Incipient Nanoscale Plasticity in Metallic Glasses
Final Report Abstract
Plastic deformation of metallic glasses (MGs) at technically relevant temperatures is mediated by the operation of shear bands. Because of their inherent tendency to localize during deformation, MGs show an undesired brittle behavior on macroscopic length scales. Understanding the emergence of shear bands from smaller deformation entities, so-called shear transformation zones (STZs), has become a topic of intense research in the past years, with the goal of ultimately mediating the brittle behavior of MGs. However, since STZs operate at time and length scales that are barely accessible experimentally, indirect access is gained via small-scale mechanical testing. In this project, spherical nanoindentation under different conditions (externally applied loads, strain rate, indenter tip radius, hydrogen charging, thermal and mechanical pre-treatments) was used to gain information about the onset of plastic deformation, i.e. the moment at which individual STZs correlate to form a larger irreversible flow event. In load-displacement nanoindentation data, this event can be identified as a strain burst or so-called pop-in. Statistical distributions of the pop-in loads and sizes have been gathered, with the aim to understand how the different conditions affect the onset of plasticity. Furthermore, changes in structure as a result of the deformation and applied conditions were investigated using fluctuation electron microscopy, which provides information about the medium range order in the glass. The combination of sensitive methods to measure early plasticity and small changes in structure have resulted in a number of advances: we now know that pop-in formation is controlled by STZs that require a critical local strain to be activated; we find that high loading rates and certain stress distributions lead to multiple triggering or overlap of pop-ins which may improve the ductility of the MG; we find that hydrogen charging of a Zr based glass increases both the strength and ductility of the glass and that this effect is due to changes in the arrangement of structural heterogeneities within the glass; generally, we find support for both the cooperative shear model and for the idea that the arrangement of structural heterogeneities is a powerful means to control shear band formation.
Publications
- (2018). Measuring structural heterogeneities in metallic glasses using transmission electron microscopy. Metals, 8(12), 1085
Tian, L., & Volkert, C. A.
(See online at https://doi.org/10.3390/met8121085) - (2019). Size-dependent failure of the strongest bulk metallic glass. Acta Materialia, 178, 249-262
Qu, R., Tönnies, D., Tian, L., Liu, Z., Zhang, Z., & Volkert, C. A.
(See online at https://doi.org/10.1016/j.actamat.2019.08.019) - (2020). Effect of hydrogen charging on pop-in behavior of a Zr-based metallic glass. Metals, 10(1),22
Tian, L., Tönnies, D., Hirsbrunner, M., Sievert, T., Shan, Z., & Volkert, C. A.
(See online at https://doi.org/10.3390/met10010022) - (2020). Environmental transmission electron microscopy study of hydrogen charging effect on a Cu-Zr metallic glass. Materials Research Letters, 8(12), 439-445
Tian, L., Yang, Y. Q., Meyer, T., Tönnies, D., Roddatis, V., Voigt, H., ... Volkert, C. A. & Shan, Z. W.
(See online at https://doi.org/10.1080/21663831.2020.1791273)
