Lattice Diffusion from Grain Boundaries

Lattice diffusion from grain boundary to neck 1 RT = CiDint A/ 1/2 / V liTai ) 3... 

Once grain growth has allowed the grain boundaries to leave the pores, the only mechanism of densification is by lattice diffusion of atoms from grain boundaries... 

Fig. 5.10 Six distinct mechanisms can contribute to the sintering of a consolidated mass of crystalline particles 1 surface diffusion (5D), 2 lattice diffusion from the surface, 3 vapor transport, 4 grain boundary diffusion, 5 lattice diffusion from the grain boundary, and 6 plastic flow. Only mechanisms i to 5 lead to densification, but all cause the necks to grow and so influence the rate of densification. Reproduced with permission from [24]. Copyright 2007, Springer...

Lattice diffusion from the grain boundary 5 3 SChrfVYjvfl kT... 

Aside from pore fluid diffusion, geochemically relevant processes in solid media are driven by diffusion along grain boundaries and through crystal lattices. However, at ambient temperatures and pressures, diffusion of even the smallest elements and molecules in solid minerals is exceedingly slow. Properties of diffusing ionic... 

Figure 19.2 shows, at a microscopic level, what is going on. Atoms diffuse from the grain boundary which must form at each neck (since the particles which meet there have different orientations), and deposit in the pore, tending to fill it up. The atoms move by grain boundary diffusion (helped a little by lattice diffusion, which tends to be slower). The reduction in surface area drives the process, and the rate of diffusion controls its rate. This immediately tells us the two most important things we need to know about solid state sintering ... 

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. 

These are depicted schematically in Figure 18.4 in the case of metal A deposited on metal B. Bulk diffusion, as noted above, is the transfer of B into A or A into B through the crystal lattice. This is characterized by the coefficient D in the figure. Defect path diffusion is the migration along lattice defects such as grain boundaries, characterized by the coefficient D in the figure. Ordered A B, possible phases are indicated between the metals. Finally, Kirkendall void porosity is indicated and will be expected to be present if the interdiffusion rates from one metal to the other are not equal in both directions. 

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