Nucleation kinetics model for primary crystallization in Al-Y-Fe metallic glass

The formation of aluminum nanocrystals during primary crystallization in Al-based metallic glasses remains poorly understood. Although high nucleation rates have been observed, existing models fail to fully capture the underlying kinetics. Structural heterogeneities, such as medium range order (MRO), have been identified in these materials but their role in nucleation remains unclear. Prior research has shown the presence of Al-like MRO regions in Al88Y7Fe5 metallic glass, but no comprehensive model has yet linked these features to the observed crystallization behavior. This gap motivated the development of a nucleation model that integrates structural and kinetic data. The absence of a complete kinetic framework has limited the ability to predict nanocrystal evolution during isothermal treatments. Researchers have proposed various mechanisms for nanocrystal formation, but none have been validated across all relevant parameters. The need for a self-consistent model that accounts for structural, thermodynamic, and kinetic factors remains unmet in the field.

This study aimed to develop a nucleation kinetics model that explains the formation of aluminum nanocrystals during primary crystallization in Al-based metallic glasses. The researchers sought to address the lack of a comprehensive model by incorporating structural data from fluctuation electron microscopy. Their goal was to establish a mechanism that accounts for the observed high nucleation rates and nanocrystal density. The study focused on the role of medium range order (MRO) in reducing the nucleation barrier. By analyzing the delay time (τ) and nanocrystal density (Nv), the team aimed to validate the model under isothermal conditions. The researchers also aimed to ensure the model satisfied constraints on critical nucleus size and interface energy. Their approach combined structural characterization with kinetic measurements to achieve a self-consistent framework. The ultimate objective was to create a model applicable to other materials with spatial heterogeneities.

The researchers used fluctuation electron microscopy to identify structural heterogeneities in Al88Y7Fe5 metallic glass. They analyzed the presence of Al-like medium range order (MRO) regions as potential nucleation sites. The team measured the delay time (τ) to determine the kinetics of nanocrystal formation. They calculated the steady-state nucleation rate (Jss) based on the observed nanocrystal density (Nv). The model incorporated structural parameters such as critical nucleus size and interface energy. Thermodynamic factors, including the volume-free energy driving force, were also considered. The researchers validated the model by comparing predicted Jss values with experimental results. They ensured the model satisfied all necessary structural, thermodynamic, and kinetic constraints.

The model predicted steady-state nucleation rates (Jss) that matched experimental results closely. The calculated nanocrystal density (Nv) aligned with observations from isothermal annealing treatments. The delay time (τ) measurements supported the proposed MRO seeded nucleation mechanism. The model accounted for the high nucleation rate (∼1018 m-3 s-1) observed in the material. The critical nucleus size and interface energy values were consistent with structural data. The volume-free energy driving force was found to be a key factor in the model. The researchers demonstrated that MRO regions act as nucleation sites, reducing the nucleation barrier. The model successfully described the evolution of nanocrystal density during primary crystallization.

The nucleation kinetics model accurately describes the formation of Al nanocrystals during primary crystallization. The model incorporates structural, thermodynamic, and kinetic parameters to achieve self-consistency. The presence of MRO regions as nucleation sites was confirmed by the researchers. The model satisfies constraints on critical nucleus size and interface energy. The calculated steady-state nucleation rates (Jss) matched experimental results. The delay time (τ) measurements supported the model's predictions. The researchers concluded that the model can be applied to other materials with spatial heterogeneities. The study provides a framework for understanding nanocrystal evolution in Al-based metallic glasses.