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.