SUBSTRUCTURE EVOLUTION OF COPPER POLYCRYSTALS UNDER DIFFERENT TESTING CONDITIONS - CONVENTIONAL STRAIN CONTROL AND RAMP LOADING

Substructure evolution has been studied by transmission electron microscopy in cyclically deformed copper polycrystals during conventional strain control testing and ramp loading. In both cases, and very similar to the substructure evolution previously reported for copper single crystals, dislocation structures develop from high and medium energy dislocation structures into low energy dislocation structures with increasing cumulative plastic strain. The two clearest differences between the substructure evolution of copper polycrystals conventionally tested and those ramp loaded are found to be the levels of homogeneity, from grain to grain, and the density of secondary dislocations within the developing loop patches. During ramp loading, the very small increment of the load per cycle promotes a much more homogeneous substructure, from grain to grain, higher densities of secondary dislocations and, therefore, lower energy loop patches than those observed in conventionally tested specimens. At stress amplitudes high enough to produce strain localization during ramp loading, substructure heterogeneities occur, which are explained by an orientation effect in which those grains with multislip behavior promote rapid evolution of low energy dislocation structures, i.e. persistent slip bands.