Evolution of the structure and properties of thermomechanically-processed metallic glasses

Metallic glasses (MGs) can assume a remarkably wide range of structures and properties depending on their thermal and processing histories. Increasing the MGs’ enthalpies through thermomechanical processing, a process typically termed rejuvenation, can provide large improvements in mechanical properties. Plastic-deformation processes induce large changes in structure; however, these processes are destructive and therefore undesirable for engineering applications. It is thus of interest to induce significant structural changes with processes involving strain within the nominally elastic regime. This can be achieved through cryogenic thermal cycling (CTC) or elastostatic loading (EL). In the present work, the structural evolution induced by CTC was explored in depth. Four MG-forming compositions and different initial states were thermally cycled with fixed parameters. Calorimetric characterization showed that the stored energy evolves in three regimes, first increasing, then decreasing, and finally reaching a steady state; the changes in stored energy are paralleled by changes in local and macroscopic mechanical properties. The effects of CTC are shown to be transient, with the stored energy and properties reverting to those of the initial state following roughly one week at room temperature. Comprehensive memory effects were demonstrated at all stages of CTC. Based on these findings, and on observations in the literature, a structural model is proposed for the effects of CTC on the structure of MGs. The evolution of structure and properties was explored also for EL. Calorimetric characterization shows pronounced rejuvenation in two prototypical MG-forming compositions following loading for 6–72 h. The rejuvenated states show an accelerated stored energy decay with a profile similar to that seen for CTC-treated states, and a memory effect is also demonstrated for EL-treated states. A novel methodology was developed to separate the different strain contributions during heating of rejuvenated MGs in thermomechanical analysis (TMA). This was achieved through combining TMA and Archimedes’ density measurements. The results show that the stored energy increases are proportional to the accumulated anelastic strain. The higher stored energies in CTC- and EL-treated states are shown to be linked to lower density, albeit with a different enthalpy-density relationship than that observed for as-cast and annealed states. It is suggested that despite the differences between treatments, the structural model proposed for CTC can be extended to EL. The evolution of structure and properties during an individual cryogenic thermal cycle was investigated with in-situ synchrotron diffraction and cryogenic resonant ultrasound spectroscopy. The results show an asymmetric structural evolution during cooling and heating, challenging the conventional view that the glassy state is isoconfigurational during cooling and heating at relatively low temperatures. A superposition of effects is observed for three MG-forming compositions in as-cast and annealed states; the underlying contributions are suggested to be from thermal hysteresis and from rejuvenation/relaxation effects related to non-affine structural rearrangements of structural features with a range of activation energies and characteristic relaxation times. An inconsistency between the structural evolution following a single cycle and multiple cycles was identified, suggesting that the first-cycle behaviour may not be representative of the structural evolution during CTC. The results show that the techniques applied are informative and useful for probing the structural evolution with temperature, and they point towards future experiments in the field.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Frausto de Brito Costa, Miguel

Advisor dc:contributor.advisor

• Greer, Alan

• Glass states
• Glass structure
• Metallic glasses
• Structural relaxation
• Thermomechanical processing

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/e94ef187-f586-4ece-9bef-76a257de68f4/download
• https://creativecommons.org/licenses/by-nc-sa/4.0/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.108396

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/368042