Structural Consequences of Plastic Deformation

The arrival at the yield point on the stress-strain curve is revealed at the most gross structural level by a permanent change in the body s shape. However, there are more subtle structural changes as well. One of the intriguing features that arise with the onset of plastic flow and a hint in favor of the claim that dislocations are the carriers of plastic change is the occurrence of micron-sized steps on the crystal faces. In particular, if an optical microscope is used to examine the crystal surface 

Higher resolution reveals surprises as well. Indeed, if a thin foil of the sample is examined via the electron microscope, it is found that the sample is seething with activity. Consider the sequence of snapshots shown in fig. 8.4. What is revealed is a series of line objects in motion, objects which ultimately it can be concluded are dislocations. 

As will be seen below, the presence of dislocations has the effect of producing local disturbances of the atomic-level geometry that are so severe as to produce scattering that is not consonant with that from the remainder of the crystal, giving rise to the contrast revealed in the figure. Evidently, the onset of plastic deformation reveals its presence at multiple scales simultaneously. Indeed, as was seen in fig. 1.9, the properties of individual dislocation cores can be observed using high-resolution transmission electron microscopy. 

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