Role of grain boundary on the sources of size effects

GZ Voyiadjis and M Yaghoobi, COMPUTATIONAL MATERIALS SCIENCE, 117, 315-329 (2016).

DOI: 10.1016/j.commatsci.2016.01.025

The present paper investigates the effects of grain boundary (GB) on the sources of size effects. Up to now, several studies have been conducted to address the role of GBs in size effects from the atomistic point of view. However, a study which addresses the effects of GB on different governing mechanisms of size effects as the sample length scale varies has not been presented yet. Here, samples with different length scales are studied to capture the role of GB in size effects as the grain size changes. The response of single and bi-crystal Ni thin films with two different sizes are studied during nanoindentation experiment using large scale atomistic simulation. Various symmetric and asymmetric tilt GBs are incorporated to study the effects of GB geometry on the response of samples during nanoindentation. The sources of size effects are analyzed in each sample using the atomistic information obtained from the simulations. The results show that the size effects mechanism influenced by GBs changes from dislocation nucleation and source exhaustion to the forest hardening mechanism as the grain size increases. In the case of small bi-crystal samples, dislocation nucleation and source exhaustion govern the size effects. The GB contributes to the dislocation nucleation beneath the indenter which reduces the strength of sample by providing required dislocations to sustain the imposed plastic deformation. Also, the GB itself is the source of defects which can affect the sample strength depending on the indentation depth at which the dislocation is nucleated from the grain boundary. Increasing the indentation depth, some of the dislocations are blocked by the GB. However, there is no noticeable additional hardness due to the dislocations blockage by GB. Furthermore, the results show that the coherent twin boundaries shows the best performance during the nanoindentation. In the case of large bi-crystal samples, the GB does not influence the size effects at lower indentation depths where the source exhaustion is the controlling mechanism of size effects. However, at higher indentation depths, the dislocation interaction with GB contributes to the forest hardening mechanism and induces some hardening. It is observed that the dislocations are firstly absorbed by the GB. Increasing the indentation depth, some dislocations start dissociating into the next grain. However, it is observed that more dislocations are nucleated in the upper grain. The results show that the main role of GB at larger length scale is to change the pattern of dislocation structure in a way that the dislocations are piled up near the GB which increases the hardness. (C) 2016 Elsevier B.V. All rights reserved.

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