Grain boundary kinetics

Babcock, S. E. and Ballufifi, R. W., Grain boundary kinetics—I. In situ observations of coupled grain boundary dislocation motion, crystal translation and boundary displacement, Acta MetalL, 37, 2357 5, 1989. 

Fig. 10. Grain boundary diffusion parameters calculated from the grain boundary kinetics portions of the shrinkage isotherms.

The following mechanisms in corrosion behavior have been affected by implantation and have been reviewed (119) (/) expansion of the passive range of potential, (2) enhancement of resistance to localized breakdown of passive film, (J) formation of amorphous surface alloy to eliminate grain boundaries and stabilize an amorphous passive film, (4) shift open circuit (corrosion) potential into passive range of potential, (5) reduce/eliminate attack at second-phase particles, and (6) inhibit cathodic kinetics. 

Yttrium, on the otlrer hand, which has a larger cation radius than Cr +, appears to affect the grain boundary cation diffusion and not the volume diffusion of Ni +. The effects of the addition of small amounts of yttrium to nickel is to decrease dre rate of tire low temperamre grain-boundary dominated oxidation kinetics. 

Fig. 19.15 Schematic representation of range of corrosion potentials expected from various chemical tests for sensitisation in relation to the anodic dissolution kinetics of the matrix (Fe-l8Cr-IONi stainless steel) and grain boundary alloy (assumed to be Fe-lOCr-lONi) owing to depletion of Cr by precipitation of Cr carbides of a sensitised steel in a hot reducing acid (after Cowan and Tedmon )...

In cases where the interfacial energy is dependent on orientation, the equilibrium condition (6.41) does not hold [19]. Some grain boundaries will then represent higher Gibbs energies than others, and if kinetics allow for reorientation, certain grain boundaries will become dominant. However, in most cases the kinetics of... 

Said this, we can let the reader to recall Fig. 1.15, where we depicted amorphous-like phase regions at grain boundaries as the pathways open for preferential diffusion of hydrogen atoms. Apparently, an alloy can benefit from some fraction of amorphous phase to improve kinetics of hydrogen absorption, but complete amorphization of crystalline lattice lowers capacity for storing hydrogen [156]. Mechanochemical activation is therefore a complex process where kinetic and thermodynamic effects must be firstly well understood, and then optimized. 

The resulting equilibrium concentrations of these point defects (vacancies and interstitials) are the consequence of a compromise between the ordering interaction energy and the entropy contribution of disorder (point defects, in this case). To be sure, the importance of Frenkel s basic work for the further development of solid state kinetics can hardly be overstated. From here on one knew that, in a crystal, the concentration of irregular structure elements (in thermal equilibrium) is a function of state. Therefore the conductivity of an ionic crystal, for example, which is caused by mobile, point defects, is a well defined physical property. However, contributions to the conductivity due to dislocations, grain boundaries, and other non-equilibrium defects can sometimes be quite significant. 

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