Thermomechanical Processing of Metallic Glasses
Creators: name: Meylan, Carolina Maria
Publisher: name: Apollo - University of Cambridge Repository
Year: 2019
Subjects: metallic glasses
relaxation
rejuvenation
nanoindentation
differential scanning calorimetry
heat treatment
mechanical treatment
elastic cycling
thermal cycling
annealing
plastic deformation
ultra-fast scanning calorimetry

description: Depending on its thermal and kinetic history, a metallic glass (MG) of a given composition can show a wide range of energetic states. One central parameter determining the initial state of a glass is the cooling rate used to form it upon quenching from the melt. A faster cooling rate will give rise to a glass of higher energy with a more disordered structure. The energy, structure and properties of a glass can also be modified after glass formation. Through various thermomechanical methods, it is possible to bring the glass either to a more relaxed, ordered state of lower energy – this process is called relaxation – or to a more unrelaxed, disordered state of higher energy, a process called rejuvenation. In the present work, the effects of several thermomechanical processing methods were explored on the properties of various MG samples. One well-known method to rejuvenate MGs is by introducing damage through plastic deformation. Here, an infrequently used approach to reach large plastic strains was tested, in which brittle fracture was avoided by deforming samples with a low aspect ratio through uniaxial compression. In this manner, large plastic strains of up to 85% were achieved in La-based MG rods, which led to an average energy increase of ≈670 J/mol, determined by ultra-fast differential scanning calorimetry (FDSC). But for this MG, room temperature (RT) is a relatively high fraction of the glass-transition temperature Tg; thus RT storage led to significant ageing and the deformation-induced energy was found to relax away with time. Another method studied here was thermal cycling (TC), which is a novel technique to rejuvenate MGs and consists of introducing non-affine thermal strains by cycling between cryogenic and room temperatures. TC was applied to three MG compositions and different aspects were investigated. From elastic bending experiments on a La-based MG ribbon, TC was observed to accelerate the relaxation of anelastic strain. Nanoindentation measurements on a CuZr-based MG plate showed that TC decreased the yield stress, but barely affected the hardness or modulus. Finally, TC was also applied to a Fe-based MG; but for this composition, around 100 cycles were necessary to observe a significant effect, whereas only 5‒10 cycles were needed to affect the other two compositions. But for all three compositions, TC was found to target mainly the individual flow units of the MG without affecting much the global structure. While rejuvenation of MGs is often desired with the aim of improving plasticity, relaxation also has advantages, e.g. it increases thermal stability and hardness. Annealing is the common method for relaxation, but may pose the risk of inducing crystallization. Therefore, there is an interest in relaxing MGs through other means. The application of loading cycles in the elastic regime has been reported to relax MGs. Elastic cycling (EC) by nanoindentation was tested here on a CuZr-based MG and compared directly to annealing. Although EC indeed increased the yield strength of the sample, its hardening effect differed from that of annealing. The obtained results show that EC did not necessarily lead to a more relaxed state of the MG; instead the induced strengthening seemed to be rather due to the build-up of anelastic strains. Finally, relaxation and rejuvenation of an Au-based MG through annealing was studied by FDSC. Due to its low Tg, storage at RT leads to significant relaxation, which gives rise to a characteristic endothermic sub-Tg peak in the heat flow curve measured by FDSC. This peak corresponds to the disordering of highly relaxed zones in the glass upon heating. This disordering event was analysed here, and its activation energy was observed to depend on the initial energy of the sample. Furthermore, by annealing a heavily aged sample for short times above its fictive temperature followed by fast cooling, it could be successfully returned to a state thermally equivalent to that of a freshly formed glass.