Discontinuous flow and continuous flow are common phenomena in metals. An example of discontinuous flow is the yield point phenomenon in steels containing a small amount of interstitial solutes. The yield point phenomenon is, however, also observed in metals where an effect of solutes can be ruled out. Such metals are characterized by a fine or ultrafine grain size produced by plastic deformation to a large strain followed by annealing and an example is aluminium. In the grain size range of 1-5 μm, a yield point phenomenon is observed, whereas for larger grain sizes, the flow is continuous. The yield point phenomenon may have its cause in depinning of dislocations from low angle boundaries, whereas the continuous flow can be related to the formation of dislocation sources within the grains. For both ultrafine and fine grain sizes the relationship between the stress and the grain size follows the Hall-Petch relationship for the yield stress and the flow stress, respectively, and it is found that Hall-Petch strengthening increases by a factor of about 3 when a yield point phenomenon is present.
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