Transmission electron microscopy deformation measurement

In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measurements in the deformation zone are required to determine local stresses and strains. In RTFs craze zones can develop that are important microscopic features around a crack tip governing strength behavior. Plastics fracture is preceded by the formation of a craze zone that is a wedge shaped region spanned by oriented micro-fibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction techniques, scanning electron microscope, and transmission electron microscopy. 

It has been possible, for metals and ceramic materials, to demonstrate by direct observation the existence of lattice defects called dislocations, using the techniques of transmission electron microscopy. These studies have shown that it is often adequate to assume that dislocation motion is responsible for the observed plastic, or permanent, deformation, and that this motion is negligible at stresses below the yield stress. Although very refined microstrain measurements and internal friction experiments have failed to define a stress range in which dislocation motion is completely absent, there is still a clear distinction for these materials between elastic and plastic strain, both on a macroscopic level, in terms of permanency of deformation, and on a microscopic level in terms of large scale dislocation motion. 

These illustrations are an indication of the contribution of transmission electron microscopy [henceforth TEM] to the understanding of plastic deformation in some oxide ceramics, including softening at the measured temperatures, most probably caused by climb. Clearly, since most of the common ceramics are ductile at high temperamres, Mitchell s [38] above measurements explain the plasticity of oxide ceramics and the recovery by climb, as observed by the softening seen as indicated in Fig. 3.74. 

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