Softening-induced plastic flow instability and indentation size effect in metallic glass

Wang, S.; Ye, Y.F.; Sun, Baoan; Liu, Chang; Shi, San-Qiang; Yang, Yong

doi:10.1016/j.jmps.2015.01.008

Cited by 37 publications

(26 citation statements)

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“…Moreover, this also agrees well with the lower m observed under macro-compression [32] than micro-pillar compression [33]. When the peak-load increases to larger than a critical value, the overall unstable plastic flow (the advent of a mature shear band) will occur immediately once the indenter presses into the metallic glass [20]. In such a scenario, the stress overshoot becomes further weak.…”

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confidence: 86%

“…However, at higher strain rates, the activation of STZs and free volume creation are delayed due to short time. This results in a higher yield stress before strain softening (stress drop), and thus a more remarkable stress overshoot is observed at higher strain rates [8,20,30,31]. Therefore, with increasing strain rate, the yield stress to activate STZs increases, although strain softening becomes more pronounced.…”

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confidence: 99%

“…It leads to the formation of one mature shear band. Then the elastic backbone will be penetrated and give rise to irreversible macroscopic yield of the metallic glass [20,27,28]. The critical stress corresponds to the yield strength or the hardness.…”

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confidence: 99%

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confidence: 95%

“…Moreover, Jang et al [19] reported a continuous reduction of hardness with increasing peak-load or indentation size, i.e., an indentation size effect (ISE). On the contrary, Wang et al [20] recently demonstrated that the ISE would disappear when the indentation size is larger than a critical value.Evidently, the dependence of hardness on the testing conditions (strain rate and peak-load) under nanoindentation is still under debate. Further exploration is required to gain more insights into these issues.…”

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Testing effects on hardness of a Zr-based metallic glass under nanoindentation

Jiang

et al. 2017

Scripta Materialia

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A series of nanoindentation tests were performed on a Zr-based bulk metallic glass at different peak-loads and strain rates. It is observed that the strain rate sensitivity index gradually decreases from positive to almost zero with increasing peak-load. Furthermore, at a fixed strain rate, the indentation hardness firstly shows a dramatic reduction and then becomes stable with increasing peak-load. These observations can be well understood in the framework of the activation and the accumulation of free volume via shear transformation zones. The current work may provide a practical way to tailor the hardness of the metallic glass.© 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords:Metallic glass Nanoindentation Strain rate Indentation size effect Metallic glasses have been long regarded as potential structural materials due to their ultra-high strength and hardness [1,2]. Therefore, a number of researchers have paid much attention to the factors that will influence the hardness of metallic glass [3][4][5]. For instance, it is demonstrated that the increase of loading rate corresponded to a decrease of hardness and the underlying mechanism was attributed to the free volume relaxation induced hardening that was more dramatic under lower strain rates [6]. However, the free volume dynamics includes not only the relaxation induced annihilation, but also stress driven creation [7][8][9]. It is believed that the free volume creation could lead to a softening effect on the hardness of metallic glasses [10,11]. Therefore, both hardening and softening effects of strain rates on the hardness is possible. For example, Xue et al. [12] reported a positive strain rate dependence of the hardness and ascribed this correlation to the stronger strain rate softening effect under lower strain rates. There are also analogous inconsistent results, but the underlying mechanism has been rarely shed light on [13][14][15][16][17]. In addition, Bhattacharyya et al. [6] demonstrated a lower strain rate sensitivity of the hardness under a higher peak-load, but the results of Ma et al. [17,18] showed a much more positive rate sensitivity when the peak-load increased by 1 order of magnitude. Moreover, Jang et al. [19] reported a continuous reduction of hardness with increasing peak-load or indentation size, i.e., an indentation size effect (ISE). On the contrary, Wang et al. [20] recently demonstrated that the ISE would disappear when the indentation size is larger than a critical value.Evidently, the dependence of hardness on the testing conditions (strain rate and peak-load) under nanoindentation is still under debate. Further exploration is required to gain more insights into these issues. In the current work, a series of nanoindentation tests were performed on a Zr-based metallic glass under different peak-loads and strain rates. A positive dependence of hardness on the strain rate is found. Meanwhile, both the strain rate sensitivity index and the indentation hardness firstly decrease and then become stabl...

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confidence: 86%