Using molecular dynamics to determine mechanical grain boundary energies and capture their dependence on residual Burgers vector, segregation and grain size
F Shuang and KE Aifantis, ACTA MATERIALIA, 195, 358-370 (2020).
The present article focuses on interpreting mechanical interface energies parameters which have been introduced within gradient plasticity and can capture the ability of grain boundaries to deform plastically. Molecular dynamics simulation compression tests were performed on body-centered cubic Fe bicrystals with tilt and twist grain boundaries of different misorientations. The simulations allowed for the formation of dislocation pile-ups, which give rise to gradients in the plastic strain, and therefore it was possible to employ interfacial gradient plasticity to interpret the stress-strain curves and obtain values for the mechanical interface parameter. The simulations showed that the atomic structure at the grain boundary changed during dislocation absorption, illustrating that the initial grain boundary static energy cannot characterize boundaries after the onset of plastic deformation. It was found that the value of the mechanical interface parameter (i) depended on the GB type and GB-dislocation interactions that led to slip transmission across the GB, and (ii) was positively related to the magnitude of residual Burgers vector, indicating that as the mechanical interface parameter increased the GB strength increased. Repeating the simulations by adding hydrogen atoms at the grain boundaries, showed that segregation increased the mechanical interface energy. Changing the sample size indicated that such mechanical energy terms are size-dependent, however their ratio with the internal length was size independent and depended only on the GB type. A detailed understanding of these mechanical grain boundary energies may pave a new way to engineering materials in the sub-micron scales. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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