Structural Relaxation in Metallic Glass and Metallic-Glass Matrix Composite: From Macro- to Nano-Scale

Like many other types of glasses, metallic glasses inevitably relax over time towards a lower energy state owing to their non-equilibrium nature. Therefore, for a long time, structural relaxation in metallic glasses has been used as an effective means to control/optimize their mechanical/physical properties. On the other hand, there is a surge of interest in recent years to improve the mechanical properties of monolithic metallic glasses by nucleating secondary crystalline phases to form metallic-glass matrix composites. However, as of today, little is known yet on how the ‘composite’ approach could possibly alter the relaxation behavior of the metallic-glass matrix in the pursuit of the superior mechanical properties in the glasses. In this project, we aim to characterize and understand the relaxation behavior of metallic glasses and their composites across the size scales ranging from ~1 nm to ~1 mm and the time scales from ~1E-5 s to ~10 s by fully exploiting the recently developed nano-scale dynamic mechanical analysis (NanoDMA) and atomic force microscopy techniques as combined with the conventional DMA. To achieve our goal, metallic-glass samples in different forms (bulk, foil and fiber) will be prepared with the controlled chemical composition and thermal/mechanical history; afterwards, we will systemically investigate the effects of chemical composition, plastic deformation and cooling rate on the global/local structural relaxation behavior of the metallic glasses and metallic-glass matrix composites. The successful implementation of this project will unfold, in a comprehensive manner, how the local structural relaxation process in metallic glasses and their composites are affected by the various external factors, such as size, cooling rate, plastic deformation, which finally leads to the global structural relaxation behavior and the resultant mechanical/physical properties typically seen in bulk metallic glasses. From the scientific perspective, the proposed research could deepen our understanding of the structural origin of the relaxation process in metallic glasses; from the application viewpoint, our research could provide the most needed information on the most efficient way for the control and optimization of the mechanical properties of bulk metallic glasses and their composites through structural relaxation. Furthermore, since sample size is the factor to be investigated in our proposed research, the outcome of our project is also very valuable to the development of metallic-glass based nano- and micro-systems, which is an emerging high-tech area recently showing great technological promise in micro- and nano-scale manufacturing, sensing and actuation, biotechnology and energy harnessing.