Grain boundaries exponent

Different growth-law exponents are obtained for other dominant transport mechanisms. Coblenz et al. present corresponding neck-growth laws for the vapor transport, grain-boundary diffusion, and crystal-diffusion mechanisms [10]. 

Another important fact which must be taken carefully into account is the presence of impurities at the grain boundaries. In the case of YTZP ceramics, the stress exponent is reported to depend on the stress in high-purity specimens, whereas it is... 

In this equation, A is a constant, Q is the apparent activation energy, R is the gas constant, G is the shear modulus, and p and n are the inverse grain size and stress exponents, respectively. The values of n and Q, measured for the fibers considered here, are listed in Table VI. The high values of n suggest that creep deformation is rate controlled by an interface reaction-limited grain boundary diffusional process [78], Mullite fibers are more resistant to creep than a-alumina fibers. For example, the strain rate of the Nextel 480 mullite fiber at 1200 C is almost two orders of magnitude lower than that of the Nextel 610 a-alumina fiber [55]. 

In an additional work, Kim [27] indicated a tensile elongation to fracture in the 200-400 % range, as shown Fig. 2.63. Here, strain hardening may be observed in the curve. The deformation mechanism in his tests was grain-boundary sliding. To calculate the grain-size exponent, p, one can use Eq. (2.20), as follows ... 

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