Study of local atomic structure of disordered materials using in-situ synchrotron radiation

J. Bednarčík^1,2

¹Institute of Physics, Faculty of Science, P.J. Šafárik University in Košice, Park Angelinum 9, 04154 Košice, Slovakia

²Institute of Experimental Physics, Slovak Academy of Science, Watsonova 45, 04001 Košice, Slovakia

jozef.bednarcik@upjs.sk

The atomistic mechanisms behind the aging and rejuvenation of bulk metallic glasses (BMGs) are still unclear. Early studies on glassy polymers in the 1950s [1] and debates in the late 1990s [2,3] highlighted the challenges of aging, which increases brittleness and limits BMG applications. Various strategies, including deformation, high-pressure torsion, ion irradiation, flash annealing, and cryogenic cooling, have been explored to control aging and rejuvenation, improving mechanical properties. However, the fundamental atomistic origins of these effects remain unresolved.

Recent advances in computer simulations have shed light on the structural and thermodynamic origins of aging and rejuvenation in metallic glasses [4,5]. However, the kinetic aspects and experimental validation of these findings, especially those linking dynamic relaxation modes to atomic-scale reorganizations, remain limited. Different studies have explored correlations between stress-driven processes like shear transformation zones and dynamic relaxations, specifically α- and β-relaxation modes. A newly identified γ- or β’-relaxation mode, active at low temperatures, may be linked to stress inhomogeneities at cryogenic temperatures, but its structural origin is still unclear due to experimental challenges.

This contribution provides a deeper understanding of the relationship between structural reorganization and dynamic relaxations in glassy materials, particularly bulk metallic glasses (BMGs). Understanding and experimentally validating the atomistic mechanisms during aging and rejuvenation are essential for improving and explaining the limited ductility of BMGs. However, many structural characterization methods struggle to detect the subtle changes associated with these processes. In this study, we use in situ synchrotron X-ray diffraction to observe structural rearrangements during annealing, from 77 K to the crystallization temperature of Cu₄₄Zr₄₄Al₈Hf₂Co₂ BMGs. We introduce a method to visualize subtle changes in topological ordering by using a configurational entropy equivalent of the experimentally determined X-ray pair distribution function (PDF). The samples were rejuvenated through high-pressure torsion (HPT) at cryogenic and room temperatures prior to annealing. Structural changes, as indicated by the X-ray-derived equivalent configurational entropy, are correlated with dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) to assess dynamic relaxations and crystallization. DMA measurements offer a detailed view of the relaxation processes, distinguishing between the well-known β- and α-relaxation modes and identifying the presence of the faster γ-relaxation mechanism in the glassy material.

1. Kovacs, A. J. Transition vitreuse dans les polymères amorphes. Etude phénoménologique. In Fortschritte der Hochpolymeren-Forschung. (eds Ferry, J. D. et al.) 394–507 (Springer Berlin Heidelberg, 1964).

2. Struik, L. On the rejuvenation of physically aged polymers by mechanical deformation. Polymer 38, 1997, pp. 4053–4057.

3. McKenna, G. B. Mechanical rejuvenation in polymer glasses: fact or fallacy? J. Phys.: Condens. Matter 15, 2003, pp. S737–S763.

4. Ding, J. et al. Universal nature of the saddle states of structural excitations in metallic glasses. Mater. Today Phys. 17, 2021, 100359.

5. Han, D. et al. Atomistic structural mechanism for the glass transition: entropic contribution. Phys. Rev. B 101, 2020, 014113.

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