GM Cheng and WW Jian and WZ Xu and YF Zhang and PC Millett and YT Zhu, NANOMETALS - STATUS AND PERSPECTIVE, 1-13 (2012).

We report grain size effect on the densities of edge and mixed dislocations in nanocrystalline Mo prepared by high-pressure torsion: with decreasing grain size the density first increases and then decreases. The trend of grain size effect on the density of edge and mixed dislocations mirrors the reported grain size effect on the strain rate sensitivity of bcc metals, suggesting that the high density of edge and mixed dislocations contribute to the reduction of strain rate sensitivity. We also observed for the first time the 1/2 < 111 > and < 001 > pure edge dislocations under high-resolution transmission electron microscopy (HRTEM). Crystallographic analysis and simulations reveal that the best way to study the edge dislocations or the edge components of mixed dislocations is to take HRTEM images along a < 110 > zone axis. The < 001 > pure edge dislocations can be easily identified from a < 100 >< 100 > zone axis. This work not only sheds light on the deformation mechanisms of nanocrystalline bcc metals, but also provides guidance on future HRTEM studies of their dislocation structures.

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