Continuous Observation of Dynamic Behaviors of Dislocations in Aluminum

Abstract

Foil specimens of about 3µ in thickness were directly stretched inside a 500 kV electron microscope, and the dynamic behaviors of dislocations were continuously observed. When the specimens are deformed, frequency of cross slip is increased with increasing amount of deformation, and a number of small prismatic dislocation loops are formed. Mechanical and thermal stability of these loops are related to their size; i.e. , small ones are fairly stable mechanically but unstable thermally. Therefore, distribution and size of the loops depend on stress, deformation temperature, and strain rate. Movement of dislocations is retarded by these loops, and dislocations often tangle around them. At regions where dislocations tangle, the loop density is increased with the increase of deformation, so that escape tendency of tangling dislocations from the loop forest is decreased. Finally the cell boundaries are formed around these loop forest, while dislocations still can move forward through the cell boundaries by repeating cross slip. Work-hardening process is closely correlative to these phenomena. When the deformed specimens are annealed, the dislocation loops disappear in the regular order of their size, and then many of tangling dislocations in cell boundaries come loose. This fact shows a good correlation with the deformation process mentioned above.