Grain elongation

As the stress is reduced, the rate of power-law creep (eqn. (19.1)) falls quickly (remember n is between 3 and 8). But creep does not stop instead, an alternative mechanism takes over. As Fig. 19.4 shows, a polycrystal can extend in response to the applied stress, ct, by grain elongation here, cr acts again as a mechanical driving force but, this time atoms diffuse from one set of the grain faces to the other, and dislocations are not involved. At high T/Tm, this diffusion takes place through the crystal itself, that... 

It was shown that, after cyclic tensile fatigue fracture at room temperature in a narrow region within a couple of micrometers of each side of the fractured surfaces, the morphology of grain elongation appeared to be general. Viewed... 

Figure 7.8 shows a high-resolution transmission electron micrograph of the grain boundary liquid phase in silicon nitride. Using such observations, Wilkinson et a/. (1991) have verified that redistribution of the liquid does occur. Grain boundary sliding can also be coupled with diffusional creep, as shown in Fig. 7.9. As grains elongate in the tensile direction by diffusional flow, adjacent...

Fig. 6.63 Seven grains are shown in a two-dimensional hexagonal array before creep deformation. Following diffusion, the grains elongate in one direction and decrease perpendicularly to the tensile axis. A void formation develops between the grains, but grain-boundary sliding, which may accompany this process, removes these voids...

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