Metallic Glass Formation Research Articles

Influence of oscillatory shear on nucleation in metallic glasses: A molecular dynamics study

The process of crystal nucleation can be accelerated or retarded by ultrasonic vibration, which is particularly attractive for the addictive manufacture and thermoplastic forming of metallic glasses, however, the effect and mechanism of oscillatory loading on the nucleation process are still elusive. Here, by using molecular dynamics simulation, the changes in the time–temperature-transformation (TTT) curve under oscillatory external loading are systematically investigated in two typical binary alloys. A glass forming ability dependent response to the external loading is found, and the shortest incubation time is insensitive to the external loading, while the corresponding temperature can be significantly shifted. Within the framework of classical nucleation theory, a fitting formula is proposed to describe the simulation data quantitatively. In contrast to stationary shear, the elastic stress, rather than the strain rate, is the key parameter to control the evolution of TTT curve under oscillatory loading. Furthermore, the model shows that oscillatory loading can decouple the mobility and nucleation in the deeply supercooled liquid, hence the deformation ability can be enhanced while the nucleation is suppressed, which is particularly helpful for the forming and manufacturing of metallic glasses.

Understanding the metallic glasses formation by applying a centrality measure based on betweenness

Key nodes have a significant impact, both structural and functional, on complex networks. Evaluating the importance of a node, from an information flow perspective, has applications in many areas but, to a great extent, they are focused on the shortest path perspective. Then, there exist measures based on the representation of the frequency in which each node plays the role of a transition node by random trajectories between two other nodes. Development of a specific kind of material, metallic glasses, has been mainly based on trial and error methods and, this approach is inefficient in terms of time and cost. Therefore, the development of new approaches for metallic glasses design remains an open topic for researchers in this field. The understanding of the role of each element could be useful for the generation of new metallic glasses and the improvement of existing ones. This work aims to study the formation capacity of metallic glass from the point of view of complex networks, using random walk betweenness centrality approaches, as well as the proposal of a new hypothesis that can serve as a roadmap for future research in this field of generation of new metallic glass. The results and discussions indicate that the proposed approach may be of great interest, for the development of metallic glasses, given the great correlation between the results of this study and the actual obtaining of metallic glasses in recent research.

Temperature-dependent effect of cooling rate on the melt-quenching process of metallic glasses

The cooling rate during the melt-quenching process is a crucial factor in the formation of metallic glasses. This study reveals the temperature- and thermal-history-dependent effects of cooling rate on the relaxation state of metallic glasses prepared by the rapid quenching of a liquid alloy. Using molecular dynamics techniques, we conducted melt-quenching simulations in which the cooling rates were varied during the quenching process. Maps were constructed to demonstrate the effect of the cooling rate on the quenching process as a function of temperature. The critical temperature range varied depending on the thermal history of the cooling process. Analyses of the local atomic order and atomic dynamics revealed the atomistic origin of the temperature- and thermal-history-dependent effects of the cooling rate. This study also discusses the physical background of the upper and lower ends of the critical temperature range based on the potential energy landscape and atomic dynamics.

Machine learning versus human learning in predicting glass-forming ability of metallic glasses

Complex materials science problems such as glass formation must consider large system sizes that are many orders of magnitude too large to be solved by first-principles calculations. The successful application of machine learning (ML) in various other fields suggests that ML could be useful to address complex problems in materials science. To test its efficacy, we attempt to predict bulk metallic glass formation using ML. Surprisingly, we find that a recently developed ML model based on 201 alloy features constructed using simple combinations of 31 elemental features is indistinguishable from models that are based on unphysical features. The 201ML-model performs better than the unphysical model only when significant separation of training and testing data is achieved. However, it performs significantly worse than a human-learning based three-feature model. The limited performance of the 201ML-model originates from the inability to accurately represent alloy features through elemental features, showing that physical insights about mixing behavior are required to develop predictable ML models.

Mechanical, electrical properties and microstructures of combinatorial Ni-Mo-W alloy films

Nickel-molybdenum-tungsten (Ni-Mo-W) alloys have been suggested for applications of advanced micro-electro-mechanical systems (MEMS) in extreme environments due to their high strength, good electrical conductivity, and excellent thermal stability. In this work, we have carried out combinatorial experiments to reveal composition-dependent phase formation, physical properties, and thermal stability of Ni-Mo-W alloy films. Combinatorial synthesis through a magnetron sputtering process fabricated thin films with a composition range of Ni 56–92 Mo 6–38 W 0.5–7.5 . Structural analysis for the as-deposited combinatorial film revealed that a nanotwinned and strongly textured nano-columnar fcc structure was formed in the Ni-rich compositions, while solute content greater than 30% resulted in the formation of metallic glasses. The specimen exhibited very high hardness (8.8–12.5 GPa) and moderate electrical resistivity (85–135 μΩ ∙ cm), and both the properties were found to increase with solute concentration. Annealing of the combinatorial film at 600 ℃ for 1 h revealed that nanotwinned structure is thermally stable for high Ni concentrations. Metallic glasses with high solute concentration also exhibited high thermal stability and were not crystallized after annealing. Alloys with intermediate solute content experienced the transformation from the amorphous to fcc nanocrystalline phase with nanotwins. The annealed combinatorial film maintained high hardness and moderate electrical resistivity after annealing. • Composition-dependent phase formation and physical properties of Ni 56–92 Mo 6–38 W 0.5–7.5 alloy films were investigated. • A nanotwinned (NT) and nano-columnar fcc structure was formed in the Ni-rich compositions. • The solute content greater than 30% resulted in the formation of metallic glasses. • The specimen exhibited high hardness (8.8–12.5 GPa) and moderate electrical resistivity (85–135 μΩ · cm). • The NT structure with high Ni content and metallic glasses with high solute concentration were thermally stable.

Atomistic insights into the effect of cooling rates on the structural and mechanical properties of Vanadium monatomic metallic glass

• Splitting in the second RDF peak is obtained for a cooling rate of 10 14 K/s. • Total energy decreases continuously for cooling rates of 5.10 13 K/s and 10 14 K/s. • Young...s modulus value of the V MG is large compared to the V in a liquid phase. Recently, metallic glasses have received much interest owing to their valuable and unique properties such as high corrosion resistance, high elasticity, fracture resistance, etc. In the present study, Molecular Dynamics (MD) simulation, based on semi-empirical potential described by the Embedded-Atom Method (EAM), was used to investigate the structural and mechanical properties of V-monoatomic Metallic glass. In order to analyze the behavior of this material, the radial distribution function (RDF) method and common neighbor analysis (CNA) were applied. The obtained results indicate a splitting in the second RDF peak with cooling rates of 10 14 K/s and 5.10 13 K/s. Furthermore, it was found that the total system energy decreases continuously when the high cooling rates of 5.10 13 K/s and 10 14 K/s are used, supporting the formation of Vanadium metallic glass (V MG). This latter one demonstrates a higher Young's modulus compared to the V-monatomic in the liquid phase.

The effect of impurities in zirconium on the formation and mechanical properties of Zr55Cu30Al10Ni5 metallic glass

Minor alloying has been developed as an effective method to modify and optimize various properties of bulk metallic glasses (BMG). In this work, as inevitable minor alloying elements, two major impurities, hafnium (Hf) and oxygen (O) were quantitatively confirmed to co-exist in two zirconium (Zr) raw materials with different grades of impurities. The effects of these two impurities on the thermal properties, crystallization behavior, glass-forming ability (GFA), and mechanical properties of the nominal Zr55Cu30Al10Ni5 BMG were systematically studied. The O impurity was found to promote the formation of the Al5Ni3Zr2 intermetallic compound phase, which could account for the poor GFA, thermal stability, and plasticity associated with BMGs with higher O content in the Zr raw material. In contrast, although the impurity of Hf has a much higher content than O, a negligible effect of Hf on the Zr55Cu30Al10Ni5 BMG was revealed. These findings were further confirmed by deliberately adding extra O and Hf with designed contents into the Zr55Cu30Al10Ni5 BMG. Our results clarify the effects of different impurities in Zr raw materials on the Zr-based BMG and will help guide industrial production and applications of Zr-based BMGs.

Non-Arrhenius behaviour of nickel self-diffusion in liquid Ni77Si23

Nickel self-diffusion was measured for a Ni77Si23 alloy in the liquid state over a temperature range of about 400 K through quasielastic neutron scattering. At the two lowest temperature points the derived diffusion coefficients deviate from a high-temperature Arrhenius-type behaviour and indicate a change in dynamics above the liquidus temperature. A fit with a power-law temperature dependence as predicted by the mode coupling theory for the liquid to glass transition can describe the diffusion coefficients quite well over the whole measured temperature range. The obtained results agree with predictions from a classical molecular dynamics (MD)-simulation, which evidenced an increasing glass forming ability with increasing silicon content. A crossover to a super-Arrhenius behaviour was reported for metallic glass formers above the liquidus temperature and the here investigated NiSi alloy demonstrates the same signature.

Compositional dependence of the fragility in metallic glass forming liquids

The viscosity and its temperature dependence, the fragility, are key properties of a liquid. A low fragility is believed to promote the formation of metallic glasses. Yet, the fragility remains poorly understood, since experimental data of its compositional dependence are scarce. Here, we introduce the film inflation method (FIM), which measures the fragility of metallic glass forming liquids across wide ranges of composition and glass-forming ability. We determine the fragility for 170 alloys ranging over 25 at.% in Mg–Cu–Y. Within this alloy system, large fragility variations are observed. Contrary to the general understanding, a low fragility does not correlate with high glass-forming ability here. We introduce crystallization complexity as an additional contribution, which can potentially become significant when modeling glass forming ability over many orders of magnitude.

Single-element amorphous palladium nanoparticles formed via phase separation

Physically vitrifying single-element metallic glass requires ultrahigh cooling rates, which are still unachievable for most of the closest-packed metals. Here, we report a facile synthetic strategy for creating mono-atomic palladium metallic glass nanoparticles with a purity of 99.35 +/- 0.23 at% from palladium-silicon liquid droplets using a cooling rate below 1000 K/s. In-situ environmental transmission electron microscopy directly detected the leaching of silicon. Further hydrogen absorption experiment showed that this palladium metallic glass expanded little upon hydrogen uptake, exhibiting a great potential application for hydrogen separation. Our results provide insight into the formation of mono-atomic metallic glass at nanoscale.

Probing the formation of ultrastable metallic glass from structural heterogeneity

• The structure underlying the unexpected stability of Zr-based TFMGs was tracked. • MGs with greater heterogeneity have a higher potential to form SMGs. • Slower deposition rates are required for ultrastability in more heterogeneous MGs. • The relaxation mechanism changes as the deposition rate decreases. • The geometry and distribution of loosely packed phases determines ultrastability. Ultrastable metallic glasses (SMGs) exhibit enhanced stability comparable to those of conventional glasses aged for thousands of years. The ability to understand why certain alloy compositions and processing conditions generate an SMG is an emerging challenge. Herein, amplitude-modulation dynamic atomic force microscopy was utilized for tracking the structure of Zr 50 Cu 50 , Zr 50 Cu 44.5 Al 5.5 and Zr 50 Cu 41.5 Al 5.5 Mo 3 thin film metallic glasses (TFMGs) that were produced by direct current magnetron sputtering at room temperature with the rate of deposition being the only variable. The transition in stability from bulk- to SMG-like behavior resides in the change of relaxation mechanism as the deposition rate is decreased. The formation of SMGs is directly linked with the degree of structural heterogeneity, whereby MGs with greater heterogeneity have a higher potential to form SMGs with more significant enhancement in stability. Slower deposition rates, however, are required to yield the more homogenous structure and lower energy state underlying the ultrastability. Ultrastability is closely linked with the geometric shape and distribution of loosely packed phases, whereby SMGs containing more slender loosely packed phases with a more skewed distribution achieve more significant improvements in stability. This work not only provides direct evidence of the structure of SMGs, but also opens new horizons for the design of SMGs. .

Microstructural evolution of shear bands formation of metallic glasses under different loading conditions and strain rates

The shear bands (SBs) evolution of Cu50Zr50 metallic glasses (MGs) by the uniaxial loading under different strain rates are investigated on the temporal and spatial scales using the molecular dynamics (MD) simulation. The stress-strain curve shows a corresponding relation to the microstructure parameter curve in the form of four-stage, together with a synchronized evolution of multiple shear bands. During the propagation of the dominate shear band (DSB), the rearrangement of global microstructure is induced by the different transformation of three categories of clusters located in different zones. The synchronized evolutions of the microstructure, the deformation and the mechanical property are found to be less affected by loading condition or strain rate. However, with the increase of strain rate, the initiation and propagation of the DSB are observably retarded. As a consequence, the fracture mode of the MGs presents a change from a sliding along the DSB to a uniform deformation.

Ag Surface and Bulk Segregations in Sputtered ZrCuAlNi Metallic Glass Thin Films.

We report on the formation of Ag-containing ZrCuAlNi thin film metallic glass (nano)composites by a hybrid direct-current magnetron sputtering and high-power pulsed magnetron sputtering process. The effects of Ag content, substrate temperature and substrate bias potential on the phase formation and morphology of the nanocomposites were investigated. While applying a substrate bias potential did not strongly affect the morphological evolution of the films, the Ag content dictated the size and distribution of Ag surface segregations. The films deposited at low temperatures were characterized by strong surface segregations, formed by coalescence and Ostwald ripening, while the volume of the films remained featureless. At higher deposition temperature, elongated Ag segregations were observed in the bulk and a continuous Ag layer was formed at the surface as a result of thermally enhanced surface diffusion. While microstructural observations have allowed identifying both surface and bulk segregations, an indirect method for detecting the presence of Ag segregations is proposed, by measuring the electrical resistivity of the films.

Atomic transport in amorphous metals

Abstract A novel mechanism of atomic transport in amorphous metallic alloys, or metallic glasses, is proposed based upon the fluctuations in the local structure. The proposed mechanism is very different from those in crystalline solids and from the free volume model, and is characterized by the bond-exchange action triggered by the local topological instability of the atomic environment. The implications of this mechanism on the liquid fragility and bulk metallic glass formation are discussed.

Atomic-scale observation of strain-induced local amorphization in face-centered cubic metals

The formation of metallic glass through melt processing proves to be the most challenging for pure monatomic face-centered cubic (FCC) metals. Though it has long been conjectured that amorphous monatomic metals can be generated through deforming a solid at room temperature, there is rarely direct evidence to prove that is indeed the case. In this study, mechanical loading was applied to nanometer-sized crystals inside an aberration-corrected transmission electron microscopy, and atomic-scale in situ evidence is provided of strain-induced amorphization in Pt and Ni near room temperature. The loading was applied in such a way that the stress state is complicated, and the strain distribution is non-uniform, restricting dislocation activities in accommodating the imposed strain. The local lattice distortion is then rendered so large and the associated strain energy is so high that the crystal collapses into the amorphous state. As such, even elemental FCC metals can be forced to become amorphous.

Virtual Development of Process Parameters for Bulk Metallic Glass Formation in Laser-Based Powder Bed Fusion.

The development of process parameters and scanning strategies for bulk metallic glass formation during additive manufacturing is time-consuming and costly. It typically involves trials with varying settings and destructive testing to evaluate the final phase structure of the experimental samples. In this study, we present an alternative method by modelling to predict the influence of the process parameters on the crystalline phase evolution during laser-based powder bed fusion (PBF-LB). The methodology is demonstrated by performing simulations, varying the following parameters: laser power, hatch spacing and hatch length. The results are compared in terms of crystalline volume fraction, crystal number density and mean crystal radius after scanning five consecutive layers. The result from the simulation shows an identical trend for the predicted crystalline phase fraction compared to the experimental estimates. It is shown that a low laser power, large hatch spacing and long hatch lengths are beneficial for glass formation during PBF-LB. The absolute values show an offset though, over-predicted by the numerical model. The method can indicate favourable parameter settings and be a complementary tool in the development of scanning strategies and processing parameters for additive manufacturing of bulk metallic glass.

Virtual Prediction of Material Properties

To start working with materials, it is most important to determine the properties to ensure its suitability but sometimes it is not only difficult but also time consuming and costly affair to arrange an experimentation with the materials. To overcome this problem some free libraries of python like pymatgen, matminer etc. are used with Materials Application Programming Interface (API)to gather and process datasets and when combined with Machine learning libraries like Sklearn, a machine Learning Model can be built. In the present work, with the aid of glass_ternary_hipt dataset, a metallic glass formation dataset for Co-Fe-Zr, Co-Ti-Zr, Co-V-Zr and Fe-Ti-Nb ternary alloy systems, a Support Vector Machine Classifier, is built as an example to predict the glass forming ability of the alloys. The accuracy of the model is checked and a heatmap is generated to show the correlation between the features and the target. The development of these program is very intuitive with these packages and python for prediction of material properties virtually.

Pressure Effects in the Unusual-Caged Dynamics of La80al20 Metallic Glass-Forming Liquid

Pressure Effects in the Unusual-Caged Dynamics of La80al20 Metallic Glass-Forming Liquid

Simulation of phase evolution in a Zr-based glass forming alloy during multiple laser remelting

Additive manufacturing by laser-based powder bed fusion is a promising technique for bulk metallic glass production. But, reheating by deposition of subsequent layers may cause local crystallisation of the alloy. To investigate the crystalline phase evolution during laser scanning of a Zr-based metallic glass-forming alloy, a simulation strategy based on the finite element method and the classical nucleation theory has been developed and compared with experimental results from multiple laser remelting of a single-track. Multiple laser remelting of a single-track demonstrates the crystallisation behaviour by the influence of thermal history in the reheated material. Scanning electron microscopy and transmission electron microscopy reveals the crystalline phase evolution in the heat affected zone after each laser scan. A trend can be observed where repeated remelting results in an increased crystalline volume fraction with larger crystals in the heat affected zone, both in simulation and experiment. A gradient of cluster number density and mean radius can also be predicted by the model, with good correlation to the experiments. Prediction of crystallisation, as presented in this work, can be a useful tool to aid the development of process parameters during additive manufacturing for bulk metallic glass formation.

Data-driven discovery of a universal indicator for metallic glass forming ability.

Despite the importance of glass forming ability as a major alloy characteristic, it is poorly understood and its quantification has been experimentally laborious and computationally challenging. Here, we uncover that the glass forming ability of an alloy is represented in its amorphous structure far away from equilibrium, which can be exposed by conventional X-ray diffraction. Specifically, we fabricated roughly 5,700 alloys from 12 alloy systems and characterized the full-width at half-maximum, Δq, of the first diffraction peak in the X-ray diffraction pattern. A strong correlation between high glass forming ability and a large Δq was found. This correlation indicates that a large dispersion of structural units comprising the amorphous structure is the universal indicator for high metallic glass formation. When paired with combinatorial synthesis, the correlation enhances throughput by up to 100 times compared to today's state-of-the-art combinatorial methods and will facilitate the discovery of bulk metallic glasses.