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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.
Visual–graphical representations are used to visualise information and are therefore key components of learning materials. An important type of convention-based representation in everyday contexts as well as in science, technology, engineering, and math (STEM) disciplines are vector field plots. Based on the cognitive theory of multimedia learning, we aim to optimize an instruction with symbolical-mathematical and visual-graphical representations in undergraduate physics education through spoken instruction combined with dynamic visual cues. For this purpose, we conduct a pre-post study with 38 natural science students who are divided into two groups and instructed via different modalities and with visual cues on the graphical interpretation of vector field plots. Afterward, the students rate their cognitive load. During the computer-based experiment, we record the participants’ eye movements. Our results indicate that students with spoken instruction perform better than students with written instruction. This suggests that the modality effect is also applicable to mathematical-symbolical and convention-based visual-graphical representations. The differences in visual strategies imply that spoken instruction might lead to increased effort in organising and integrating information. The finding of the modality effect with higher performance during spoken instruction could be explained by deeper cognitive processing of the material.
Plasticity in metallic glasses depends on their stoichiometry. We explore this dependence by molecular dynamics simulations for the case of CuZr alloys using the compositions Cu64.5Zr35.5, Cu50Zr50, and Cu35.5Zr64.5. Plasticity is induced by nanoindentation and orthogonal cutting. Only the Cu64.5Zr35.5 sample shows the formation of localized strain in the form of shear bands, while plasticity is more homogeneous for the other samples. This feature concurs with the high fraction of full icosahedral short-range order found for Cu64.5Zr35.5. In all samples, the atomic density is reduced in the plastic zone; this reduction is accompanied by a decrease of the average atom coordination, with the possible exception of Cu35.5Zr64.5, where coordination fluctuations are high. The strongest density reduction occurs in Cu64.5Zr35.5, where it is connected with the partial destruction of full icosahedral short-range order. The difference in plasticity mechanism influences the shape of the pileup and of the chip generated by nanoindentation and cutting, respectively.