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.
Highlights
Plastic strain of metallic glasses well below the glass transition temperature is known to be localized into shear bands [1,2,3,4]
We denote the entire rebound as viscoelastic, encompassing elastic and viscous processes. This figure shows the temperature dependence of the recovery. For this particular quenching rate, the recovery remained high at all temperatures up to the glass transition temperature; for a discussion of the quenching-rate dependence of this quantity, see we studied the effects of the indentation rate on the shear transformation zones (STZs)’ intermittency—i.e., the effects on the serration—during loading and unloading
The hardness, calculated as the average of contact pressures for indentation depths ≥ 60 Å, grew as a power law with indentation rate, with exponent ∼0.04. This strain-rate sensitivity was previously reported in nano-indentation experiments [11] and can be explained in the framework of the so-called cooperative-shear model (CSM) [35] where the strain-rate sensitivity is inversely proportional to the activation volume and the STZ volume
Summary
Plastic strain of metallic glasses well below the glass transition temperature is known to be localized into shear bands [1,2,3,4]. After accommodating a certain plastic strain, shear bands have the tendency to form cracks eventually leading to failure and at temperatures well below the glass transition, limiting the applications of metallic glasses as structural materials. The statistics of the STZs during unloading follow the same power law found during loading These findings add new aspects in the understanding of deformations of metallic glasses and highlight the influence of the unloading process on the post-test deformation pattern
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
