Relaxation spectrum of glass-forming alloys: Mechanical and microscopic insights

This work reviews recent findings on the dynamics of metallic glasses, focusing on the relationship between mechanical relaxation and microscopic dynamics, the latter explored by means of coherent x-ray scattering experiments. We first summarize the fundamental features of the mechanical relaxation spectrum of metallic glasses, encompassing α relaxation as well as secondary processes (such as β and γ). We examine the specificities and limitations of various characterization methods, and we survey what we currently know about how the relaxation spectrum evolves during cooling—from the supercooled liquid through the glass transition—and during physical aging of the glass. Subsequently, we contrast these macroscopic results with the microscopic dynamics. While macroscopic and microscopic observations appear consistent in the SCL, a notable divergence emerges in the glass state, with the emergence of compressed nonexponential relaxation functions. We discuss the possible origins of these differences in view of recent microscopic observations. Stress relaxation curves of a metallic glass at different temperatures. Overall relaxation map of metallic glasses showing the time scales expected for the primary and secondary relaxation processes. SCL, supercooled liquid