Grains and Boundaries

Grain size distributions for sediments and soils are used to determine the amount of sand, silt, and clay present in a sample. For example, a grain size of 2 mm serves as a boundary between gravel and sand. Grain size boundaries for sand-silt and silt-clay are given as 1/16 mm and 1/256 mm, respectively. 

Grain boundaries have different corrosion properties from the grain and may corrode preferentially, giving cracks that then propagate by stress corrosion or corrosion fatigue (Fig. 23.10). 

We saw in Chapter 2 that, when boundary energies were the dominant factor, we couid easiiy predict the shapes of the grains or phases in a materiai. Isotropic energies gave tetrakaidecahedrai grains and sphericai (or iens-shaped) second-phase particies. 

The sputtering process is frequendy used in both the processing (e.g., ion etching) and characterization of materials. Many materials develop nonuniformities, such as cones and ridges, under ion bombardment. Polycrystalline materials, in particular, have grains and grain boundaries that can sputter at different rates. Impurities can also influence the formation of surface topography. ... 

There apparently exists a critical amount of liquid phase for the optimization of grain/interface boundary sliding during superplastic deformation. The optimum amount of liquid phase may depend upon the precise material composition and the precise nature of a grain boundary or interface, such as local chemistry (which determines the chemical interactions between atoms in the liquid phase and atoms in its neighboring grains) and misorientation. The existence of an equilibrium thickness of intergranular liquid phase in ceramics has been discussed [14]. This area of detailed study in metal alloys has not been addressed. 

Fig. 4.18 Corroded grain in ZREl showing the grain boundaries still intact. Attack occurs on the periphery of the grains and thus is not intergranular...

The decomposition process can be significantly intensified by the mechanical activation of the material prior to chemical decomposition. Based on a thermodynamic analysis of the system, Akimov and Chernyak [452] showed that the mechanical activation initiates dislocations mostly on the surface of the grains, and that heterogeneities in the surface cause the predominant migration of iron and manganese to the grain boundaries. It is noted that this phenomenon is more pronounced for manganese than it is for iron. 

Out of this volume, only half of the particles will be ejected towards the rim of the grain and only a half of those will have trajectories which actually cross the grain boundary (Fig. 5). Thus, for a porous media with porosity ( ) and density ps the number of daughter... 

Thuvander and Andren (2000) have reviewed APFIM studies of grain and phase boundaries and demonstrate that the technique has played a vital role in the understanding of interfacial chemistry in many important materials including... 

I. Kaur and W. Gust Fundamentals of Grain and Interphase Boundary Diffusion, Ziegler, Stuttgart, 1988. 

P.Y. Huang, C.S. Ruiz-Vargas, A.M. van der Zande, W.S. Whitney, M.P. Levendorf, ).W. Kevek, et al., Grains and grain boundaries in single-layer graphene atomic patchwork quilts, Nature. 469 (2011) 389-392. 

Several models for diffusive transport in and among minerals have been discussed in the literature one is the fast grain boundary (FGB) model of Eiler et al. (1992, 1993). The FGB model considers the effects of diffusion between non-adjacent grains and shows that, when mass balance terms are included, closure temperatures become a strong function of both the modal abundances of constituent minerals and the differences in diffusion coefficients among all coexisting minerals. 

---