical rates (7), so again it may be concluded that the stress fields of dislocations have little effect cp dissolution at moderate undersaturations. 

Fig. 11.14. Schematic of the pile-up of dislocations at a grain boundary. Enhancement in the stress field in an adjacent grain as a result of the presence of the pile-up is also indicated schematically.

Physical traps are generally associated with discontinuities in the lattice, such as voids, particle-matrix interfaces, and grain boundaries. A hydrogen atom near a purely physical trap is not specifically attracted but, once trapped, has difficulty escaping. Most traps can probably be considered to have both attractive and physical characteristics. An edge dislocation is a good example of a mixed trap with both sets of characteristics the tensile stress field provides some attractive character, while the core region provides some physical character. 

Pande and Suenaga [ ] have recently claimed that grain boundary flux pinning is caused by the elastic interaction between the dislocations constituting the grain boundaries and the fluxoids. The interactions between dislocations and fluxoids have long been the subject of studies. The two modes of interaction are (1) the first-order, or volume difference, effect, and (2) the second-order, or shear modulus difference, effect. The former usually dominates The Peach-Koehler equation [ ] can be used to calculate the interaction force between the stress field of the fluxoid lattice (a calculation of which has recently become available [ " ]) and the strain field of the dislocations. In the experiments of this study, the calculation of fpL... 

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