Abstract
Abstract In this paper, the mechanical behavior of gradient nano-grained copper under uniaxial deformation was investigated using molecular dynamics simulations. The stress response was found to be different in the regions with different grain sizes, which was attributed to the different dislocation activities due to the dislocation-grain boundary synergies. The phenomenon of grain rotation was observed and a program was developed to accurately evaluate the grain rotation and explore its dependence on the grain size and the initial crystal orientation. It is found that all grains tend to rotate to the 30° orientation, consistent with the activation theory of the slip systems under the uniaxial deformation. The rotation magnitude is larger for larger grains, but the rotation rate is more diversely distributed for smaller grains, indicating more disturbance from grain boundary mechanisms such as the grain boundary sliding and the grain boundary diffusion for smaller grains. The effect of temperature on the grain rotation is also investigated, showing an increase of the dispersion of grain rotation distribution with the increase of temperature. This paper aims at providing insights into the synergistic deformation mechanisms from dislocations and grain boundaries accounting for the exceptional ductility of the gradient nano-grained metals.
Highlights
Grain rotation, referred to as the change of lattice orientation of grains, has received increasing attention since the ignition of research interests in nanocrystalline metals in 1990s
Grain rotation is not treated as an important mechanism in coarse-grained metals, it is believed to play an important role in the plastic deformation of nanocrystalline metals in which the volume percentage of grain boundaries upsurges as the grain size decreases to nanometers [1,2,3]
From the in situ tensile test of Pt thin films under a high-resolution TEM, Wang et al [4] directly showed that when the grain size is below 6 nm, the plastic deformation mechanism transits from dislocation glide to grain rotation coordinated in multiple grains mediated by the climb of grain boundary (GB) dislocations
Summary
Grain rotation, referred to as the change of lattice orientation of grains, has received increasing attention since the ignition of research interests in nanocrystalline metals in 1990s. Experiments and simulations [6,7,8] have indicated that as the grain size decreases from micrometer to a few nanometers, the dominating mechanism of plastic deformation evolves from full dislocation activity in large grains, to partial dislocation activity in smaller grains, and to GB-mediated mechanisms including grain creep, grain sliding, and grain rotation. It indicates that the Hall–Petch relation [1,9] that prevails for the conventional metal materials can no longer be applied for predicting the strength of the nanocrystalline metals [10].
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