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Shear-Transformation Zone Activation during Loading and Unloading in Nanoindentation of Metallic Glasses

1
Physics Department, University Kaiserslautern, Erwin-Schrödinger-Straße, D-67663 Kaiserslautern, Germany
2
Research Center OPTIMAS, University Kaiserslautern, Erwin-Schrödinger-Straße, D-67663 Kaiserslautern, Germany
*
Author to whom correspondence should be addressed.
Materials 2019, 12(9), 1477; https://doi.org/10.3390/ma12091477
Received: 17 April 2019 / Revised: 1 May 2019 / Accepted: 2 May 2019 / Published: 7 May 2019
(This article belongs to the Section Materials Physics)
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Abstract

Using molecular dynamics simulation, we study nanoindentation in large samples of Cu–Zr glass at various temperatures between zero and the glass transition temperature. We find that besides the elastic modulus, the yielding point also strongly (by around 50%) decreases with increasing temperature; this behavior is in qualitative agreement with predictions of the cooperative shear model. Shear-transformation zones (STZs) show up in increasing sizes at low temperatures, leading to shear-band activity. Cluster analysis of the STZs exhibits a power-law behavior in the statistics of STZ sizes. We find strong plastic activity also during the unloading phase; it shows up both in the deactivation of previous plastic zones and the appearance of new zones, leading to the observation of pop-outs. The statistics of STZs occurring during unloading show that they operate in a similar nature as the STZs found during loading. For both cases, loading and unloading, we find the statistics of STZs to be related to directed percolation. Material hardness shows a weak strain-rate dependence, confirming previously reported experimental findings; the number of pop-ins is reduced at slower indentation rate. Analysis of the dependence of our simulation results on the quench rate applied during preparation of the glass shows only a minor effect on the properties of STZs. View Full-Text
Keywords: molecular dynamics; nanoindentation; metallic glass molecular dynamics; nanoindentation; metallic glass
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Avila, K.E.; Küchemann, S.; Alabd Alhafez, I.; Urbassek, H.M. Shear-Transformation Zone Activation during Loading and Unloading in Nanoindentation of Metallic Glasses. Materials 2019, 12, 1477.

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