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Kinetics of Relaxation in Zr55Cu30Ni10Al5 Bulk Metallic Glass Rejuvenated by Elastostatic Loading

Emami, Soheil | 2025

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 58543 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Tavakoli, Roohollah; Gholamipour, Reza
  7. Abstract:
  8. This study investigates the structural and mechanical evolution of Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glass (BMG) subjected to elastostatic rejuvenation under 90% of its yield stress for 24 hours. The goal is to understand the time-dependent relaxation processes that follow mechanical rejuvenation and their influence on the internal structure and performance stability of the glass. A comprehensive suite of techniques—including Differential Scanning Calorimetry (DSC), Positron Annihilation Lifetime Spectroscopy (PALS), Atomic Force Microscopy (AFM), nanoindentation, and microhardness testing—was employed to capture both atomic-scale and continuum-level changes during structural aging over a 45-day period at ambient temperature. Immediately after rejuvenation (Day 4), the material enters a high-energy metastable state characterized by a significant drop in hardness, elevated creep displacement, surface heterogeneity, and increased strain rate sensitivity. However, PALS results reveal that the free volume in this state is not maximized, and DSC data show a surprisingly low enthalpy release—suggesting that although the structure is disordered, it is not yet in a kinetically favorable configuration for large-scale atomic rearrangement or STZ activation. Over time, particularly by Day 10, the material undergoes a pronounced relaxation phase marked by an increase in enthalpy release, growth in STZ volume, reduction in creep rate, and stabilization of S and W parameters in PALS. AFM imaging shows a concurrent decline in surface energy contrast, confirming homogenization of the local structure. The nanoindentation data were further used to compute the strain rate sensitivity (m), STZ volume (Ω), and number of atoms involved in each transformation event. Notably, although creep behavior is most pronounced on Day 4, the calculated STZ volume is initially low, reaching its peak only after sufficient structural relaxation. This reflects a delay between rejuvenation-induced disorder and the formation of fully activated shear transformation zones. By Day 30, the system reaches a semi-stable equilibrium with restored hardness, reduced plastic activity, and structural coherence. Overall, this work provides new insight into the transition from mechanical rejuvenation to structural relaxation in metallic glasses. It highlights the nontrivial relationship between internal disorder, STZ development, and macroscopic mechanical behavior. The findings have direct implications for designing thermomechanically stable glassy materials and managing the long-term reliability of BMG-based components in advanced applications
  9. Keywords:
  10. Bulk Metallic Glass ; Shear Transformation Zone ; Structural Relaxation ; Elastostatic Rejuvenation ; Rejuvenation Treatment

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