STRESS RELAXATION OF La₃₀Ce₃₀Al₁₅Co₂₅ METALLIC GLASS

Metallic glass is a well-known engineering material that has been attracting tremendous research interest in materials science and condense matter physics. Early studies of the properties and structures of the metallic glasses showed that the dynamic heterogeneity is closely linked to the viscoelasticity and plasticity of metallic glasses. However, the physical landscape between the macroscopic stress relaxation behavior and the mechanical relaxation is still obscure. Different from the deformation mechanism of their crystalline counterparts, the deformation mechanism of metallic glasses is more complicated. To fully understand the mechanical properties of metallic glass, it is necessary to ascertain the structural characteristics of different spatial scales of metallic glass and evolution of structural characteristics with time. The significant importance is the connection between the macroscopic stress relaxation behavior and the dynamic mechanical relaxations (β relaxation, or α relaxation) in metallic glasses. Stress relaxation is a robust technique to characterize the viscoelastic and plastic mechanisms in glasses which can reflect their structural and dynamic heterogeneities. In the current research, La₃₀Ce₃₀Al₁₅Co₂₅ metallic glass was used as a model system, dynamic mechanical processes and stress relaxation behavior were studied. Compared with other traditional metallic glasses, La₃₀Ce₃₀Al₁₅Co₂₅ metallic glass shows a pronounced β relaxation process. The analysis based on the Kohlarausch-Willams-Watts (KWW) equation suggests that the stress relaxation process of metallic glass is a heterogeneous dynamic process. We observed an unusual two-stage stress relaxation phenomenon, consisting of the fast stress-driven event and the slow thermally activated event. The two-stage stress relaxation behavior is attributed the stress-driven event and thermally activated event to short-range atomic rearrangement, and long-range atomic diffusion, respectively. In addition, the analysis of the activation energy spectrum shows that the activation of the stress relaxation unit is not uniform, which corresponds to fluctuations in energy. This research is a step towards building a bridge linking the structural and dynamic heterogeneity of metallic glasses, and strongly supports the physical scenario of β to α relaxation.