Thermodynamic predicting and atomistic modeling the favored compositions for Mg–Ni–Y metallic glasses

For Mg–Ni–Y system, glass formation is jointly studied by thermodynamic calculations and atomistic simulations. The prediction results have extensive implications for the Mg-based family and could be of great help for guiding the composition design.

Interfacial free energy controlling glass-forming ability of Cu-Zr alloys

Glass is a freezing phase of a deeply supercooled liquid. Despite its simple definition, the origin of glass forming ability (GFA) is still ambiguous, even for binary Cu-Zr alloys. Here, we directly study the stability of the supercooled Cu-Zr liquids where we find that Cu₆₄Zr₃₆ at a supercooled temperature shows deeper undercoolability and longer persistence than other neighbouring compositions with an equivalent driving Gibbs free energy. This observation implies that the GFA of the Cu-Zr alloys is significantly affected by crystal-liquid interfacial free energy. In particular, the crystal-liquid interfacial free energy of Cu₆₄Zr₃₆ in our measurement was higher than that of other neighbouring liquids and, coincidently a molecular dynamics simulation reveals a larger glass-glass interfacial energy value at this composition, which reflects more distinct configuration difference between liquid and crystal phase. The present results demonstrate that the higher crystal-liquid interfacial free energy is a prerequisite of good GFA of the Cu-Zr alloys.

Combinatorial development of bulk metallic glasses

The identification of multicomponent alloys out of a vast compositional space is a daunting task, especially for bulk metallic glasses composed of three or more elements. Despite an increasing theoretical understanding of glass formation, bulk metallic glasses are predominantly developed through a sequential and time-consuming trial-and-error approach. Even for binary systems, accurate quantum mechanical approaches are still many orders of magnitude away from being able to simulate the relatively slow kinetics of glass formation. Here, we present a high-throughput strategy where ∼3,000 alloy compositions are fabricated simultaneously and characterized for thermoplastic formability through parallel blow forming. Using this approach, we identified the composition with the highest thermoplastic formability in the glass-forming system Mg-Cu-Y. The method provides a versatile toolbox for unveiling complex correlations of material properties and glass formation, and should facilitate a drastic increase in the discovery rate of metallic glasses.

Favored composition design and atomic structure characterization for ternary Al-Cu-Y metallic glasses via proposed interatomic potential

A realistic interatomic potential is constructed for the Al-Cu-Y system under a newly proposed formulism and applied to perform atomistic simulations, leading to predicting a hexagonal composition region within which metallic glass formation is energetically favored and the region is defined as the quantitative glass formation ability of the system. Amorphization driving force of a glassy alloy is then calculated to correlate the readiness of its forming ability in practice, and a local optimized stoichiometry is pinpointed to be Al74Cu14Y12, of which the metallic glass could be most stable or easiest obtainable. The predictions are well supported by the experimental observations reported so far in the literature. Further structural analysis indicates that adding Y extends the short-range landscape and facilitates developing a hybridized icosahedral- and fcc-like packing in the medium-range, eventually enhancing the glass formation ability of the system.

Reactive cluster model of metallic glasses

Though discovered more than a half century ago metallic glasses remain a scientific enigma. Unlike crystalline metals, characterized by short, medium, and long-range order, in metallic glasses short and medium-range order persist, though long-range order is absent. This fact has prompted research to develop structural descriptions of metallic glasses. Among these are cluster-based models that attribute amorphous structure to the existence of clusters that are incommensurate with crystalline periodicity. Not addressed, however, are the chemical factors stabilizing these clusters and promoting their interconnections. We have found that glass formers are characterized by a rich cluster chemistry that above the glass transformation temperature promotes exchange as well as static and vibronic sharing of atoms between clusters. The vibronic mechanism induces correlated motions between neighboring clusters and we hypothesize that the distance over which these motions are correlated mediates metallic glass stability and influences critical cooling rates.

First-principles prediction and experimental verification of glass-forming ability in Zr-Cu binary metallic glasses

In the field of metallic materials with amorphous structures, it is vitally important to understand the glass formation and to predict glass-forming ability (GFA) in terms of constituent elements and alloy compositions. In this study, an expression has been formulated from first-principles calculations to predict the trend of GFA by hybridizing both internal energies and atomic-scale defect structures. The prediction of GFA from compositions has been verified successfully by available experimental data in the model Zr-Cu alloy system. The physical scenario revealed here has extensive implications for the design of bulk metallic glasses with superior GFA.

Super elastic strain limit in metallic glass films

On monolithic Ni-Nb metallic glass films, we experimentally revealed 6.6% elastic strain limit by in-situ transmission electron microscopy observations. The origin of high elastic strain limit may link with high free volume in the film, causing the rearrangement of loosely bonded atomic clusters (or atoms) upon elastic deformation. This high elastic limit of metallic glass films will shed light on new application fields for metallic glasses, and also trigger more studies for deformation mechanism of amorphous materials in general.

Volume expansion measurements in metallic liquids and their relation to fragility and glass forming ability: an energy landscape interpretation

Recent studies of Cu-Zr glasses have reported a rapid variation in the amorphous phase density near the optimal glass forming compositions, supporting the belief that the densest liquids are also the best glass formers. Here, we show that the measured densities of the Cu-Zr liquids at higher temperatures are not peaked sharply near these compositions, but the volume expansivities are. Theoretical studies have shown that the expansivity correlates with fragility near T(g); the experimental results presented here show that at high temperature they become anticorrelated. From energy landscape arguments, this indicates the existence of a crossover temperature for the expansivity-fragility correlation that scales inversely with the liquid fragility. These results lead to an improved understanding of the high temperature properties of liquids that form glasses and suggest a new method for identifying the best glass forming compositions within an alloy system from the properties of the equilibrium liquids.

Atomic-scale mechanisms of the glass-forming ability in metallic glasses

The issue, composition dependence of glass-forming ability (GFA) in metallic glasses (MG), has been investigated by systematic experimental measurements coupled with theoretical calculations in Cu-Zr and Ni-Nb alloy systems. It is found that the atomic-level packing efficiency strongly relates to their GFA. The best GFA is located at the largest difference in the packing efficiency of the solute-centered clusters between the glassy and crystal alloys in both MG systems. This work provides an understanding of GFA from atomic level and will shed light on the development of new MGs with larger critical sizes.

Predicting the solid state phase diagram for glass-forming alloys of copper and zirconium

The free energies of six crystal structures associated with Cu-Zr alloys-Cu (face centred cubic), Cu(2)Zr, Cu(10)Zr(7), CuZr, CuZr(2) and Zr (hexagonal close packed)-are calculated using the embedded atom potential of Mendelev et al (2009 Phil. Mag. 89 967). We find that the observed low temperature stability of the Cu(10)Zr(7) and CuZr(2) phases is not reproduced. Instead, the model predicts that the CuZr phase remains stable down to T = 0 K. This discrepancy is largely removed when the interaction potentials are cut off at a short distance, such as that used by Duan et al (2005 Phys. Rev. B 71 224208). We present evidence, however, that the cut-off distance necessary to achieve the change in phase stability results in pathological artefacts in the energetics of some crystal phases.

Processing of bulk metallic glass

Bulk metallic glass (BMG) formers are multicomponent alloys that vitrify with remarkable ease during solidification. Technological interest in these materials has been generated by their unique properties, which often surpass those of conventional structural materials. The metastable nature of BMGs, however, has imposed a barrier to broad commercial adoption, particularly where the processing requirements of these alloys conflict with conventional metal processing methods. Research on the crystallization of BMG formers has uncovered novel thermoplastic forming (TPF)-based processing opportunities. Unique among metal processing methods, TPF utilizes the dramatic softening exhibited by a BMG as it approaches its glass-transition temperature and decouples the rapid cooling required to form a glass from the forming step. This article reviews crystallization processes in BMG former and summarizes and compares TPF-based processing methods. Finally, an assessment of scientific and technological advancements required for broader commercial utilization of BMGs will be made.