Residual stress of metal glass originates from the localization of strain or casting or manufacturing process, which has a significant impact on its performance. High temperature annealing is a common method to release residual stress, but due to the disordered atomic filling structure of metal glass, it is very difficult to directly measure the evolution of its residual stress. Studying the evolution of external stress during annealing of is an effective way to explore residual stress relaxation behavior. It also helps to understand the mechanical properties of metallic glass, including viscoelastic deformation and viscoplastic deformation. The stress relaxation process can be well fitted with multiple non-exponential equations, showing the hierarchical dynamics process. Meanwhile, the non-equilibrium glass state exhibits complex relaxation dynamics such as α relaxation, β relaxation and γ relaxation. It is generally believed that the elastic recovery of strain originates from the reversible rearrangement event of metallic glass, while macroscopic flow corresponds to plastic deformation. Therefore, stress relaxation is a convenient method to detect non-uniform evolution. From a practical point of view, a lot of research is dedicated to improving the performance of metallic glass by adjusting relaxation. For example, the activation energy of the shear transition zone is consistent with β relaxation, reflecting that β relaxation is related to plasticity. Furthermore, beta relaxation was identified as having reversible and irreversible portions. Alpha relaxation represents a larger scale irreversible rearrangement, which is usually related to vitrified transition behavior and uniform flow. Therefore, it is urgent to reveal the structural evolution in the stress relaxation process.
Based on this, the team of Wang Junqiang, a researcher at Ningbo Institute of Materials, Chinese Academy of Sciences, explored the evolution of physical quantities during stress relaxation of Fe-based metal glass and discovered the transformation mechanism of dynamic relaxation behavior during stress relaxation. The relevant paper is published on Scripta Materialia under the title "Continuous transition from gamma to beta dynamics during stress relaxation".
Paper link:
https://www.sciencedirect.com/science/article/pii/S1359646222006091
Figure 1 Thermodynamics parameters, relaxation behavior and deformation mechanism of metal glass.
Figure 2 a) Stress relaxation behavior of metal glass; extension index b) and relaxation time c) Evolution with relaxation time
Figure 3a-b) Strain evolution during metal glass recovery process; c) Evolution of normalized viscoelastic strain with relaxation time at different temperatures.
Figure 4 a) The evolution of relaxation time with temperature; b) The evolution of activation energy with relaxation time; c) Schematic diagram of the evolution of atom in the stress relaxation process.
In this study, the researchers characterized the relaxation kinetics and mechanical responses of Fe76Si9B10P5 metal glass through stress relaxation and subsequent strain recovery tests. The modified KWW equation was used to obtain instantaneous dynamic parameters. Revealing the evolutionary laws of relaxation, kinetic inhomogeneity and strain recovery over time. Locally isolated motion through g relaxation showed more kinetic uniformity, resulting in reversible non- elastic deformation of . The local rearrangement of the irreversible portions corresponding to slow β relaxation reduces uniformity and triggers viscoplastic flow. This research results provide a new idea for clarifying the deformation behavior of metal glass. (Text: Keep real)
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