Assessment of grain size distributions in nanocrystalline copper and their effect on mechanical behavior

Grain size measurements by X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM) observations were carried out on nanocrystalline copper processed by inert gas condensation. Grain size distributions obtained by XRD and TEM follow a log normal relationship. The distributions of both number and volume fractions of grain sizes were evaluated. The mode of the latter was found to be much larger than that of the former. The results of XRD and TEM matched closely in cases of finer mean grain size with a relatively narrow distribution. It was observed that extensive twinning could lead to underestimation of grain size through XRD methods. Grain sizes increased significantly on use of higher compaction temperature during processing or on extended room temperature exposure. Finally, a micromechanical model was used to understand the effect of grain size (volume) distribution on the yielding of nanocrystalline metals. An increase in the mean grain size or standard deviation, which means an increase in the volume fraction of relatively coarse grains, reduces the yield stress significantly. This implies it is not just the average grain size, but the character of the grain size distribution, that controls the mechanical behavior of nanocrystalline metals.