Evolution of coupling modes between α and β relaxations in metallic glass-forming liquids revealed by nano-calorimetry

The coupling behavior of alpha and beta relaxations in supercooled liquids (SLs) is a key scientific issue that relates to the essence of glass transition and the modulation of glass properties. Here, we observe the evolution of alpha and beta relaxations in Au50Ag7.5Cu17.5Si25 alloys from glass to SLs by nano calorimetric technique. We find that all the beta participates in alpha relaxation in these metallic glasses (MGs) upon heating. Two typical coupling (or decoupling upon cooling) modes of alpha and beta relaxations in Au -based MG SLs have been discovered, dependent on the degree of coupling between alpha and beta relaxations in glasses. The coupling degree increases during the isothermal annealing process as the energy state of the glass decreases. Above a critical degree the decoupling mode changes from "beta splits from alpha" to "alpha splits from beta". A survey including metallic glasses, polymer glasses and oxide glasses shows the general existence of the critical degree of coupling in glasses. The differences in local structure feature in glasses with different coupling degree is elucidated by analysis on atomic nonaffine displacements using molecular dynamics simulations. Further our work demonstrates that the plasticity of MGs can be effectively modulated by altering the coupling behavior between alpha and beta relaxation.