Environmental Engineering Reference
In-Depth Information
3.14 Microstructure of NaCl corresponding to the power-law
breakdown regime. 72
number of dislocation cores generated at high applied stresses. Sherby and
Burke 16 suggest that vacancy diffusion due to vacancy supersaturation at
high applied stresses can be associated with the rate controlling mechanism.
Others have considered breaking down of subgrain walls 70 and cross slip or
cutting of forest dislocations 71 instead of dislocation climb as the rate con-
trolling mechanism. In any case PLB remains poorly understood and this is
due to the relatively small number of studies that have been carried out in
this regime.
Microstructural features
The PLB regime has been described as similar to a normal tension test. This
is primarily due to the high strain rates of deformation experienced in the
PLB regime. Naturally the microstructural features associated with such a
regime should have similarities to those observed under normal tensile test
conditions. TEM studies have shown the presence of high dislocation den-
sity. There is a tendency to develop dislocation cell structures. Figure 3.14
provides the deformation microstructure of NaCl crept in the PLB creep
regime. 72
￿ ￿ ￿ ￿ ￿ ￿
3.4
Rate controlling mechanisms and activation
energy
The activation energy of deformation is dependent on the rate controlling
mechanism of creep. As shown in Table 3.1, the activation energy changes
with the underlying creep mechanism. For example, the activation energy of
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