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The Emergence of Bulk Metallic Glasses
Bulk Metallic Glasses (BMGs) are a class of advanced materials that have garnered significant attention due to their amorphous atomic-scale structure. Unlike traditional crystalline metals, BMGs lack a long-range ordered pattern, which endows them with a combination of physical and mechanical properties that are highly desirable in various industrial applications. These properties include high strength, fracture toughness, an impressive elastic limit, and excellent wear and corrosion resistance (Huang et al., 2016; Trachenko, 2008).

Superior Glass Forming Ability of Zr-Based Alloys
Zirconium (Zr)-based BMGs are particularly notable for their superior Glass Forming Ability (GFA), allowing them to be processed into large parts exceeding several centimeters in thickness using conventional melting and casting methods (Liu et al., 2002). The net-shape as-cast form of BMGs offers reduced processing costs and enables the production of custom tools for a wide range of industries.

The Critical Role of Cooling Rates
To successfully produce BMGs, cooling rates exceeding 10^3 K/s are required to prevent the formation of crystalline structures and maintain the amorphous microstructure during solidification (Huang et al., 2016; Petrescu et al., 2015). This rapid solidification is crucial in preserving the unique characteristics of BMGs.

Microscopic Insights into BMG Structures
The internal structure of BMGs is a key differentiator from conventional metals. While ordinary metals exhibit a periodic lattice, BMGs display a short-range organization typical of glass materials, including ceramics, polymers, and metals (Aversa et al., 2015; Petrescu et al., 2016).

Investigating Morphological Features
Ion and electron microscopy techniques have been employed to conduct morphological microscopic observations of BMGs. These analyses reveal the presence of surface defects and crystalline phases, providing insights into the manufacturing process and the resulting material properties.

Differential Scanning Calorimetry Analysis
Differential Scanning Calorimetry (DSC) is used to analyze the calorimetric properties of BMG alloys. For instance, a Zr44Ti11Cu10Ni10Be25 alloy exhibits a glass transition between 380-395°C and multiple crystallization peaks, indicating the presence of different crystalline phases (Aversa and Apicella, 2016; Lewandowski et al., 2005).

Surface Defects and Their Origins
Surface defects, such as grooves, can occur due to flow instabilities during the injection molding process. These defects are often filled with foreign materials, such as silicates, from the cutting process. The removal of these materials allows for a clearer understanding of the defect’s origin (Mirsayar et al., 2016).

The Role of Short-Range Order Clusters

Microscopic observations aim to examine the arrangement of atoms and the presence of short-range order clusters. These clusters play a significant role in the material’s properties and behavior during processing.

Compositional Analysis of BMG Alloys
Energy Dispersive Spectrometry (EDS) is utilized to analyze the chemical composition of BMGs. The analysis reveals variations in the distribution of elements such as Zr, Ti, Cu, and Ni, which are influenced by the thermal and rheological behavior of the melt during injection molding. The presence of crystalline inclusions and their distribution within the material can be attributed to factors such as oxygen impurity, microalloying elements, and manufacturing process parameters (Liu et al., 2002).

Conclusion

The study of BMGs involves a deep understanding of material properties and the influence of manufacturing processes. The presence of crystalline phases and their distribution within BMGs can significantly affect the material’s performance. Advanced microscopic techniques provide valuable insights into the internal structure and behavior of these materials, paving the way for further innovations in the field.

Acknowledgements
The authors express their gratitude to Liquid Metals Technologies Inc, California, USA, for providing the samples used in this research.

References
For a comprehensive list of references and further reading on the topic of Bulk Metallic Glasses and their properties, please refer to the original article and the cited works within.

The Unique Properties of Bulk Metallic Glasses
BMGs are distinguished by their disordered atomic-scale structure, which lacks the long-range order found in crystalline materials. This unique microstructure imparts a combination of physical, chemical, and mechanical properties that are not typically observed in conventional metals. For instance, BMGs possess high mechanical strength, fracture toughness, and an impressive elastic limit. They also demonstrate excellent deformability and ductility, a low coefficient of thermal expansion, and outstanding resistance to corrosion and wear [1].

Zr-Based BMGs: A Technological Marvel
Among the various BMG systems, Zr-based alloys have garnered significant attention due to their broad super-cooled liquid region and high glass-forming ability (GFA), which allow for the production of larger parts through conventional melting and casting techniques [2]. These alloys often include a combination of elements such as Cu, Al, Ti, Ni, and occasionally Be, which contribute to their superior GFA and desirable properties [3].

Processing Challenges and Solutions
The absence of crystalline structure in BMGs presents both opportunities and challenges in processing. The key issue lies in the rheology of the melt during cooling procedures. To maintain the amorphous structure, it is crucial to cool the material at a rate that prevents crystallization. This requires a deep understanding of the intermolecular and interatomic interactions occurring in the liquid phase [4].

Rheological Models and Molecular Dynamics
Various models have been developed to predict the viscosity of liquids, with modifications to better suit metallic melts. These models are based on atomic/molecular dynamics rather than hydrodynamics, focusing on interatomic force laws to derive the effects of inertial and viscous forces governing melt flow [5].

Flow Instabilities and Microfluidic Turbulence
Flow instabilities can occur at different scales in fluids in motion, leading to patterns such as Kelvin-Helmholtz and Kármán vortex streets. In BMGs, these instabilities can be quantitatively evaluated using molecular dynamics, which considers the interactions between alloy atoms [6].

Observations from Microscopy
Microscopic observations of injection-molded BMG plates reveal surface defects characteristic of flow instabilities, similar to those found in polymeric parts. These defects can be attributed to high temperature gradients and viscosity changes during the molding process, which can lead to the formation of micro-grooves and ripples [7].

The Role of Computational Tools
Advancements in computing power have enabled more complex simulations without necessarily reducing the system size. This has led to increased fidelity in simulations, allowing for detailed modeling of human cells and other biological structures [8].

Conclusion
The processability of BMGs involves managing the rheology of the melt to prevent crystallization and maintain the amorphous structure. Understanding the interplay between temperature gradients, shear flow stresses, and the resulting segregation of micro-fluidic phases is crucial for optimizing the manufacturing process and minimizing defects.

Acknowledgements
The authors express gratitude to Liquid Metals Technologies Inc, California, USA, for providing the samples for characterization and to Dr. Francesco Tatti (FEI Company Application Specialist SEM-SDB) for his contribution to the SEM analyses.

Author Contributions
All authors contributed equally to the experimental work and the preparation of the paper.

Ethics Statement
The authors declare that there are no ethical issues that may arise after the publication of this manuscript.