Is plastic flow always controlled by dislocation mobility? An answer from in situ transmission electron microscopy straining tests

In situ transmission electron microscopy (TEM) straining experiments are us ed to illustrate in two extreme cases the possible role of dislocation nucl eation and exhaustion as a controlling factor in plastic flow. In the first example (FeAl intermetallic compounds), a thermally activated dislocation exhaustion is responsible for an anomalous stress-temperature dependence an d an associated small strain rate sensitivity, the latter being evidenced d uring in situ experiments through unstable localized slip. The second examp le (heavily drawn pearlite) shows specific dislocation loop nucleation proc esses that may account for the Hall-Petch law breakdown characteristic of f ine scale nanostructures.