Bonding characters of Al-containing bulk metallic glasses studied by 27Al NMR

Contrasting Bonding-Interaction-Induced Distinct Relaxation in La65Ni35 and La65Al35 Glass-Forming Alloys

The α and β relaxations are two fundamental processes in glass-forming materials, and quite important for many of the properties. Although intensive studies have found that α and β relaxations can be tuned by changing the constituent elements, the underlying structural basis is still elusive. Here, we explored the effect of two key elements of Al and Ni on distinct β and α relaxations in La₆₅Al₃₅ and La₆₅Ni₃₅ glass-forming alloys via classical and ab initio molecular dynamics simulations combined with dynamical mechanical spectroscopy. Unexpected coupling of relaxation in both β and α relaxation time scales is observed for La and Al atoms in the La₆₅Al₃₅ system, which drastically suppresses the relaxation dynamics. It is revealed that the dynamic coupling of La and Al results from the covalent-like bonding interaction between Al atoms, which connect Al together, forming a network-like structure. The bonding network not only drastically slows down the dynamics of Al but also couples the motion of La and Al together. This finding elucidates the underlying basis of Al and Ni elements for distinct β and α relaxation and sheds light on tuning the formation and properties of metallic glasses by minor alloying.

On the use of atomistic simulations to aid bulk metallic glasses structural elucidation with solid-state NMR

Solid-state nuclear magnetic resonance (ssNMR) experimental ²⁷Al metallic shifts reported in the literature for bulk metallic glasses (BMGs) were revisited in the light of state-of-the-art atomistic simulations. In a consistent way, the Gauge-Including Projector Augmented-Wave (GIPAW) method was applied in conjunction with classical molecular dynamics (CMD). A series of Zr-Cu-Al alloys with low Al concentrations were selected as case study systems, for which realistic CMD derived structural models were used for a short- and medium-range order mining. That initial procedure allowed the detection of trends describing changes on the microstructure of the material upon Al alloying, which in turn were used to guide GIPAW calculations with a set of abstract systems in the context of ssNMR. With essential precision and accuracy, the ab initio simulations also yielded valuable trends from the electronic structure point of view, which enabled an overview of the bonding nature of Al-centered clusters as well as its influence on the experimental ssNMR outcomes. The approach described in this work might promote the use of ssNMR spectroscopy in research on glassy metals. Moreover, the results presented demonstrate the possibility to expand the applications of this technique, with deeper insight into nuclear interactions and less speculative assignments.

NMR signature of evolution of ductile-to-brittle transition in bulk metallic glasses

The mechanical properties of monolithic metallic glasses depend on the structures at atomic or subnanometer scales, while a clear correlation between mechanical behavior and structures has not been well established in such amorphous materials. In this work, we find a clear correlation of (27)Al NMR isotropic shifts with a microalloying induced ductile-to-brittle transition at ambient temperature in bulk metallic glasses, which indicates that the (27)Al NMR isotropic shift can be regarded as a structural signature to characterize plasticity for this metallic glass system. The study provides a compelling approach for investigating and understanding the mechanical properties of metallic glasses from the point of view of electronic structure.