Symposium F.SF02—Bulk Metallic Glasses

We have undertaken to develop multiscale models of plastic flow and failure processes in amorphous solids, materials that exhibit a complete lack of crystalline order. In this we have taken metallic glass as an exemplar material and applied molecular dynamics simulation to examine the physics at the atomic scale. This has allowed us to uncover the statistics of the shear transformation zone (STZ) defects that control plastic deformation. We have then sought to relate this defect structure to statistical mechanics models of “effective temperature” that characterize the degree of glass disorder in an effort to build a constitutive model of plastic deformation. The constitutive model has been incorporated into a high-fidelity viscoplastic finite differencing scheme that adapts techniques originally developed for solving the Navier-Stokes equation. A novel machine learning algorithm then utilizes the atomistic data to guide the parameterization of the constitutive model so as to inform the continuum method. In doing so we have begun to ask some fundamental questions about the concept of “effective temperature.”
The concept of fictive-temperature has long been utilized to characterize the processing dependence of glass structure, and has recently been shown to be predictive of metallic glass ductility. Some theories have hypothesized that it is actually a real temperature related to the configurational degrees of freedom of the glass, i.e. an “effective-temperature,” notably the shear-transformation-zone (STZ) and soft-glassy-rheology (SGR) theories. We derive a thermodynamic integration scheme for calculating effective-temperature based on a 2-temperature hypothesis. To test this scheme we simulate a binary Cu-Zr metallic glass modeled with an EAM potential. Measures of the energy fluctuations associated with both the fast and slow degrees of freedom are measured during the glass quench. The resulting effective-temperature is consistent with estimates of fictive-temperature obtained from simulation in more heuristic ways. The results indicate that effective-temperature can be understood as a purely structural quantity. The method provides a means to measure the effective-temperature in the absence of fluctuations induced by shear and without resorting computationally expensive and impractical methods for explicitly measuring the configurational entropy.

In crystals atomic mobility in transport and deformation is governed by structural defects, such as vacancies and dislocations. This was carried over to the study of glasses and liquids, and various defect-like concepts have been proposed, including free-volume, cooperatively rearranging region and shear-transformation-zone (STZ). However, whereas in crystals defects are topologically protected by the lattice and preserve their identity after motion, the same does not apply for liquid and glass. For this reason, today these defects in glass are considered to be transient; they do not pre-exist and disappear after the action. In this talk this view is advanced even further to suggest that the dynamics in liquid and glass is governed not by defects but by the structural medium-range order (MRO) of the bulk. The MRO is defined by the coherence length which characterizes the decay of the oscillation in the pair-distribution function (PDF) beyond the first peak. Because the higher-order peaks of the PDF are broad, ~ 1 Å in width, the MRO does not describe the order at the atomic level. It represents the strength of the coarse-grained density wave, represented by the first peak in the structure function, S(Q). We show that the coherence volume is proportional to the fragility coefficient, m, at the glass transition temperature, Tg, and also to the activation energy of viscosity above Tg. In crystal atoms are mobile only at defects of which density is low. But in liquid all atoms are mobile and participate in transport. Whereas the defect-like sites have a higher propensity to start deformation or atomic transport, as soon as deformation starts extensive atomic rearrangement ensues, and the structure of the final state is very different from the initial one. Furthermore, at the saddle point of transition the effective temperature becomes so high that the system locally melts for a short time, and the system locally loses its thermal history. Therefore, the mobility is not determined by the initial structure but by the resistance to local collective motion which is determined by the MRO. For this reason, the structural coherence, which is a bulk property in equilibrium, controls the dynamics in liquid and glass. Defects are relevant only as outliers in the equilibrium statistical distribution where action could start.

Depending on their thermal history, bulk metallic glasses can fail in a brittle or ductile fashion. However, the nature of the link between structure and propensity for ductility has remained elusive. Here, based on molecular dynamics simulations, we investigate how the degree of structural disorder affects the fracture of bulk metallic glasses. As expected, we observe that structural disorder results in an increase in ductility. By applying the activation-relaxation technique (an open-ended saddle point search algorithm), we demonstrate that the propensity for ductility is controlled by the topography of the energy landscape. Interestingly, we observe a power-law relationship between the particle non-affine displacement upon fracture and the average local energy barrier. This reveals that the dynamics of the atoms upon fracture is encoded in the static energy landscape, i.e., before any load is applied. Besides bulk metallic glasses, this relationship is shown to apply to several classes of non-crystalline materials (oxide glasses, amorphous solids, and colloidal gels), which suggests that it may be a generic feature of disordered materials.

It is now possible to routinely perform atomistic simulations at the microsecond timescale. In the present work, we exploit this for a model binary Lennard-Jones glass to study structural relaxation at a timescale spanning several tens of microseconds, and the collective plastic shear activity under simple shear spanning six orders of magnitude strain rate. It is found that at these longer time-scales, significant mobility and relaxation occurs. Under load, such structural activity during collective and system-spanning plasticity plays an important role, producing a rich spectrum of atomic-scale activity in terms of spatial and temporal correlation, both in the elastic and plastic regimes of deformation.

Understanding the fracture toughness of glasses is a fundamental problem of prime theoretical and practical importance. We develop a theory of the notch fracture toughness of glasses using a non-equilibrium thermodynamic approach to the elastoplastic deformation of glassy materials. We show that the notch fracture toughness results from a competition between the initial plastic relaxation timescale, which depends on initial non-equilibrium structural state of the glass (e.g. its age and its preparation protocol), and an effective loading timescale, which depends on the notch radius, the system's geometry and the external loading rate. The initial glass structure, which controls the initial plastic relaxation timescale, is characterized by a non-equilibrium (effective/fictive) temperature. The theory predicts a rather abrupt increase in the toughness – a brittle-to-ductile transition – as a function of the ratio between these two timescales. Novel experiments on various Bulk Metallic Glasses (BMGs), which allow to carefully controlling the initial structural state of the glass and accurately measure the toughness, verify the theoretical predictions. In particular, they show that BMGs can be significantly toughened by carefully controlling their initial non-equilibrium state. These results highlight the importance of timescales competition and far from steady-state plastic deformation dynamics for understanding the toughness of glasses, and open up the way for a broader usage of BMGs as tough, highly-reproducible structural materials.

Over time, a variety of environmental descriptors have been proposed to indentify and characterize the atomic scale heterogeneities and to guide us towards an inherent structure-property relationship for metallic glasses. By using the STZ-vortex coupling the atomic-level mechanisms underlying elastic and plastic deformation in metallic glasses are disclosed. Removing the thermal noise by simulating athermal quasistatic shearing processes even small elastic/structural fluctuations can be identified and analysed. Given to complex morphology and a long range signature of STZs the activation of low number of events could strongly perturb the initial configuration and explain the observed weak correlations between structure and dynamics. The rotation field calculation reveals that elastic and plastic STZs have similar signature but different dynamics. Additionally, the observed deformation mechanisms have significant implications for understanding relaxation phenomena in amorphous metals and allow distinguishing between the different relaxation mechanisms. These results highlight that the classical definition of STZ should be reconsidered and a full understanding of this elementary event must be assured as they play a decisive role in controlling plasticity in metallic glasses

Plastic deformation of glasses is governed by local shear transformation zones (STZs). We need to know their concentration, their size, their transformation rate, how they nucleate, whether they are reversible or not, and if there are correlations between them. STZs can be studied by deforming colloidal glasses and tracking the particles in space and time. As modeling systems, colloidal glasses have the advantage of being dynamic, three-dimensional and large (>50,000 particles). The data in this study were obtained during cyclic deformation in pure shear up at 10^-5 s^-1 between 0 and 10%. To study how the dynamics and properties of the STZs are related to the evolution of the glassy structure, we have employed three machine learning techniques: Support Vector Machines (SVM), Convolutional Neural Networks (CNN) and Graph Neural Networks (GNN). We will discuss how these methods can be applied to experimental data with physical boundary conditions. We use them to predict the evolution of local strain, which particles are most likely to move, and which particles undergo irreversible rearrangements. The results of the three methods for each of these tasks will be compared.

Recent research^1-3 has proposed that structural order present in the liquid is related to glass-forming ability in metallic alloys suggesting that high order in the metallic liquid makes crystallization easier due to the lack of atomic motion needed for this process to occur in comparison to a liquid with little order in the liquid. However, the effective quantification of this liquid structure is extremely challenging due to the inherent long-range disorder present. While Voronoi tessellation is a common method for describing the short-range order of disordered systems, there are many drawbacks to this technique. Namely, Voronoi indices describe the topology of the polyhedral volume associated with a particular atom, but provide little insight into the relative similarity of the various polyhedra that make up the material. That is, a minor distortion in atomic positions may result in vastly different Voronoi indices without dramatically impacting the local environment of an atom or the overall degree of ordering in the system. A more rigorous approach would ask if local environments are similar enough to be considered the same. Here, we show that a density-based machine learning approach (HDBSCAN) provides such characterization of structure in the liquid. Out of this method emerge groups of atoms with similar atomic arrangements. The relative degree of order is described by the variance in the group sizes; a low variance indicates that most atoms belong to a small number of groups, implying a high degree of order. To demonstrate the effectiveness of this approach, we analyze the structures of simulated Cu-Zr and Al-Sm alloys to show that this variance is inversely correlated to experimentally observed glass-forming ability. This work has the potential to fundamentally change our understanding of glass formation. While the historical approach to glass formation is to consider the competition between the thermodynamic driver and the kinetic mobility of atoms within the super-cooled liquid, this data suggests that the ease of glass formation in a given material is predetermined in the liquid state, before ever reaching the melting point or the glass transition temperature.

¹N. A. Mauro, M. Blodgett, M. L. Johnson, A. J. Vogt, and K. F. Kelton, “A structural signature of liquid fragility,” Nat. Commun., vol. 5, Aug. 2014.
²X. J. Liu et al., “Static atomic-scale structural heterogeneity and its effects on glass formation and dynamics of metallic glasses,” Intermetallics, vol. 101, pp. 133–143, Oct. 2018.
³J. Wang, A. Agrawal, and K. Flores, “Are hints about glass forming ability hidden in the liquid structure?,” Acta Mater., vol. 171, pp. 163–169, Jun. 2019.

Determining metal compositions that have good Glass Forming Ability (GFA), and in particular that yield bulk metallic glasses through cooling, has been an outstanding grand challenge in metallic glass research. Previous attempts to predict GFA for metals often include some functional form of T_g, T_x, and T_l, known as the glass transition, the onset of crystallization, and the liquidus temperatures, respectively. For example, the reduced glass transition temperature, T_rg=T_g/T_l, is a well-known GFA indicator. Another well-known GFA indicator is the liquid fragility, m, which is measured by finding the slope of viscosity as a function of temperature near T_g for an alloy. Multiple researchers have shown that direct measures of GFA, in particular the critical cooling rate (R_c) and critical casting diameter (d_max), can be written as relatively simple functions of T_g, T_x, T_l, and m [1, 2]. However, these relationships have limited applicability for new materials discovery because a material must be synthesized and measured in order to obtain these features, and they cannot be readily predicted accurately from molecular simulations. However, there are other characteristic temperatures and kinetic properties that have similar physics to that represented in the above features and which are accessible to molecular simulations.

Our study explores to what extent characteristic temperatures of metallic alloys readily accessible to molecular dynamics (MD) can be correlated with d_max. We use MD to predict a set of characteristic temperatures {T_i} for 73 metal alloys and fit d_max as a function of {T_i} using experimental d_max values for 19 of the alloys. We find a 3-fold cross-validation root mean squared error (RMSE) score of 0.512 for the model, well below the standard deviation of 0.848 for the 19 dmax values. The remaining 54 alloys are used as a test set and the d_max predictions are used to determine if the alloy should form a glass under melt spinning, and this categorization was then compared to experimental results. We find a maximum F1 score of 0.75 for the categorization model. These cross-validation and F1 scores suggest that the model has significant predictive ability, providing a potential pathway to predicting d_max directly from MD.

References
[1] W. L. Johnson, J. H. Na and M. D. Demetriou, "Quantifying the origin of metallic glass formation," Nature Communications, vol. 7, no. 1, p. 10313, 20 12 2016.
[2] Z. Long, H. Wei, Y. Ding, P. Zhang, G. Xie and A. Inoue, "A new criterion for predicting the glass-forming ability of bulk metallic glasses," Journal of Alloys and Compounds, vol. 475, no. 1-2, pp. 207-219, 2009.

Critical Cooling Rate (R_c) is the lowest rate a liquid can be cooled to avoid crystallization and vitrify into a glass. While it is a key factor to quantify the glass forming ability (GFA), direct measurement of Critical Cooling Rate (R_c) of metallic glass has been cumbersome and challenging and only determined for a few alloys.
The development of science and technologies raise demand for new materials including new Bulk Metallic Glasses. However, it has been estimated that only about 10% of the composition space of potential BMG formation has been considered thus far. To address the large potential compositional space more effectively, combinatorial approaches have been used for the fabrication of large number of alloys in thin film alloy libraries. But attempts to directly quantify critical cooling rates of alloys in such libraries have been limited to the as-sputtered state which forms under a cooling rate exceeding 10⁸ K/s. Although scientifically important, this high cooling rate is not able to probe practical BMGs, of which the R_c is always below 10³ K/s. Some progress was made recently by reheating the thin films and reducing the cooling rate to ~10⁵ K/s. However, this is with a still high cooling rate rage.
We will report a fast screening method which is based on Single Pulse Laser Annealing to directly measure R_c in thin film alloy libraries. Cooling rates ranging from 10² to 10⁶ K/s can be realized during solidification of the alloys. To demonstrate this method, we determined R_c for a large number of alloys in the Al-Ge-Ni system, and identified the best glass forming composition as Al₅₁Ge₃₅Ni₁₄ with a critical cooling rate of 10⁴ K/s.

Bioresorbable materials have been of increased interest in the last two decades due to their potentially transforming impact on human body implants. However, even with a large number of groups working on it, there is data available only for a very small number of alloy compositions (<20) in the ternary amorphous alloy systems of magnesium (Mg)–zinc (Zn)–calcium (Ca) and iron (Fe)–Mg–Zn. The optimal alloy compositions for biomedical applications should be chosen from a large variety of available alloys with best combination of mechanical properties (modulus, strength, hardness) and biological response (in-situ degradation rates, cell adhesion and proliferation). This research presents high-throughput screening of more than 150 different compositions each in two different bioresorbable alloy systems. We demonstrate that this new approach will allow future researchers to develop patient- and implant-specific bioresorbable alloys based on specific application requirements. As a first step towards establishing a database designed to enable such targeted material selection, amorphous alloy composition libraries were fabricated employing a combinatorial magnetron sputtering approach where Mg, Zn, and Ca/Fe are co-deposited from separate sources onto a silicon wafer substrate, followed by composition analysis using energy dispersive X-ray spectroscopy and X-ray diffraction measurements and mechanical properties characterization using nanoindentation.
[1] Amit Datye et al., Journal of Materials Chemistry B 7, 5392 (2019).

When, in the regime of inhomogenous flow, a metallic glass is constrained during plastic deformation, all aspects of the kinetics of shear banding, including initiation, advance of the tip, and "shear," or displacement across its thickness, become coupled. An example is the indentation experiment, in which surrounding material, loaded elastically, constrains deformation within the plastic zone and leads to a self-similar, hemispherical pattern of shear bands falling along multiple orientations surrounding the indent. Likewise, in metallic glass composites the reinforcement phases introduce constraints because they disrupt shear band propagation and
shear. In each case, the effect of the constraint is to introduce a back stress, which leads to an increase in deformation resistance similar to kinematic hardening in crystalline composites. From indentation experiments, we can measure the strength of this coupling in terms of a "hardening modulus," the magnitude of which can be determining from the fluctuations in slope of the load-depth curve during serrated loading. It is expected that the hardening modulus will depend on both material properties and the geometry of the constraint.
Using nanoindentation experiments with a Berkovich indenter, we measure
the hardening modulus in ZrCuAl metallic glass ribbons and ingots across 2 decades of strain rate during loading and 6 decades of strain rate during constant load creep. Hardening modulus is nearly independent of load despite a size effect in the serrations. Relaxing the glass near T influences the hardening modulus. The magnitude of the constraint is the same during constant load creep and during loading. Work arguments allow us to arrive at a model for coupled deformation, from which we estimate the magnitude of the constraint modulus in terms of the shear modulus of the glass, G. We compare this estimate with experimentally measured values.

Bicontinuous nanoporous metallic glasses (MG) synergize the outstanding properties of MGs and open cell nanoporous materials. The low-density and high-specific-surface-area of bicontinuous nanoporous structures have the potential to enhance the applicability of MGs in catalysis, sensors, and lightweight structural designs. In this work we generated stochastic bicontinuous structure using an efficient direct method based on Cahn’s idea and Fibonacci grids. We then perform molecular-dynamics simulations of tensile loading on a bicontinuous nanoporous Cu₆₄Zr₃₆ and characterize the deformation mechanisms dictating its elastic and plastic response preceding failure. Results indicate an anomalous MG mechanical behavior featuring extensive plastic deformation preceding a typically ductile failure. Results show progressive necking of ligaments aligned with the loading direction and progressive alignment of randomly oriented ligaments during the loading. Failure occurs by fracture following successive necking and rupture of ligaments. This work indicates that a bicontinuous nanoporous design is able to effectively delocalize strain localization in a bulk MG by utilizing the size effect of MG nanopillars and nanowires that induces a brittle to ductile transition in their mechanical behavior as ligament sizes are reduced to less than 100 nm.

The intricate deformation and failure of a shape memory alloy-bulk metallic glass composites (BMGC) under tensile loading is investigated with large-scale molecular dynamics simulations. By carefully choosing the volume fraction and the arrangement of the crystalline phase the failure of the BMGC is shifted from the propagation of a critical shear band in the amorphous matrix to yield and failure of the crystalline second phase. Results show that a brick and mortar design with staggered crystalline bricks (stg-BMGC) is able to further synergize the mechanical properties of the amorphous and crystalline phases. By effectively distributing the local stress and preventing the generation of a critical shear band the stg-BMGC design displays enhanced ductility while preserving the strength of the metallic glass matrix.

Crystallization induced microstructure plays a central role in tailoring materials properties, yet our understanding of its formation process is limited due to its complexity. Classical crystallization theories assume that nucleation and growth are separate processes in which thermodynamically unstable clusters fluctuate in the nucleation stage, but do not participate in the later growth. Here, we apply in situ aberration-corrected transmission electron microscopy (TEM) technique to directly observe the crystal growth of metallic glass nanorods at the atomic resolution and test the classical theories. Surprisingly, we show that, at the atomic scale, the isothermal growth rate for a single crystalline phase is asymmetric upon thermal history. This observation is in direct contrast to what would be expected based on classical theories. We further observe that the asymmetric growth rate depends on the diameter of the nanorod. Thus, our in situ TEM observations mark departure from classical theories in metallic glass system^1,2,3.

1. Sohn, S., Jung, Y., Xie, Y., Osuji, C., Schroers, J. and Cha, J.J., 2015. Nanoscale size effects in crystallization of metallic glass nanorods. Nature communications, 6, p.8157.
2. Sohn, S.*, Xie, Y*., Jung, Y., Schroers, J. and Cha, J.J., 2017. Tailoring crystallization phases in metallic glass nanorods via nucleus starvation. Nature communications, 8(1), pp.1-8. (co-first author)
3. Xie, Y., Sohn, S., Wang, M., Xin, H., Jung, Y., Shattuck, M.D., O’Hern, C.S., Schroers, J. and Cha, J.J., 2019. Supercluster-coupled crystal growth in metallic glass forming liquids. Nature communications, 10(1), pp.1-9.

Metastable metallic glasses experience significant structural evolution and phase transition in supercooled liquid regions at high temperatures. In this research, we employed a MEMS based ultra-stable heating stage to in situ investigate the coalescence and crystallization of metallic glasses in a Cs-corrected high-resolution transmission electron microscope. The atomic-scale in situ observations reveal the significant kinetic difference between crystals and glasses in particle coalescence, demonstrating that the fast surface dynamics of metallic glasses is associated with the isotropic atomic structure and supercooling state. On the other hand, the kinetics-controlled crystallization process at temperatures close to T_g is found to be manipulated by both liquid diffusivity and chemical defects at liquid/crystal interfaces. These results provide new insights into the relation between structural stability and surface/interface dynamics of metallic glasses.

Metallic glasses can have widely varying thermodynamic and kinetic stability at constant composition depending on their processing history. For bulk glasses, fast quenching or post-synthesis deformation result in a high free energy, less stable states and slow quenching or post-synthesis sub-T_g annealing result in low free energy, more stable states.

For thin films and nanostructures, surfaces play a larger role. We have used electron correlation microscopy experiments and molecular dynamics simulations to show that metallic glass surfaces have approximately ten times faster dynamics than the bulk at the same temperature for temperatures near T_g. The fast surface layer has a glass transition temperature that is suppressed with respect to the bulk by ~20 K. We have extended the understanding of diffusion in supercooled liquids based on caging and hopping of atoms to surfaces to explain this behavior.

Enhanced surface dynamics enable deposition of highly stable thin films, since every atom at some point has been a surface atom and the surface layer maintains sufficient mobility for the atoms to find low-energy configurations even at temperatures well below the bulk T_g. We have deposited Zr₆₅Cu_27.5Al_7.5 metallic glass thin films with varying stability by varying the deposition rate at a substrate temperature of 0.7T_g. The stability of the films is demonstrated by calorimetry and creates changes in films’ modulus and hardness. We show using electron nanodiffraction that more stable films have greater nanometer-scale structural order, demonstrated by an increase in Friedel speckle pairs associated with Bragg diffraction from ordered structures. Analysis of rotational symmetries in nanodiffraction patterns show that the order is dominated by icosahedra, but that crystal-like rotational symmetries are also present. The ability to create glasses with widely varying stability at constant composition enables disentangling the influence of atomic rearrangements created by processing and changes in chemistry.

We have applied electron correlation microscopy (ECM) to study near-surface layer dynamics in metallic glass forming liquids as a function of temperature using in situ heating. Previous ECM studies showed that the near-surface layer relaxation dynamics in Pt_57.5Cu_14.7Ni_5.3P_22.5 metallic glass nanowires are an order of magnitude faster than bulk dynamics at the same temperature [1]. We have performed ECM experiments with higher spatial resolution and at temperatures lower than the bulk glass transition temperature, where the surface layer dynamics are still active but the bulk is vitrified. Our investigations reveal a ~1 nm layer with faster dynamics near the surface of the nanowire, which has a glass transition temperature 20 K lower than the bulk. Coating the nanowire with a thin layer of amorphous carbon suppresses the fast near-surface layer, which indicates that access to free volume available at the surface is responsible for the enhanced mobility.

Parallel investigations were performed via molecular dynamics simulations on a Ni₈₀P₂₀ system. Simulated Ni₈₀P₂₀ has a ~1 nm thick surface layer with faster dynamics and a glass transition temperature suppressed by 40 K compared to the bulk. With the surface layer held immobile in the simulation, the near-surface layer dynamics become slower than the bulk, but speed up with distance away from the surface, converging to the bulk dynamics. Based on these observations, we will briefly present a mechanism of diffusion in supercooled liquids based on caging and hopping of atoms, and how caging barriers control atomic diffusion and relaxation phenomena even in the near-surface region.

[1] Zhang, P., Maldonis, J.J., Liu, Z. et al. Spatially heterogeneous dynamics in a metallic glass forming liquid imaged by electron correlation microscopy. Nat Commun 9, 1129 (2018). https://doi.org/10.1038/s41467-018-03604-2

While minor alloying is used extensively to manipulate stability and mechanical properties of metallic glasses (MGs), a mechanistic understanding of these effects is still lacking, limiting rational design of new MGs. In crystalline metals, alloying elements can, for instance, pin dislocations and thereby have a large effect on the overall mechanical response, but MGs do not have a long-range order and they do not exhibit dislocation plasticity. It is therefore puzzling how a small amount of impurities can lead to significant changes in mechanical properties of an amorphous material. To understand the role of minor alloying on stability and mechanical properties of MGs, we have investigated Al-Sm glasses microalloyed with transition metal (TM) elements. Al-based glasses were chosen because of the many excellent properties of these materials, such as light weight, high specific strength, good corrosion resistance, and thermoplastic forming ability.

We have found experimentally that TMs increase both the glass forming ability (GFA) and the strength of Al-Sm-TM glasses, and we have used atomistic simulations to discover the underlying mechanisms for these effects. First, we have found that in the binary Al-Sm glasses, the experimentally determined GFA (quantified based on the critical cooling rate) is correlated with a local topology determined from classical molecular dynamics (MD) simulations. Specifically, the GFA was found to be higher for glasses that have a higher fraction of icosahedral clusters. We have then used this topological descriptor combined with ab initio MD simulations to predict ternary Al-Sm-TM glasses with high GFA. The predictions were then verified experimentally. This result shows that, similarly to what has been reported in commonly studied Cu-Zr MGs, the topology plays a dominant role in determining stability of Al-Sm MGs. The results also demonstrate that ab initio simulations can be used directly to design new glass compositions with high GFA.

Interestingly, we have found the topology to be correlated with mechanical strength of binary Al-Sm glasses, but not with the strength of Al-Sm-TM glasses. A commonly accepted explanation for the strengthening of MGs with minor alloying is based on the impurity-induced increase in the topological order and a more efficient packing of atoms. It has been confirmed to be the case for instance in Cu-Zr glasses and by us in Al-Sm binary glasses. However, in the case of Al-Sm-TM, we discovered that a small amount of impurities can increase the strength of MGs by changing the chemical bond strength alone. In fact, we find that the favorable changes in the chemical bond strength overcome the effect of impurity-induced unfavorable changes in the topological order.

Our results are important for design of new MGs, as they demonstrate that the effect of bond chemistry and the effect of topology should be separated from each other and they could potentially compete with each other in controlling mechanical response of MGs. These results also highlight the importance of investigating structure-property relations in a number of different MGs, beyond the Cu-Zr model system, in order to extract generally applicable trends.

Plastic deformation of metallic glasses is usually carried by local plastic instabilities, eventually leading to shear banding that is coupled to work softening. As a result, metallic glasses often show macroscopically brittle behaviour, thus limiting practical applications. This short description already signifies that plastic deformation of metallic glasses inherently involves vastly different length scales from the scale of locally ordered regions (so-called medium range order, MRO) to the macroscopic and system-spanning dimensions of shear bands. In this contribution, emphasis is laid on the local MRO structure of metallic glasses and specifically its modifications upon applying an external shear stress.
For this reason, the MRO structure of both as-cast and deformed states of different metallic glasses were analysed using variable resolution fluctuation electron microscopy. After deformation, significant changes of the MRO structure were observed inside the shear bands and also inside the adjacent matrix; the MRO structure was altered in terms of types, sizes and volume fractions. The changes in the matrix upon deformation confirm the existence of a shear affected zone around shear bands.
To understand the coupling between medium range order structure and local deformation behaviour further, sputtered nanolaminates of a binary metallic glass, Cu₄₀Zr₆₀, and nanocrystalline Cu were synthesized and subsequently co-deformed. The nanolaminates deform in shear zones, including nanocrystalline Cu layers, amorphous layers and the surrounding matrix. Due to the confinement of the glass, large plastic strains can be realized without premature macroscopic failure, allowing to assess deformation states in one sample that have carried between several ten percent and up to several hundred percent of strain. Analyses of nanobeam-diffraction patterns reveal an extraordinary modification of the MRO structure of amorphous Cu₄₀Zr₆₀ after deformation. This transformation shows similarities to a reverse martensitic transformation in the corresponding crystal from the B19 to the B2 structure, as shown by fluctuation electron microscopy and a symmetry analysis of nanobeam-diffraction patterns. Since the local ordered domain size surprisingly stays similar, this phenomenon is attributed to transformation-induced plasticity of the local units in the glass that carry the shear.
The current work illustrates the significance of the MRO structure for the deformation of metallic glasses and further suggests that the plasticity of monolithic BMGs might be improved by tailoring the topological order based on MRO types, sizes and volume fractions.

Damage accumulation as a function of plastic strain is well understood in crystalline metals but not in metallic glasses, where strain localizes into nano-scale shear bands. Here we aim at revealing how the nanoscale and mesocopic signature of strain localization evolves during deformation. Using high-energy x-ray tomography, we first address how the gradual emergence of shear-band cavities leads to a large reduction in the load bearing area as a function of strain (Scripta Materialia 170 (2019) 29). Determining the true flow stress on the basis of the tomography data indicates significantly larger stresses than the yield stress of the material. This apparent strain hardening can be rationalized by taking into account the shear-plane roughness and its contribution to the friction condition between the two shearing sample parts. As a second step, we pay attention to the shear band itself with the interest in tracing any structural or chemical changes as a function of shear strain. To this end, we use a combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM,), synchrotron-based nano-beam x-ray fluorescence, atom probe tomography (APT), scanning electron nano-beam diffraction (SEND) and energy-dispersive x-ray spectroscopy (EDS). Apparent chemical changes in the shear band are found that can be shown to be due to the density change of the shear band relative to the matrix (Scripta Materialia 169 (2019) 23). We further discuss if the shear-band width and density change are related and how the depend on position (Acta Materialia 140 (2017) 206). Finally, we provide first evidence for that the locally admitted shear strain determines the locally determined density change of the shear-band material. This body of work highlights the locallay varying complexity of shear-band structure in bulk metallic glasses.

It would be interesting to study the atomic-scale structure response to deformation for better understanding the structure-property relationship for metallic glasses. The atomic-scale structure evolution during plastic deformation of Pd₈₂Si₁₈ metallic glasses was studied using the high-energy X-ray pair-distribution function. Our experimental results revealed that the change of the cluster connectivity on the medium-range length scale plays an essential role during plastic deformation. It was found that the transformation between 2-atom and 3-atom connection modes would be responsible for the densification process during tension. Our findings would be helpful in deepening the understanding of the plastic deformation mechanism of metallic glasses.

Current understanding of plastic deformation of crystalline solids has benefitted greatly from electron-microscopic imaging of lattice defects. In the absence of periodicity, atomic-scale imaging of glasses is far more limited. However, similar to crystalline materials, physical analogs have provided important initial insights. Bubble-raft experiments by Argon et al. indicated that time-dependent deformation of a glass, permanent or reversible, is accommodated by localized rearrangements, termed shear transformations. Using Eshelby’s theory of deformation of inclusions embedded in an elastic medium, Argon developed a theory of thermally activated, stress-biased, shear transformation kinetics. It is based on a single size of a shear transformation zone (STZ). Since the activation free energy for a shear transformation is proportional to the STZ volume, the spread in time constants associated with different STZ sizes can be significant. Dynamic-mechanical analysis results have, indeed, demonstrated a range of time constants. However, with the loss modulus peak associated with a single time constant being a Cauchy function, details cannot be readily resolved.
We have used quasi-static anelastic relaxation measurements spanning more than ten orders of magnitude of time, 10^-3 – 6x10⁷ s. Non-instrumented curvature measurements at stable room temperature, with automated curvature determination from images, provide relaxation curves with small error bars. The inverse problem of obtaining relaxation-time spectra from the seemingly featureless experimental curves was solved computationally. The spectra exhibited distinct peaks, which we showed to correspond to an atomically-quantized hierarchy of STZs. The resolution level achieved is assisted by the relatively low temperature of the experiment. Using Argon’s theory, the spectra yield the volume of each STZ type as well as the volume fraction occupied by the corresponding potential STZs.
We have used the methodology detailed above to obtain the following results:
1) At small radii of curvature, for which the strain is large and the anelastic strain rate is not linear in the stress, we obtained the transformation shear strain and volume of the largest STZs independently. The value of the former is 0.18, in agreement with physical analogs. This result indicates that reports of STZ volume values of hundreds of atoms are unrealistically high, as they were based on a transformation strain value equal to the much-smaller macroscopic yield strain.
2) Structural relaxation increases each of the time constants and reduces the volume fraction occupied by the largest active potential STZs. The former effect is a result of an increase in shear modulus. Rejuvenation by cryogenic cycling reverses the increase in time constants, suggesting a reversal of densification. However, the volume fraction occupied by the largest active potential STZs is not restored.
3) Some metallic glasses exhibit a distinct high-frequency β relaxation. While it has been suggested to correspond to a mechanism distinct from that of the α relaxation, we find that both α and β are consistent with the STZ model. However, they exhibit different properties – either the STZ volume increment in the hierarchy or the corresponding shear modulus is different for the two mechanical relaxation modes. Both possibilities are consistent with composition fluctuations.
4) While it has been suggested that an intense β relaxation correlates with alloy plasticity, a comparison between La₇₀Cu₁₅Al₁₅ and La₇₀Ni₁₅Al₁₅ shows the opposite – the former exhibits far greater plasticity and its β relaxation intensity is smaller. We find that the overall anelastic response, which is equal to the integrated area of the relaxation-time spectrum, can be used as a predictor of plasticity.
_____
This work was funded by the U.S. National Science Foundation, Grant DMR-1708043

Shear banding is the main mechanism dominating plastic deformation of bulk metallic glasses (BMGs). Under constrained loading conditions, such as bending or compression, BMGs generally deform through serrated flow at room temperature. A serration event is accomplished by fast nucleation and propagation, together with the arrest of a shear band. A shear band forms by cooperative shear of nanometer-size shear transformation zones (STZ), developing into a highly localized deformation band with a thickness of ~10 nm. The dynamics of serrated flow in BMGs have been intensively studied, and the cumulative distribution of shear-avalanche sizes follows a universal power-law relation multiplied by an exponentially decaying scaling function as depicted by the mean-field theory. In contrast, under uniaxial tension, BMGs exhibit almost zero plasticity due to the fast runaway of a detrimental shear band.
In order to improve the tensile plasticity of BMGs, bulk metallic glass composites (BMGCs) containing in-situ formed crystals during solidification have been developed. Zr/Ti-based BMGCs containing beta-Zr/Ti phases and CuZr-based BMGCs containing B2 CuZr phases are the two most intensively investigated kinds of BMGCs. Generally, macroscopic serrated flow of BMGCs under tension disappears, because the shear bands are well restricted in the inter-crystalline glassy regions. BMGCs often exhibit strain-softening, because hardening caused by dislocation entanglement in the crystals cannot sufficiently compensate shear-softening in the glassy matrix. Recent research has shown that this drawback can be overcome and strain-hardening can be obtained via incorporating deformation-induced phase transformations or twinning of metastable crystals.
This talk explores the possibilities for tuning shear band initiation and propagation in metallic glasses and composites considering structural heterogeneity, phase transformation and size effects for glasses and composites with different composition and inherent stability. The findings will be discussed with respect to short- and medium-range order modulation, local stress and strain states, and their impact on shear band nucleation, bifurcation, blunting and modulation of cooperative shear events in the presence of crystals or triggered by the precipitation of secondary phases. The structure changes will be correlated with plastic deformability and strain-hardening capacity, and the effectiveness of composition tuning and structure modulation for ductility improvement will be discussed to derive guidelines for property optimization.

Deformation of nanoscale metallic glasses has attracted increasing attention but remains controversial due to difficulty in fabrication and testing of small samples. Here, we report the effects of systematic variation in sample-size and testing temperature. Pt-based metallic glass samples with varying diameters (100 nm -500 mm) were prepared by thermoplastic drawing. Multiple samples were deformed at different temperatures and the fracture morphologies were characterized using SEM. The results show that the fraction of vein-like morphology decreases with decreasing sample size and mirror-like fracture surface was observed in samples smaller than about 2 mm in diameter. With further decreasing diameter, transition from shear-localized to necking was observed and the extent of neck increased with decreasing sample diameter. The lowering in temperature had similar effect as decrease in sample diameter. This temperature-size equivalence is discussed in terms of temperature rise and formation of liquid-like layer in shear bands.

Usually, plastic deformation of large-sized metallic glasses (MGs) is highly localized into shear bands under uniaxial stress state. Because of this, the plastic deformation is heterogeneous and strain softening would occur. Over the past few years, we have studied the plastic behaviour of metallic glasses under triaxial stress state. Under this condition, shear banding is suppressed, and metallic glasses are capable homogeneous plastic flow at room temperature under either tension or compression. Our talk will present a summary on the mechanical behaviour of BMGs in a deep notched cylindrical bar. We will show that the plasticity and strength of Zr-based BMG can be significantly enhanced by suppressing shear banding under tension condition due to strain hardening and densification (e.g. free volume annihilation) due to mechanical relaxation. Meanwhile, the samples also exhibit different fracture behaviour with new fracture features. Under the compression condition, however, metallic glasses can be rejuvenated and extreme softening is seen in the sample. Such softened glass enables subsequent strain hardening. The mechanism for these observations will be discussed in terms of annihilation and generation of free volume under triaxial stress state.

Conventional polycrystalline alloys show strain-hardening. This is a key factor underpinning their usefulness in structural applications, delocalizing plastic deformation, enabling ductility, and preventing catastrophic failure. In contrast, metallic glasses (MGs) generally show strain-softening, leading to extreme localization of plastic flow in thin (10–20 nm) shear bands. This shear banding leads to early catastrophic failure in tension, and to unsightly surface steps when significant plastic strains can be achieved in other deformation modes, notably bending.

There have been many attempts at ‘shear-band engineering’, mainly to stimulate as many shear bands as possible to promote more uniform plastic flow, and by such means uniquely high damage tolerance has been achieved [1]. Nevertheless, shear-banding is the key obstacle to the wider use of MGs in mechanical applications [2]. Elimination of shear bands would be highly desirable. Yet there is the concern that shear-banding might be inevitable, given that the yield stress of MGs is a high fraction of the theoretical strength, so that any capacity for hardening must be limited.

In recent years, there has been a focus on the range of MG states that can be obtained at a given composition. As annealing and relaxation often lead to embrittlement, there is particular interest in the reverse process (‘rejuvenation’), by which the MG is taken to higher-energy, less dense states. It is speculated that a sufficiently rejuvenated MG could show strain-hardening [3], but it has been unclear whether the required rejuvenation could be achieved without destabilizing the MG against, for example, crystallization.

Thermomechanical processing, particularly in compression, can induce extreme rejuvenation in significant (mm-scale) volumes of MG [4]. When these rejuvenated volumes are subjected to uniaxial stress, in tension or compression, they do show strain-hardening, and correspondingly shear-banding is suppressed [5]. With this finding, it is possible that the aim of MG research should shift from shear-band engineering to the elimination of shear bands.

The first well accepted explanation of strain-hardening in polycrystalline metals was that proposed by G.I. Taylor in 1934 [6]: plastic strain involves the generation of dislocations, and as the imposed macroscopic strain increases, internal local strain gradients increasingly impede the motion of those dislocations. In general terms, this has remained the only explanation for strain-hardening until that now offered for MGs. In Taylor’s model, it is intrinsic that strain-hardening involves increased defect density and a higher energy for the system. In contrast, the strain-hardening of MGs involves a lower energy and, effectively a decreased density of defects. We survey the strain-hardening rate of metallic materials, and find that this is particularly high for MGs.

[1] M.D. Demetriou, M.E. Launey, G. Garrett, J.P. Schramm, D.C. Hofmann, W.L. Johnson, R.O. Ritchie, Nature Mater. 10 (2011) 123–128.
[2] A.L. Greer, Y. Q. Cheng and E. Ma, Mater. Sci. Eng. R 74 (2013) 71–132.
[3] Y.H. Sun, A. Concustell, A.L. Greer, Nature Rev. Mater.1 (2016) 16039.
[4] J. Pan, Y.X. Wang, Q. Guo, D. Zhang, A.L. Greer, Y. Li, Nature Comm. 9 (2018) 560.
[5] J. Pan, Yu.P. Ivanov, W.H. Zhou, Y. Li, A.L. Greer, Nature 578 (2020) 559–562.
[6] G.I. Taylor, Proc. R. Soc. Lond. A 145 (1934) 362–387.

In the last decades, researchers have been developing amorphous metals with bulk size, so-called bulk metallic glasses (BMGs) through carefully designing the alloy compositions and the quenching processes. Although the critical cooling rates required to bypass crystallization have been much reduced, BMGs still suffer from the constraints in dimensions and geometries when they are produced by casting. Here, we use the selective laser melting (SLM) technique to 3D-print the near-net-shape bulk metallic glasses. Their microstructures and mechanical properties are characterized. The defects such as porosity, pore size and morphology are analyzed in the relation with their tensile fracture behaviors of large-scale fully amorphous 3D-printed BMGs. The results show that the characteristics of porosity in 3D-printed BMGs play a crucial role in the ultimate tensile strength (UTS). The UTS decreases drastically from ∼ 1.5 GPa for pore-free samples to ∼ 1.25 GPa for samples with a porosity of 0.26%. Yet, the Young’s modulus is found to be insensitive in the low-porosity regime. Finite element simulations show different deformation behaviors of pore-free and pore-containing models, where the former fractures with multi shear bands initiated at the material weak point, while the latter experiences plastic deformation around pores with the shear transformation zone (STZs) perpendicular to the normal stress. The understanding of microstructure defects enables us to engineer the defects for controlled tensile fracture behaviors in 3D-printed BMGs.

The in-depth understanding of the mechanisms of plastic deformation in metallic glasses is still missing despite years of dedicated experimental and theoretical effort. Here we demonstrate how polymer-supported films can be utilized to capture different stages of shear band formation and propagation opening new perspectives to uncover plasticity mechanisms in metallic glasses.
By means of in-situ resistance measurements, in-situ optical microscopy as well as quasi-in-situ SEM and FIB characterization, different stages of evolution of shear bands and cracks with increasing strain are detected and described. Two distinct types of shear bands appear in polymer-supported sputter deposited Pd₈₀Si₂₀ and Au₆₀Ag₂₀Si₂₀ metallic glasses with increasing strain: (i) the “out-of-plane” shear bands (the direction of shear is not in the film plane) which are formed at about 2% strain and develop quickly into through-thickness cracks and (ii) in-plane shear bands (the direction of shear is within the film plane) which appear after crack density saturation (at about 10% strain) and do not lead to crack formation. If the film thickness is above 500-700 nm the propagation of single out-of-plane shear bands which do not develop into cracks is observed. If the film thickness is reduced below 15 nm, the formation of shear bands is generally suppressed and the film can deform up to strains of about 6% elasto-plastically (i.e. without cracking) showing formation of homogeneously distributed short nanocracks at higher strains. It is demonstrated, that with increasing applied strain, new in-plane shear bands can easily intersect the existing ones, whereupon the intersected shear bands become inactive and cannot carry further plastic deformation. This mechanism can lead to effective strain hardening of metallic glasses. Furthermore, the existence of droplet-like extrusion directly on top of the traces of in-plane shear bands provides a direct evidence of specific material state characterized by high free volume and low viscosity during the shear banding event.
Reported results demonstrate that polymer-supported films offer an alternative way to investigate plasticity of metallic glasses through higher control over generation, propagation and topology of shear bands. It is believed that new insights into such controversial topics as the propagation mode, propagation speed as well as temperature of operating shear bands can be gained.

Micron sized colloidal particles, which interact with nearly hard-sphere potentials, can be used to form dense amorphous packings using a variety of well established techniques. Due to the large size and slow dynamics of colloidal particles, confocal microscopy can be used to measure the 3D structure and dynamics of these densely packed colloidal glasses. For example, previous work has directly visualized, as a function of applied strain, both the inhomogeneous particle level rearrangements that occur within a colloidal glass, and the emergence of continuum elastic strain fields that surround these rearrangements. While microscopic visualization of colloidal glasses provides real-space information not readily obtainable in atomic systems, measuring the stress response of colloidal glasses, in addition to visualization, is a significant challenge. The large, micron-size of the colloidal particles and their thermal interactions produce solids with exceptionally small elastic moduli on the order to 10-100 mPa. We introduce a new technique, traction rheoscopy, to directly measure the mechanical response of these hghly compliant colloidal glasses under simple shear strain while simultaneously visualizing their microstructure. The method consists of placing a bilayer of colloidal glass atop a well calibrated soft polymer gel of similar shear modulus. The composite bilayer undergoes strain-controlled shear; shear stresses are inferred from the displacement of embedded tracer particles in the calibrated polymer gel. Using these stress measurements, we show that under applied shear the colloidal glass goes through as sequence of reversible and irreversible microscopic rearrangements as the stress develops correlated spatial heterogeneities. The stress measurements with simultaneous real space visualization of the particle dynamics corroborate previously reported microscopic flow mechanisms and present new insight into the shear response of the colloidal glass.

Metallic glass composites employ extrinsic approaches to improving the ductility and toughness of classically brittle amorphous alloys through the introduction of a distributed crystalline phase upon solidification. The crystalline heterogeneities have been shown to distribute the process of shear localization in the amorphous matrix while simultaneously promoting dislocation plasticity, manifesting as a more homogeneous plastic response. Although studies have demonstrated such mechanisms, there exists a disparity in the effective mechanical length scales of delocalization as well as a lack of a definitive approach for quantifying the crossover from incipient to homogeneous flow as a function of the underlying composite microstructural length scales. Here, we use nanoindentation combined with a bonded interface indentation technique to quantify individual shear banding events and map the role of composite microstructure on the shift in the governing yield criterion. Our results reveal a reduction in the stress required to nucleate a shear band when mechanical length scales were coincident with the composite microstructural length scales, i.e., the nucleating shear band interacts with the amorphous-crystalline interfaces. At larger indentation depths producing a fully developed plastic zone, a reduction in the propensity for shear localization was observed and attributed to strain accommodation by the crystalline dendrites suppressing shear band propagation. A crossover from the shear plane to maximum shear stress yield criterion is uncovered at a crystalline fraction of approximately 60%, thus signaling an effective transition from incipient to homogeneous flow as the dominant deformation mode of the composite.

The rise of nanotechnology has been propelled by low dimensional metals. Albeit the long-perceived importance, synthesis of freestanding metallic nanomembranes (or the so-called 2D metals), however, has been mainly restricted to elemental metals with a very limited in-plane size (< 10 µm). In this talk, I would like to talk about the development of a low-cost method to synthesize 2D metals through polymer surface buckling enabled exfoliation (PSBEE). The 2D metals so obtained could be as chemically complex as conventional metallic glasses while possessing in-plane dimensions at the scale of bulk metals (> 1 cm). With our approach, we successfully synthesized a variety of 2D metallic glasses with controllable geometries and morphologies. Interestingly, our results clearly show that, as the thickness of the 2D metallic glasses reduces to a few nanometers, its modulus is significantly reduced to only a fraction of the modulus of their bulk counterparts. Aside from rather low elastic modulus, these 2D metallic glasses also exhibit unusual properties, such as good stretchability and tunable optical properties. Finally, we demonstrate that 2D ceramic glasses and 2D metallic composites can be also fabricated through PSBEE aside from 2D metallic glasses.

Micro- and nanoscale metallic glass systems offer exciting opportunities for both fundamental research and applications in healthcare, micro-engineering, optics and electronics. In addition to their functional attributes, exceptional mechanical properties and corrosion resistance, a singular feature of bulk metallic glasses (BMGs) is their ability to be processed in the supercooled liquid state thanks to their sluggish crystallization kinetics and relatively low viscosity. However, thus far the scientific challenges and technological opportunities associated with exploiting viscous deformation to realize advanced micro- and nanoscale MG architectures remain to a large extent unexplored. Moreover, integrating nano-scale MG structures into well-functioning devices still poses great challenges.
Here, we present a scalable method to produce fibers containing continuous and well-ordered micro- and nanoscale Pt-based MG conductors via their thermal co-drawing in the supercooled liquid state within a polymer matrix [1]. Designing the parts in a macroscopic scaled-up preform enables the fabrication of structured MGs with arbitrary transverse geometries and controllable feature size down to a few tens of nanometers, previously unachievable. This approach allows us to deepen our fundamental understanding of deformation and size effects on crystallization kinetics, and engineer innovative MG-based devices within advanced fibers and smart textiles.
First, we discuss how we selected the right combination of MG and polymer for a successful co-drawing. In situ heating transmission electron microscopy (TEM) on drawn MG ribbons is then presented to reveal the influence of the thermal drawing process on the crystallization kinetics, which ultimately determines the achievable feature size. After establishing this fundamental understanding, we show a variety of complex MG nanostructures in flexible polymer matrices. Finally, we demonstrate the potential of BMGs in comparison to more traditional drawable materials by presenting applications in optoelectronic and neuroscience. We show that the MG ribbons can serve as electrodes to enhance the performance of optoelectronics fibers and fabrics. We then present two implantable MG-based fiber probes integrating multiple MG conductors, and in one case a microfluidic channel for local chemical delivery. Chronic experiments in rats show the ability to elicit robust behavioral responses when delivering electrical neurostimulation in deep structures, to record neuronal activity during unconstrained locomotion, and to deliver pharmacological agents to manipulate local circuits.

Reference
[1] W. Yan, I. Richard et al., Structured nanoscale metallic glass fibres with extreme aspect ratios, Nature Nanotech., 2020 (Accepted, in Press)

Tremendous effort has been taken during the last two decades in the development of nano fabrication techniques for materials exhibiting desirable functional properties. However, both bottom-up and top-down fabrication approaches are limited in some critical aspects such as material choice, geometry, and scalability.
A highly versatile and widely used fabrication method is molding, which is generally associated with a soft state of a material. Nanomolding has been realized, based on creep flow, for polymers, gels, and some glasses that soften at elevated temperatures, but not for crystalline materials that remain hard in their crystalline state.
Recently, we discovered that nanomolding is possible with crystalline materials. Processing usually around ~0.5 T_m, such thermomechanical nanomolding (TMNM) results in very high aspect ratio up to 1000 and nanowires as small as 5 nm in diameter. It offers itself as a versatile nanomolding technique for metals, alloys and even ordered phases.
By tracking the atomic moving in TMNM with multilayer samples and analyzing scaling experiments, we studied the underlying mechanism to be based on interface atomic diffusion and dislocation. They dominate TMNM in different temperature ranges as an interface diffusion mechanism works most effectively with a temperature above ~0.4 Tm and a dislocation mechanism works at lower temperature. A diffusion dominated mechanism is also found effective to form nanowires through amorphous phases (BMGs), with a processing temperature below T_g.
We will discuss the underlying mechanisms in TMNM, and their effect on structure and composition of formed nanowires in solid solutions, ordered phases and amorphous phases (BMGs).

Since the first synthesis of a bulk metallic glass (BMG) by copper mold casting in 1989, a large number of BMGs with centimeter-size diameter were synthesized in a variety of multicomponent alloy systems such as La, Mg, Zr, Ti, Hf, Fe, Co, Ni, Cu, Pd, Pt and Au metal bases. These BMGs are composed of multicomponent with negative and nearly zero heats of mixing and significant atomic size mismatches, while there has been little information on the influence of additional metallic elements with positive heat of mixing on the formation, thermal stability, crystallization behavior and fundamental properties for BMGs. Very recently, the pseudo-high entropy (PHE) BMGs containing solute atomic pairs with positive heats of mixing, which are different from the component rule for ordinary BMGs, have attracted an increasing interest because of their unique properties¹⁾. In particular, the PHE BMGs exhibit two exothermic peaks on the continuously heating DSC curve and the temperature interval between the two peaks is as large as 200-400 K. Besides, no crystalline phase is observed even after annealing for 1-5 h at the temperatures well above the first exothermic peak. The unusual high stability to the progress in crystallization has been reported for various PHE BMGs such as Zr-Al-(Ni,Cu)-Nb, Zr-Al-(Ni,Cu)-Ag, Zr-Al-(Co,Cu)-Ag, Zr-(Fe,Co,Ni,Cu), Ti-Zr-(Cu,Pd)-Nb and Fe-Cr-Mo-C-B-Y systems. These results indicate the possibility of developing a heat resistant BMG by utilizing this novel phenomenon. Here we present the formation, thermal stability, crystallization behavior and mechanical properties of PHE BMGs and to investigate the criterion for the synthesis of BMGs with ultrahigh thermal stability.

1) A. Inoue, F.L. Kong, S.L. Zhu, A.L. Greer, Multicomponent bulk metallic glasses with elevated-temperature resistance, MRS Bull. 44 (2019) 867-872.

This presentation will start by introducing the new Cu-based bulk metallic glasses (CBMGs) in the Cu-Zr-Hf-Al system that my group has developed recently. These new CBMGs are free of rare earths, precious metals or toxic elements, and yet possess the highest glass-forming ability known to any copper-based alloys. The presentation will then shift to focus on a computation study of the energy state of icosahedral clusters inside undercooled metallic liquids in some simpler metallic systems, in order to gain fundamental insights into glass formation and glass-forming ability.

Icosahedral clusters have been experimentally confirmed to exist in undercooled metallic liquids. Their existence is generally understood using the argument about an isolated icosahedron Frank made almost 70 years ago. This argument establishes an icosahedron as the most energy-efficient packing for an isolated 13-atom cluster, but fails to consider the effects of surrounding atoms in a liquid and temperature on the energy state of the cluster. The argument suggests core stabilization as the main mechanism for the origin of icosahedral clusters - since the core of an isolated cluster has lower potential energy than the shell.

Using large scale atomistic simulations and statistical analysis of several BCC (body-centered-cubic) and FCC (face-centered-cubic) metals, we have found that the shells of icosahedrons spontaneously formed inside deeply undercooled metallic liquids or glasses in fact have lower (averaged) potential energy than the cores. The shell potential energy deficiency occurs only to the icosahedral clusters but not to the equilibrium-crystal clusters, and, for icosahedral clusters, this deficiency grows with decreasing temperature. Compared with FCC metals, BCC metals exhibit greater potential energy deficiency on the icosahedral shells and produce significantly more icosahedral clusters upon liquid quenching, which explains the higher tendency of BCC metals to be vitrified observed in ultrafast cooling experiments. Inspecting the potential energy deficiency on the icosahedral shells through computation could provide a new avenue to the search for amorphous metals (i.e. metallic glasses) with high glass forming ability and processability.

Prediction of the glass forming ability (GFA) of alloys remains a major challenge. We are not able to predict the composition dependence of the GFA of even binary alloys. To investigate the effect of each element’s propensity to form particular crystal structures on glass formation, we focus on binary alloys composed of elements with the same size, but different atomic symmetries using the patchy-particle model. For mixtures with atomic symmetries that promote different crystal structures, the minimum critical cooling rate R_c is only a factor of 5 lower than that for the pure substances. For mixtures with different atomic symmetries that promote local crystalline and icosahedral order, the minimum R_c is more than 3 orders of magnitude lower than that for pure substances. Results for R_c for the patchy-particle model are in agreement with those from embedded atom method simulations and sputtering experiments of NiCu, TiAl, and high entropy alloys.

Bulk metallic glasses (BMGs) are generally defined as a subset of metallic glasses which can be synthesized with dimensions exceeding 1 mm while maintaining their amorphous structure. Metallic glasses possess a wide range of mechanical, magnetic, and chemical properties making them exciting for many commercial applications, particularly if they can be made in larger sizes as is possible for BMGs. Unfortunately, today there is no robust method for determining which new alloy compositions will yield BMGs without synthesizing and cooling the alloy, greatly inhibiting materials design in this area.
Perhaps the most direct measure of glass forming ability (GFA) is critical cooling rate (Rc), which is the slowest rate at which a glass can be cooled and stay fully amorphous. In this work we focus on predicting Rc from easily obtained compositional information using machine learning. There are on the order of 100 measured values of Rc in the literature, which is too little data to machine learn Rc as a function of composition. In this work we dramatically extent this database by using other sources of approximate Rc values derived from robust correlations, including correlation of Rc with (i) characteristics temperatures (about 480 new values), (ii) critical casting diameters (about 550 new values), and (iii) results of melt spinning experiments (about 2000 new values). We obtain a database of 2884 approximate Rc values which we used to build a Random Forest model for Rc as a function of elemental properties, which in turn can be derived from composition. This model is validated and then used to predict GFA on several new glassy alloys. 5-fold cross validation performance of the model gives RMSE errors of around 0.66 log(K/s) in Rc (30% of the Rc database standard deviation of 2.4 log units). This value is low enough to design BMGs if it can be maintained across new systems, but we will also show that the model has significant challenges in treating new compositions.

While metastability is undoubtedly a key-characteristic of metallic glasses, its role is often underappreciated for multicomponent alloys in general, and High Entropy Alloys (HEAs) in particular. The HEA field has long focused on equilibrium conditions, aiming to stabilize single-phase solid solutions (SPSS). However, the high entropy effect is often insufficient to fully stabilize SPSS, yielding only few, mostly metastable SPSS. While the prospective space of favorable SPSS formation is smaller than commonly assumed, only larger degrees of metastability can open up the vast High Entropy Alloy space. We argue that HEAs form metastable SPSS through polymorphic solidification upon rapid cooling. To quantify a HEA’s metastability, we propose the critical cooling rate of SPSS formation R_C^SPSS, which is analogous to the critical cooling rate of glass formation R_C^Glass. On this unified basis, we can compare multicomponent alloys by their metastability and tendency to form SPSS or glasses, which is determined by their underlying atomic dispersity.

Metallic glasses constitute a class of engineering materials having an enormous
potential for many fields of application due to their superior properties. Here, we report on a new family of sulfur-bearing bulk metallic glasses. So far, sulfur was not considered as alloying element for the synthesis of bulk metallic glasses. We observe bulk glass formation in a variety of sulfur-containing systems, including titanium-based bulk glass-forming systems with an extremely high titanium content of 70 at.%. These findings allow the development of a whole new class of amorphous metals, having good processability and consisting of alloying elements suitable for industrial applications.

Among the novel advanced metallic alloy systems, metallic glass (MG) is a potential candidate for long-term use hydrogen interactions owing to its loosely packed structure promoting the number of interaction sites. The hydrogen storage and electrocatalytic activity of Pd-Si-based MG nanofilms and Si/SiO₂ hybrid structures were evaluated by combining electrochemical (chronoamperometry, cyclic voltammetry, electrochemical impedance spectroscopy), structural (aberration-corrected high-resolution transmission electron microscopy, X-ray diffraction), composition (X-ray photoelectron spectroscopy, energy dispersive X-ray) and morphologic (scanning electron microscopy, atomic force microscopy) techniques. Recently we have shown that maximum total hydrogen charge stored in Pd-Si-Au MG nanofilm is equal to that in polycrystalline Pd films with 1 µm thickness [1]. Hydrogen to Palladium ratio of Pd-Si-Cu MG was found to be 1.56 (c.f. 0.61 in polycrystalline Pd + Si/SiO₂ of the same film thickness) [2, 3]. The volume expansion of a palladium hydride unit cell obtained from HRTEM images due to hydrogenation of the Pd-MG nanofilms is around 6 times larger than that of the Pd-polycrystalline counterpart hydrogenated under the same conditions. The same Pd-Cu-Si MG based hybrid structure has a Tafel slope of 109 mV/dec, which is less than the half of the crystalline PdNF electrode of the same thickness. Besides, it shows the highest overpotential at 10 mA/cm², indicating a better electrocatalytic activity [4]. We have also established a general evaluation criterion with practicality in assessment and high accuracy for electrocatalytic reactions applicable to different metallic alloy systems.

[1] Sarac, et al., Chem. Eur. J., DOI: 10.1002/chem.202001596
[2] Sarac, et al., ACS Appl. Energy Mater., 1 (2018) 2630-2646.
[3] Sarac, et al., Mater. Horiz., 6 (2019) 1481-1487.
[4] Sarac, et al., Electrocatalysis, 11 (2020) 94-109.

Additive manufacturing (AM) has emerged as a unique method for producing highly specialized metal alloys and composites. The high cooling rates and customizable cooling rates afforded by AM are particularly useful in the creation of bulk metallic glasses (BMGs) and BMG metal matrix composites. While AM allows for net-shaped fabrication of even the weakest glass-forming alloys, macroscale parts still suffer from embrittlement due to annealing, porosity, partial crystallization and oxygen. In contrast, thin coatings and overlays made from BMGs and BMG composites may be ideal for some wear-resistant applications. This talk will provide an overview of how BMG printing and deposition technologies might be matured and infused into applications. It will cover Fe-based, Zr-based, Cu-based, and Ti-based BMG alloys.

Understanding how glasses form remains a challenge in material science. Vitrification is the thermal transition from a supercooled liquid in metastable equilibrium into a non-equilibrium glass. One of the major questions is whether vitrification in viscous liquids is related to sluggish atomic mobility exclusively via the primary structural α-relaxation process or rather other atomic motions play a role. Here, we present experimental evidences that multicomponent bulk metallic glasses can display heterogeneity of vitrification kinetics and that the cooling rate dependency of the vitrification kinetics is decoupled from the temperature dependence of the α-relaxation [1-3]. The evidences are based on exhaustive studies of the atomic mobility of an Au-based metallic glass former by dynamic mechanical analysis (DMA), x-ray photon correlation spectroscopy (XPCS), and fast scanning calorimetry (FSC); the latest by means of a step response analysis via the temperature and frequency dependence of the complex specific heat. The vitrification kinetics is characterized in FSC in terms of the limiting fictive temperature, i.e., the temperature at which a glass formed after cooling at a given rate would be at equilibrium. The separation of these two aspects of the glassy dynamics (vitrification kinetics and α-process) becomes more pronounced at larger undercooling, suggesting a larger contribution of additional dynamical processes to the vitrification process, which are not connected to the α-process. The observed heterogeneity of vitrification kinetics hints toward the existence of multiple relaxation mechanisms, which are observed during aging experiments [3].
[1] X. Monnier, D. Cangialosi, B. Ruta, R. Busch, I. Gallino, Sci. Adv. 6 (2020) eaay1454
[2] S. Hechler, B. Ruta, …R. Busch and I. Gallino, Phys. Rev. Mat., 2 (2018) 085603
[3] I. Gallino, D. Cangialosi, et al., Acta Mater., 144 (2019) 400

Highly unrelaxed structural states of metallic glasses have often advantageous mechanical properties. Since metallic glasses continuously relax with time (age) or inherently are well relaxed after processing, methods to uniformly rejuvenate the material are needed. One approach that has received attention is the so-called cryogenic-cycling method, during which a metallic glass is repeatedly immersed into liquid nitrogen. In some cases, cryogenic cycling is truly efficient in increasing the stored excess enthalpy of metallic glasses, but it does not seem to be universally applicable to all alloys and structural states. The origins for these differences remain unclear due to our limited understanding of the underlying structural evolution. In order to shed more light onto the fundamental structural processes of cryogenic cycling, we pursue in-situ x-ray photon correlation spectroscopy (XPCS) to trace the atomic-scale structural dynamics of a Zr-based metallic glass in two different structural states (ribbon and bulk metallic glass). This method allows calculating the relaxation times as a function of time throughout the thermal cycling. It is found that the investigated glasses exhibit heterogeneous structural dynamics at 300 K, which changes to monotonic aging at 78 K. Cryogenic cycling homogenizes the relaxation time distribution for both structural states. This effect is much more pronounced in the ribbon, which is the only structural state that rejuvenates upon cycling. We furthermore reveal how fast atomic-scale dynamics is correlated with long-time average structural relaxation times irrespective of the state, and that the ribbon exhibits unexpected additional fast atomic-scale relaxation in comparison to the plate material. Overall, a picture emerges that points towards heterogeneities in fictive temperature as a requirement for cryogenic energy storage.

The surface diffusion kinetics has been measured on an Au₆₀Cu_15.5Ag_7.5Si₁₇ metallic glass using the method of surface grating decay from room temperature up to 20 K below the glass transition temperature (T_g). In the early stage of gratings decay the surface diffusion coefficients were evaluated as varying in the range from 10^-20 to 10^-18 m²/s over the temperature range of measurement and the corresponding average activation energy is about 0.67 eV. During longer time annealing the surface gratings decay rate slowed down significantly. The slowing of the initial decay was a result of the surface dynamics and influenced by the surface segregation of silicon at long time which induced a surface crystallization of a silicon-rich phase.

Metallic materials generally show rapid nucleation and growth kinetics, which makes studies of their phase transitions and metastable phase formation generally more difficult. Bulk metallic glasses (BMGs), on the other hand, reveal very sluggish crystallization kinetics. By applying ultrafast differential scanning calorimetry to slowly transforming BMG-forming systems at heating and cooling rates of several 10,000 K/s, we are able to determine phase transitions of metallic systems more or less in slow motion. In this way we find that many metallic systems reveal solid-to-solid phase transitions that proceed via melting of a metastable phase [1]. Ultrafast calorimetry also allows us to interrupt rapid cooling after (metastable) phase formation and then to "up-quench" the frozen structure via rapid heating. Via surpassing the metastable-to-stable solid phase transition, the formed metastable phase can then be fully melted and its thermophysical properties, such as temperature and enthalpy of melting, be investigated in detail [2]. In this way, we are able to discover hidden transient phases, determine the stochastics of nucleation [3], and study multistep crystallization and melting pathways in metals. In fact, we show that even simple binary alloys can reveal multiple melting points [4]. Applying ultrafast calorimetry, we are thus able to construct energy-temperature diagrams including metastable phases, with the final aim of generating complete metastable phase diagrams.
[1] S. Pogatscher, D. Leutenegger, J. E. K. Schawe, P. J. Uggowitzer, J. F. Löffler, ‘Solid-solid phase transitions via melting in metals’, Nature Comm. 7 (2016) 11113.
[2] G. Kurtuldu, K. F. Shamlaye, J. F. Löffler, ‘Metastable quasicrystal-induced nucleation in a bulk glass-forming liquid’, PNAS 115 (2018) 6123 – 6128.
[3] J. E. K. Schawe, J. F. Löffler, ‘Existence of multiple critical cooling rates which generate different types of monolithic metallic glass’, Nature Comm. 10 (2019) 1337.
[4] G. Kurtuldu, J. F. Löffler, ‘Multistep crystallization and melting pathways in the free-energy landscape of a Au–Si eutectic alloy’, Adv. Science (2020), doi:10.1002/advs.201903544.

Thermodynamic and kinetic stabilities in glasses can be varied by physical vapor deposition. We investigated the structure and properties in Zr-Cu-Ni-Al and Zr-Cu-Al thin films sputtered from single alloy targets as a function of substrate temperature and deposition rate. For both compositions, we discovered a pseudo-phase boundary between amorphous and crystalline films as a function of rate and temperature. A semi-quantitative model based on surface-diffusion limited crystallization predicts the amorphous / crystalline growth boundary. Kinetic stability in Zr₆₅Cu_27.5Al_7.5 metallic glass thin films, as indicated by their glass transition onset temperature, was significantly increased by lowering deposition rate at constant temperature. The most kinetically stable film’s reduced modulus was 20% higher than a glass of the same composition quenched from the liquid by melt spinning, consistent with increased density and improved thermodynamic stability. Fluctuation electron microscopy and angular correlations in coherent electron nanodiffraction reveal that the more stable glasses show enhanced icosahedral medium range structural order. Our results suggest that enhanced nanoscale icosahedral order contributes to resistance of glass to both glass transition and mechanical deformation.

Understanding how glasses form remains a challenge in material science. Vitrification is the thermal transition from a supercooled liquid in metastable equilibrium into a non-equilibrium glass. One of the major questions is whether vitrification in viscous liquids is related to sluggish atomic mobility exclusively via the primary structural α-relaxation process or rather other atomic motions play a role. Here, we present experimental evidences that multicomponent bulk metallic glasses can display heterogeneity of vitrification kinetics and that the cooling rate dependency of the vitrification kinetics is decoupled from the temperature dependence of the α-relaxation [1-3]. The evidences are based on exhaustive studies of the atomic mobility of an Au-based metallic glass former by dynamic mechanical analysis (DMA), x-ray photon correlation spectroscopy (XPCS), and fast scanning calorimetry (FSC); the latest by means of a step response analysis via the temperature and frequency dependence of the complex specific heat. The vitrification kinetics is characterized in FSC in terms of the limiting fictive temperature, i.e., the temperature at which a glass formed after cooling at a given rate would be at equilibrium. The separation of these two aspects of the glassy dynamics (vitrification kinetics and α-process) becomes more pronounced at larger undercooling, suggesting a larger contribution of additional dynamical processes to the vitrification process, which are not connected to the α-process. The observed heterogeneity of vitrification kinetics hints toward the existence of multiple relaxation mechanisms, which are observed during aging experiments [3].
[1] X. Monnier, D. Cangialosi, B. Ruta, R. Busch, I. Gallino, Sci. Adv. 6 (2020) eaay1454
[2] S. Hechler, B. Ruta, …R. Busch and I. Gallino, Phys. Rev. Mat., 2 (2018) 085603
[3] I. Gallino, D. Cangialosi, et al., Acta Mater., 144 (2019) 400

Highly unrelaxed structural states of metallic glasses have often advantageous mechanical properties. Since metallic glasses continuously relax with time (age) or inherently are well relaxed after processing, methods to uniformly rejuvenate the material are needed. One approach that has received attention is the so-called cryogenic-cycling method, during which a metallic glass is repeatedly immersed into liquid nitrogen. In some cases, cryogenic cycling is truly efficient in increasing the stored excess enthalpy of metallic glasses, but it does not seem to be universally applicable to all alloys and structural states. The origins for these differences remain unclear due to our limited understanding of the underlying structural evolution. In order to shed more light onto the fundamental structural processes of cryogenic cycling, we pursue in-situ x-ray photon correlation spectroscopy (XPCS) to trace the atomic-scale structural dynamics of a Zr-based metallic glass in two different structural states (ribbon and bulk metallic glass). This method allows calculating the relaxation times as a function of time throughout the thermal cycling. It is found that the investigated glasses exhibit heterogeneous structural dynamics at 300 K, which changes to monotonic aging at 78 K. Cryogenic cycling homogenizes the relaxation time distribution for both structural states. This effect is much more pronounced in the ribbon, which is the only structural state that rejuvenates upon cycling. We furthermore reveal how fast atomic-scale dynamics is correlated with long-time average structural relaxation times irrespective of the state, and that the ribbon exhibits unexpected additional fast atomic-scale relaxation in comparison to the plate material. Overall, a picture emerges that points towards heterogeneities in fictive temperature as a requirement for cryogenic energy storage.

Understanding the fracture toughness of glasses is a fundamental problem of prime theoretical and practical importance. We develop a theory of the notch fracture toughness of glasses using a non-equilibrium thermodynamic approach to the elastoplastic deformation of glassy materials. We show that the notch fracture toughness results from a competition between the initial plastic relaxation timescale, which depends on initial non-equilibrium structural state of the glass (e.g. its age and its preparation protocol), and an effective loading timescale, which depends on the notch radius, the system's geometry and the external loading rate. The initial glass structure, which controls the initial plastic relaxation timescale, is characterized by a non-equilibrium (effective/fictive) temperature. The theory predicts a rather abrupt increase in the toughness – a brittle-to-ductile transition – as a function of the ratio between these two timescales. Novel experiments on various Bulk Metallic Glasses (BMGs), which allow to carefully controlling the initial structural state of the glass and accurately measure the toughness, verify the theoretical predictions. In particular, they show that BMGs can be significantly toughened by carefully controlling their initial non-equilibrium state. These results highlight the importance of timescales competition and far from steady-state plastic deformation dynamics for understanding the toughness of glasses, and open up the way for a broader usage of BMGs as tough, highly-reproducible structural materials.

Deformation of nanoscale metallic glasses has attracted increasing attention but remains controversial due to difficulty in fabrication and testing of small samples. Here, we report the effects of systematic variation in sample-size and testing temperature. Pt-based metallic glass samples with varying diameters (100 nm -500 mm) were prepared by thermoplastic drawing. Multiple samples were deformed at different temperatures and the fracture morphologies were characterized using SEM. The results show that the fraction of vein-like morphology decreases with decreasing sample size and mirror-like fracture surface was observed in samples smaller than about 2 mm in diameter. With further decreasing diameter, transition from shear-localized to necking was observed and the extent of neck increased with decreasing sample diameter. The lowering in temperature had similar effect as decrease in sample diameter. This temperature-size equivalence is discussed in terms of temperature rise and formation of liquid-like layer in shear bands.

In crystals atomic mobility in transport and deformation is governed by structural defects, such as vacancies and dislocations. This was carried over to the study of glasses and liquids, and various defect-like concepts have been proposed, including free-volume, cooperatively rearranging region and shear-transformation-zone (STZ). However, whereas in crystals defects are topologically protected by the lattice and preserve their identity after motion, the same does not apply for liquid and glass. For this reason, today these defects in glass are considered to be transient; they do not pre-exist and disappear after the action. In this talk this view is advanced even further to suggest that the dynamics in liquid and glass is governed not by defects but by the structural medium-range order (MRO) of the bulk. The MRO is defined by the coherence length which characterizes the decay of the oscillation in the pair-distribution function (PDF) beyond the first peak. Because the higher-order peaks of the PDF are broad, ~ 1 Å in width, the MRO does not describe the order at the atomic level. It represents the strength of the coarse-grained density wave, represented by the first peak in the structure function, S(Q). We show that the coherence volume is proportional to the fragility coefficient, m, at the glass transition temperature, Tg, and also to the activation energy of viscosity above Tg. In crystal atoms are mobile only at defects of which density is low. But in liquid all atoms are mobile and participate in transport. Whereas the defect-like sites have a higher propensity to start deformation or atomic transport, as soon as deformation starts extensive atomic rearrangement ensues, and the structure of the final state is very different from the initial one. Furthermore, at the saddle point of transition the effective temperature becomes so high that the system locally melts for a short time, and the system locally loses its thermal history. Therefore, the mobility is not determined by the initial structure but by the resistance to local collective motion which is determined by the MRO. For this reason, the structural coherence, which is a bulk property in equilibrium, controls the dynamics in liquid and glass. Defects are relevant only as outliers in the equilibrium statistical distribution where action could start.

We have undertaken to develop multiscale models of plastic flow and failure processes in amorphous solids, materials that exhibit a complete lack of crystalline order. In this we have taken metallic glass as an exemplar material and applied molecular dynamics simulation to examine the physics at the atomic scale. This has allowed us to uncover the statistics of the shear transformation zone (STZ) defects that control plastic deformation. We have then sought to relate this defect structure to statistical mechanics models of “effective temperature” that characterize the degree of glass disorder in an effort to build a constitutive model of plastic deformation. The constitutive model has been incorporated into a high-fidelity viscoplastic finite differencing scheme that adapts techniques originally developed for solving the Navier-Stokes equation. A novel machine learning algorithm then utilizes the atomistic data to guide the parameterization of the constitutive model so as to inform the continuum method. In doing so we have begun to ask some fundamental questions about the concept of “effective temperature.”
The concept of fictive-temperature has long been utilized to characterize the processing dependence of glass structure, and has recently been shown to be predictive of metallic glass ductility. Some theories have hypothesized that it is actually a real temperature related to the configurational degrees of freedom of the glass, i.e. an “effective-temperature,” notably the shear-transformation-zone (STZ) and soft-glassy-rheology (SGR) theories. We derive a thermodynamic integration scheme for calculating effective-temperature based on a 2-temperature hypothesis. To test this scheme we simulate a binary Cu-Zr metallic glass modeled with an EAM potential. Measures of the energy fluctuations associated with both the fast and slow degrees of freedom are measured during the glass quench. The resulting effective-temperature is consistent with estimates of fictive-temperature obtained from simulation in more heuristic ways. The results indicate that effective-temperature can be understood as a purely structural quantity. The method provides a means to measure the effective-temperature in the absence of fluctuations induced by shear and without resorting computationally expensive and impractical methods for explicitly measuring the configurational entropy.

Metallic glasses can have widely varying thermodynamic and kinetic stability at constant composition depending on their processing history. For bulk glasses, fast quenching or post-synthesis deformation result in a high free energy, less stable states and slow quenching or post-synthesis sub-T_g annealing result in low free energy, more stable states.

For thin films and nanostructures, surfaces play a larger role. We have used electron correlation microscopy experiments and molecular dynamics simulations to show that metallic glass surfaces have approximately ten times faster dynamics than the bulk at the same temperature for temperatures near T_g. The fast surface layer has a glass transition temperature that is suppressed with respect to the bulk by ~20 K. We have extended the understanding of diffusion in supercooled liquids based on caging and hopping of atoms to surfaces to explain this behavior.

Enhanced surface dynamics enable deposition of highly stable thin films, since every atom at some point has been a surface atom and the surface layer maintains sufficient mobility for the atoms to find low-energy configurations even at temperatures well below the bulk T_g. We have deposited Zr₆₅Cu_27.5Al_7.5 metallic glass thin films with varying stability by varying the deposition rate at a substrate temperature of 0.7T_g. The stability of the films is demonstrated by calorimetry and creates changes in films’ modulus and hardness. We show using electron nanodiffraction that more stable films have greater nanometer-scale structural order, demonstrated by an increase in Friedel speckle pairs associated with Bragg diffraction from ordered structures. Analysis of rotational symmetries in nanodiffraction patterns show that the order is dominated by icosahedra, but that crystal-like rotational symmetries are also present. The ability to create glasses with widely varying stability at constant composition enables disentangling the influence of atomic rearrangements created by processing and changes in chemistry.

Additive manufacturing (AM) has emerged as a unique method for producing highly specialized metal alloys and composites. The high cooling rates and customizable cooling rates afforded by AM are particularly useful in the creation of bulk metallic glasses (BMGs) and BMG metal matrix composites. While AM allows for net-shaped fabrication of even the weakest glass-forming alloys, macroscale parts still suffer from embrittlement due to annealing, porosity, partial crystallization and oxygen. In contrast, thin coatings and overlays made from BMGs and BMG composites may be ideal for some wear-resistant applications. This talk will provide an overview of how BMG printing and deposition technologies might be matured and infused into applications. It will cover Fe-based, Zr-based, Cu-based, and Ti-based BMG alloys.

Since the first synthesis of a bulk metallic glass (BMG) by copper mold casting in 1989, a large number of BMGs with centimeter-size diameter were synthesized in a variety of multicomponent alloy systems such as La, Mg, Zr, Ti, Hf, Fe, Co, Ni, Cu, Pd, Pt and Au metal bases. These BMGs are composed of multicomponent with negative and nearly zero heats of mixing and significant atomic size mismatches, while there has been little information on the influence of additional metallic elements with positive heat of mixing on the formation, thermal stability, crystallization behavior and fundamental properties for BMGs. Very recently, the pseudo-high entropy (PHE) BMGs containing solute atomic pairs with positive heats of mixing, which are different from the component rule for ordinary BMGs, have attracted an increasing interest because of their unique properties¹⁾. In particular, the PHE BMGs exhibit two exothermic peaks on the continuously heating DSC curve and the temperature interval between the two peaks is as large as 200-400 K. Besides, no crystalline phase is observed even after annealing for 1-5 h at the temperatures well above the first exothermic peak. The unusual high stability to the progress in crystallization has been reported for various PHE BMGs such as Zr-Al-(Ni,Cu)-Nb, Zr-Al-(Ni,Cu)-Ag, Zr-Al-(Co,Cu)-Ag, Zr-(Fe,Co,Ni,Cu), Ti-Zr-(Cu,Pd)-Nb and Fe-Cr-Mo-C-B-Y systems. These results indicate the possibility of developing a heat resistant BMG by utilizing this novel phenomenon. Here we present the formation, thermal stability, crystallization behavior and mechanical properties of PHE BMGs and to investigate the criterion for the synthesis of BMGs with ultrahigh thermal stability.

1) A. Inoue, F.L. Kong, S.L. Zhu, A.L. Greer, Multicomponent bulk metallic glasses with elevated-temperature resistance, MRS Bull. 44 (2019) 867-872.

The rise of nanotechnology has been propelled by low dimensional metals. Albeit the long-perceived importance, synthesis of freestanding metallic nanomembranes (or the so-called 2D metals), however, has been mainly restricted to elemental metals with a very limited in-plane size (< 10 µm). In this talk, I would like to talk about the development of a low-cost method to synthesize 2D metals through polymer surface buckling enabled exfoliation (PSBEE). The 2D metals so obtained could be as chemically complex as conventional metallic glasses while possessing in-plane dimensions at the scale of bulk metals (> 1 cm). With our approach, we successfully synthesized a variety of 2D metallic glasses with controllable geometries and morphologies. Interestingly, our results clearly show that, as the thickness of the 2D metallic glasses reduces to a few nanometers, its modulus is significantly reduced to only a fraction of the modulus of their bulk counterparts. Aside from rather low elastic modulus, these 2D metallic glasses also exhibit unusual properties, such as good stretchability and tunable optical properties. Finally, we demonstrate that 2D ceramic glasses and 2D metallic composites can be also fabricated through PSBEE aside from 2D metallic glasses.