Dynamic relaxations and relaxation-property relationships in metallic glasses

Dynamic relaxation is an intrinsic and universal feature of glasses and enables fluctuation and dissipation to occur, which induces plentiful behaviour, maintains equilibrium, and achieves evolution in glass systems. Relaxation covers a broad time, frequency, and temperature ranges and determines the functions, behaviour, properties and applications of glassy system. Investigations of dynamic relaxation are significant for understanding the nature of glasses, liquids, and the critical issues of glass formation and transition, dynamic and structural heterogeneities, flow behaviour and flow units, various crossover temperatures, deformations, aging and rejuvenation, stability, crystallization, and the mechanical and physical properties of glasses. Metallic glasses (MGs) with unique microstructure and mechanical and functional properties, offer a simple but effective system for study of relaxation and related issues in glass science. In this review, a panoramic view of the state of the art of various aspects of dynamic relaxation in metallic glassy system, as well as a comparison with other glassy systems, is presented. The features and mechanisms of each known relaxation mode including primary a-relaxation, slow and fast 7 g-relaxations, nearly constant loss, and boson peak, as well as their coupling in MGs, are reviewed and summarized. Emphasis is presented to the microstructural origin of these dynamic relaxation modes and their connection with the dynamic and structural heterogeneities in MGs. The factors which determine and affect the relaxation modes and behaviour in low-dimensional MGs are also introduced. It is shown that the relaxation in MGs is connected with their structural characteristics, heterogeneity, formation, glass transition, flow behaviour, physical and mechanical properties, crystallization, stability, and the localized atomic diffusion. The roles and the importance of dynamic relaxation in understanding many crucial issues in glassy physics are demonstrated. The correlations between dynamic relaxation and various properties of MGs are established and summarized. With this review on dynamic relaxation in metallic glasses, relaxation in MG can provide an effective perspective for understanding nearly all issues in metallic glasses. It is demonstrated that the relationship of relaxation to various properties, similar to the relationship of structure-property of crystalline materials, can be applied to control and design of new glassy materials with multiple functionalities, superior mechanical performance, and other extraordinary physical and chemical properties. Finally, the key unsolved questions regarding dynamic relaxation in metallic glasses are listed, and several emerging research directions in this still-evolving field are highlighted for future investigations.