In this work, the Shear Transformation Zone (STZ) dynamics model is adapted to capture the transitions between different regimes of flow serration in the deformation map of metallic glass. This was accomplished by scaling the STZ volume with a log-linear fit to the strain rate, and also adjusting the activation energy of an STZ with a log-linear fit to maintain constant yield strength at differing strain rates. Twelve simulations are run at each of six different strain rates ranging from 10-5 to 100 s-1, and statistics are collected on simulation behavior and shear band nucleation and propagation rates. The simulations show shear band nucleation has a positive correlation to strain rate, and shear band propagation has a negative correlation to strain rate. This shows that in STZ dynamics, the regime of reduced flow serration arises due to competing rates of nucleation and propagation, supporting the hypothesis proposed by Schuh. A positive correlation between critical shear band nucleus size and strain rate is proposed as an underlying cause of these rate dependencies.

Metallic glass matrix composites (MGMCs) have been developed to improve upon the ductility of monolithic metallic glass. These composites utilize a secondary crystalline phase that is grown into an amorphous matrix as isolated dendritic trees. This work seeks to understand the mechanisms underlying strain delocalization in MGMCs in order to better direct efforts for continual progress in this class of material. A mesoscale modelling technique based on shear transformation zone (STZ) dynamics is used to do so. STZ dynamics is a coarse grained technique that can provide insight into the microscopic processes that control macroscopic behavior, but which can be difficult to resolve experimentally. A combined simulated-experimental approach to extract the individual material properties of the amorphous and crystalline phases is presented. Numerically, STZ dynamics is used to simulate nanoindentation of the crystalline and amorphous phases respectively. The indented phases are modelled as discs with varying thickness embedded in the other phase. Indentation depths are held constant. Experimentally, nanoindentation is carried out on DH2 and DH3 MGMC composites under varying loads at Stony Brook University (SBU). Specimens are cross-sectioned and using scanning electron microscopy, indentation sites are chosen so that the indenter targets individual phases. For both experimental and simulated nanoindentation, hardness and modulus values are calculated from the load-displacement data. The experimental and simulated values are normalized and compared. Good agreement between results suggests accurate characterization of the individual phases at low loads on both DH2 and DH3 composites. Length scales at which indentations begin sampling outside the intended phase are presented. Work is then presented on simulated uniaxial tensile loading of MGMCs. Dendritic microstructural sizes are varied and shear banding characteristics are measured. A competition of shear band nucleation and propagation rates that previously had only been seen in monolithic metallic glasses under certain loading conditions is found to exist in MGMCs as well. The stages of shear banding in MGMCs are presented and the influence of dendrites on shear band nucleation and propagation are discussed. It is proposed that the introduction of dendrites into the amorphous matrix work to inhibit shear band propagation and encourage shear band nucleation to delocalize strain in MGMCs. In particular, it was found that smaller dendrite sizes and spacings are better at doing so.

In this study, the time-dependent deformation behavior of several model bulk metallic glasses (BMGs) was studied. The BMGs were obtained in different structural states by thermal relaxation below their glass transition temperature, cryogenic thermal cycling, and chemical rejuvenation by micro-alloying. The creep behavior of Zr52.5Ti5Cu17.9Ni14.6Al10 BMG in different structural states was investigated as a function of peak load and temperature. The creep strain rate sensitivity (SRS) indicated a transition from shear transformation zone (STZ) mediated deformation at room temperature to diffusion dominated mechanisms at high temperatures. The relaxation enthalpy of Zr47Cu46Al7 BMG was found to increase significantly with the addition of 1 at% Ti, namely for Zr47Cu45Al7Ti1. Comparison of their respective free volumes indicated that chemical rejuvenation had a more pronounced effect compared to cryogenic thermal rejuvenation. Micro-pillar compression tests supported the improved plasticity with increase in free volume from the rejuvenation effect. Effect of chemistry change on mechanical response and time-dependent deformation was investigated for topologically equivalent Pt-Pd BMGs, where the Pt atoms were systematically replaced with Pd atoms (Pt42.5-xPdx)Cu27Ni9.5P21: x=0, 7.5, 20, 22.5, 35, 42.5). The hardness and reduced modulus increased while the degree of plasticity decreased with increase in Pd-content, which was attributed to the increase in stiffer 3-atom cluster connections. STZ volume was calculated for all the BMGs using cooperative shear model (CSM) for fundamental understanding of the underlying deformation mechanisms.