  The grain boundary network of nanocrystalline Cu foils was modified by the
systematic application of cyclic loadings and elevated temperatures having a
range of magnitudes. Most broadly, the changes to the boundary network were
directly correlated to the applied temperature and accumulated strain,
including a 300% increase in the twin length fraction. By independently varying
each treatment variable, a matrix of grain boundary statistics was built to
check the plausibility of hypothesized mechanisms against their expected
temperature and stress/strain dependences. These comparisons allow the field of
candidate mechanisms to be significantly narrowed. Most importantly, the effect
of temperature and strain on twin length fraction were found to be strongly
synergistic, with the combined effect being ~150% that of the summed individual
contributions. Looking beyond scalar metrics, an analysis of the grain boundary
network showed that twin related domain formation favored larger sizes and
repeated twin variant selection over the creation of many small domains with
diverse variants. Taken together, the evidence indicates that shear-coupled
boundary migration twinning is the most likely explanation for grain boundary
engineering in nanocrystalline Cu.
