Atomistic simulation of size effects in single-crystalline metals of confined volumes during nanoindentation

M Yaghoobi and GZ Voyiadjis, COMPUTATIONAL MATERIALS SCIENCE, 111, 64-73 (2016).

DOI: 10.1016/j.commatsci.2015.09.004

In single-crystalline metals, the sources of size effects depend on the sample length scale. In bulk samples, the interaction of dislocations with each other is responsible for size effects which is commonly termed forest hardening. The Taylor-like hardening models are usually incorporated to capture the forest hardening which states that the strength increases as the dislocation density increases. As an example, the fact that the nanoindentation hardness increases as the indentation depth decreases is justified as an increase in the density of geometrically necessary dislocations. In the cases of small length scales, several experiments on whiskers, wires, and micropillars have demonstrated that the sources of size effects are different from those of bulk material. In the case of nanoindentation of nanoscale samples, it has been experimentally shown that the hardness decreases as the density of geometrically necessary dislocations increases in the region of small indentation depths. It shows that the size effects theory of bulk material cannot be extended to the indentation of nanoscale samples. The present work incorporates the large scale atomistic simulation to investigate the size effects in a nanoscale single crystal Ni thin film during indentation. The results show that the hardness decreases as the dislocation density increases, and the forest hardening model cannot capture the strength size effects during nanoindentation at small length scales. It is observed that the size effects are initially controlled by dislocation nucleation and source exhaustion. As the indentation depth increases, the dislocation length and density increase. Consequently, the number of dislocation sources and their characteristic length increase which decreases the material strength. Finally, increasing the dislocation length and density, the dislocation interaction mechanism also becomes important. (C) 2015 Elsevier B.V. All rights reserved.

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