Size effects in fcc crystals during the high rate compression test

M Yaghoobi and GZ Voyiadjis, ACTA MATERIALIA, 121, 190-201 (2016).

DOI: 10.1016/j.actamat.2016.09.010

The present work studies the different mechanisms of size effects in fcc metallic samples of confined volumes during high rate compression tests using large scale atomistic simulation. Different mechanisms of size effects, including the dislocation starvation, source exhaustion, and dislocation source length effect are investigated for pillars with different sizes. The results show that the controlling mechanisms of size effects depend only on the pillar size and not on the value of applied strain. Dislocation starvation is the governing mechanism for very small pillars, i.e. pillars with diameters less than 30 nm. Increasing the pillar size, the dislocation exhaustion mechanism becomes active and there is no more source limited activations. Next, the average dislocation source length is obtained and compared for pillars with different sizes. The results show that in the case of high rate deformations, the source length does not depend on the sample size, and the related size effects mechanisms are not active anymore. Also, in the case of high rate deformations, there are no size effects for pristine pillars with the diameters larger than 135 nm. In other words, increasing the strain rate decreases the pillar size at which there is no more size effects in the absence of strain gradient. The governing mechanisms of plastic deformation at high strain rate experiments are also different from those of the quasi-static tests. First, the diameter in which the dislocation nucleation at the free surface becomes the dominant mechanism changes from around 200 nm-30 nm. Next, in the case of the pillars with larger diameters, the plastic deformation is governed by the cross-slip instead of the operation of truncated dislocation sources, which is dominant at slower rates of deformation. In order to study the effects of pillar initial structure on the controlling mechanism of size effects, an initial loading and unloading procedure is conducted on some samples prior to the compression test. In the case of the nanopillars with the height smaller than 45 nm, the results show that the pre-straining does not change the controlling size effects mechanism except for the initial phase of dislocation nucleation. In the case of the pillar with the height of 0.3 gm and diameter of 0.15 gm, however, increasing the initial dislocation density leads to the activation of the forest hardening mechanism. In other words, as the strain increases, the dislocation density increases, which activates the mechanism of dislocation interaction with each other and increases the pillar strength. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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