Symposium MB4 : Glassy, Nanocrystalline and Other Complex Alloy Systems and Their Applications

Most of today’s technologies are based on crystalline metallic, ceramic or semiconducting materials with a wide variety of atomic structures, microstructures and/or chemical compositions. If one would be able to produce non-crystalline metallic, ceramic or semiconducting materials with a similarly wide variety of atomic structures, microstructures and/or chemical compositions, these new kinds of non-crystalline materials may be expected to open the way to new technologies utilizing the new properties of these new non-crystalline materials.
Recently, a new kind of non-crystalline materials called, nano-glasses have been developed that permit the production of non-crystalline materials with atomic structures, microstructures and/or chemical compositions that may be varied in a similarly wide variety of ways as in the crystalline materials on which today’s technologies are based. Contrary to today’s glasses with a homogeneous microstructure, these nano-glasses consist of nanometer-sized glassy regions connected by glass/glass interfaces with new non-crystalline structures (different from the structure of the glassy regions). By varying the sizes and/or the chemical compositions of the glassy regions, the properties of nano-glasses could be varied in partially spectacular ways. For example, FeSc nano-glasses were revealed to be strong ferromagnets although melt-quenched glasses (with the same chemical compositions) were paramagnetic. In the cases of other nano-glasses. the ductility, the biocompatibility, the catalytic properties of nano-glasses were noted to be improved by one or several orders of magnitude in comparison to the corresponding properties of melt quenched glasses with the same chemical compositions. All of the results obtained so far indicate that nano-glasses are new kinds of non-crystalline metallic, ceramic or semiconducting materials with a similarly wide variety of atomic structures, microstructures and/or chemical compositions that may open the way to new technologies utilizing the new properties of nano-glasses.

Compositional patterning in alloys subjected to energetic particle irradiation or severe plastic deformation at elevated temperatures is now understood in terms of a dynamical competition between athermal, ballistic mixing and thermally activated diffusion. In immiscible alloys, the former tends to homogenize the alloy composition whereas the latter causes them to phase separate. We have found recently that while most immiscible alloys behave this way, strongly immiscible alloys, such as Ag-Ni and Cu–W, show compositional patterning even at very low temperatures when thermally activated diffusion is completely suppressed. Irradiated dilute Cu-W, for example, forms a steady state microstructure consisting of 2nm W nanoprecipitates in a highly supersaturated Cu matrix. Shear mixing of dilute Cu-Nb at low temperatures, similarly forms 15 nm Nb nanoprecipitates. These nanoscale compositional patterns are independent of the initial alloy microstructure and thus they appear to be true steady states of the driven system. In this presentation we review these results and offer a model to account for their observation.

While most high entropy alloys have been prepared by melting and casting methods, a limited number of studies involving the preparation by powder metallurgy – mechanical alloying – techniques have been reported, first by Murty and co-workers. Most of this work on mainly 3d transition metals with various additions revealed nanocrystalline microstructures in as-milled powders. These nanocrystalline high entropy alloys exhibited good thermal stability and promising mechanical properties. Our group has studied nanocrystalline high entropy alloys and this talk will describe results on : 1. influence of stacking fault energy on mechanical properties in nanocrystalline NiFeCrCoMn with varying stoichiometry; 2. Influence of nanocrystalline structure and the ordering of Cr in NiFeCrCo; 3. the synthesis of a low density nanocrystalline high entropy alloy with remarkable hardness. The possibility of high thermal stability in nanocrystalline high entropy alloys will be discussed.

Developing classes of metal alloys, such as stabilized nano-crystalline alloys and multicomponent High Entropy Alloys (HEA), exhibit extraordinary mechanical and chemical properties. The structure of conventional alloys based on a single host element derive primarily from the chemical interactions of the components and the free energy as depicted in the equilibrium phase diagram. In nano-crystalline alloys or HEAs minimizing strain, minimizing surface energy, or increasing configurational entropy become primary factors controlling the crystal structure and microstructure often with surprising results. A multitude of alloys can be made using various combinations of elements known to be compatible, and many have already been created. However, for some alloys, chemical incompatibility leads to separation of elements in the liquid phase and makes production by conventional casting or splat quenching difficult[1]. We demonstrate a new method to form alloys from the liquid phase via irradiation of periodic thin films with a femtosecond laser.
We will present Transmission Electron Microscopy (TEM) of nano-crystalline NiW alloy, similar to those produced by electrodeposition[2], that was produced by irradiating a 23 nm thick film composed of 12 alternating layers of 1.4 nm W and 2.4 nm Ni. Femtosecond laser pulses are absorbed in the near surface heating the top 40 nm layer to extreme temperatures and pressures on the order of 6000 C and 50 GPa within a few picoseconds so the layer remains at solid density. The thermodynamic relaxation of the film passes into the “vapor dome” in the Temperature-Density phase diagram, a region of liquid-vapor coexistence where the bonding energy between atoms is low and the kinetic energy is very high. We propose under these extreme conditions Ni and W are allowed to mix thoroughly, then thermal transport into the substrate quenches the mixed layer within a few nanoseconds. It should be noted metals irradiated with a single ultrashort pulse do not resolidify nano-crystalline, but instead regrow epitaxially from the substrate. The thermal and mechanical relaxation of femtosecond laser irradiated multilayer films includes unique, extreme thermodynamic states and thereby provides a new route to synthesize stable nano-crystalline alloys or multicomponent HEAs from the liquid phase.

1. Yeh, J.-W. et al. “Nanostructured High-Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes.” Advanced Engineering Materials 6, 299–303 (2004).

2. Detor, AJ. and Schuh, CA., “Tailoring and patterning the grain size of nanocrystalline alloys.” Acta Mater. 55, 371–379 (2007).

Nanocalorimetry is a calorimetry technique with unprecedented sensitivity and dynamic range, allowing measurements at scanning rates from isothermal to ~10⁵ K/s. We have used this technique to investigate the diffusion kinetics in glass-forming metallic alloys, both in the solid state (SS) and in the supercooled liquid state (SLS). The diffusion kinetics in the SS was evaluated by preparing polycrystalline multilayers that form a metallic glass upon intermixing. The solid-state amorphization process is controlled by diffusion in the metallic glass, and follows Arrhenius kinetics. Both the diffusivity and the activation energy are readily determined from the calorimetry measurements. The diffusion kinetics in the SLS was evaluated indirectly by measuring the crystallization kinetics of the alloys over a wide range of temperatures (T_g - 1.5 T_g). In contrast to diffusion in the SS, diffusion in the SLS does not follow Arrhenius kinetics. Instead, the behavior is well described by a fragility-based model of growth-controlled kinetics that takes into account breakdown of the Stokes-Einstein relationship.

Thermal stabilization of nanocrystalline (NC) metallic alloys against grain growth via grain boundary segregation is a promising design strategy which allows for a retention of their superior mechanical properties when subjected to elevated temperatures during either their fabrication, processing or service life. The exceptional strength predicted for NC Fe is of particular interest, and constitutes the main motivation for this work. Among non-compound-forming solutes, Mg has been identified as a promising candidate for stabilizing NC-Fe due to its very high segregation tendency, as well as its potential for substantial lightweighting. Here we study the microstructure evolution and thermal stability of NC Fe-Mg alloys at both the meso- and nano-scales. Electron microscopy and x-ray diffraction were employed to characterize the grain size, grain boundary character and segregation behavior before and after annealing. These results were compared with Monte Carlo simulations, as well as with the results for unalloyed iron. In-situ annealing in the x-ray diffractometer revealed the unique role of the allotropic α↔γ transformation in NC Fe. Additionally, the role of oxidation and its contribution to grain size retention during prolonged annealing of the alloys is discussed. These results, along with preliminary work on other solutes, help elaborate the design space for NC alloys.

Conventional nanoglasses (NG) are amorphous materials prepared by consolidation method using nanometer-size glassy powders [1]. We have recently synthesized high-quality Ni-P NG using pulse electro-deposition. We studied the thermal behavior and microstructure using differential scanning calorimetry (DSC), small-angle x-ray scattering (SAXS) and time-resolved synchrotron diffraction. The DSC scans revealed higher Tx of NG than those of as-cast Ni-P metallic glass (MG).Model fitting for SAXS profile of NG shows the presence of spheroid particles with a diameter of 6-24 nm. Time-resolved synchrotron diffraction upon heating illustrated that the appearance of the Bragg peaks for NG is ~ 33 K higher than that for MG. Careful pair distribution function (PDF) analysis revealed that the coordination shells at medium-range length scale of NG have larger atomic distance than those of MG. The above results indicate that there exists a glass/glass interfacial structure with unique medium range ordering (MRO), which may play an important role for resisting atomic re-arrangement during crystallization of NG. Our observations also support the findings of a recent simulation paper [2] for ultra-stability of NG.

[1] H. Gleiter, T. Schimmel, H. Hahn, Nanostructured solids–From nano-glasses to quantum transistors [J]. Nano Today, 2014, 9(1): 17-68.
[2] D. Danilov, H. Hahn, H. Gleiter, et al., Mechanisms of Nanoglass Ultrastability [J]. ACS nano, 2016, 10(3): 3241-3247.

To simulate the metallic glasses behaviour there were proposed different models, among which are the free-volume and the quasi-point defects (QPD) models. Another approach based on a phenomenological model is adopted in this study. For future industrial applications (automotive industry, aerospace...), it is essential to have a model which is able to predict the hot forming and the structural behaviour of metallic glasses. Therefore, the purpose of the present work is to create a constitutive model which describes the bulk metallic glasses behaviour at high temperatures (close to the glass transition temperature). Initially the model was based on the generalized Maxwell viscoelastic model. The improved model takes into account the particularities of the Zr_52.5Cu₂₂Al₁₀Ni₁₃Ti_2.5 metallic glass. The behaviour law is implemented in Zébulon software using the finite element method. Additionally, compression tests at high temperature are performed on cylindrical specimens. Finally, simulation results and experimental tests are compared. The parameters matching the behaviour law are found by fitting the simulation results to the experimental data.

High entropy alloys (HEAs) having equimolar multi-components has been being developed with a new concept of structural materials. It is attractive materials for fundamental science and application engineering both. One of the interesting properties of HEAs is excellent phase stability against irradiation damage due to shorter defect lifetime because of multiplicity of chemical species surrounding the defects. In this study, radiation-induced microstructure change and subsequent recovery behavior, that is self-healing, was systematically investigated under two radiation sources; electron and ion, which is a preliminary research to explore the neutron damage.
In order to compare the resistance to irradiation of high entropy phase to low entropy phase simultaneously in one sample, CrFeCoNiCu alloy was selected since it has a composite structure consisting of two phases with sample crystal structure (FCC) but different compositions in um scale. One of them is high entropy phase of CrFeCoNi and the other is conventional phase of Cu solid solution. The hardening effect under ion radiation was examined by indentation test and the corresponding microstructural evolution including swelling, sub-grain formation and defects was analyzed utilizing GIXRD, SEM and TEM(Talos) equipped with super X-EDS. In order to understand the kinetics of defect formation and annihilation of HEA under irradiation, in-situ observation under high energy electron beam was performed. Quick self-recovery and slower defect evolution observed in this study will be discussed with terms of thermal conductivity, distorted lattice structure, sluggish diffusion.

The ideal entropy of mixing for N constituent elements in equimolar concentration is log(N). In reality, differences in atomic size and chemical bonding preferences lead to deviations from ideality that necessarily reduce the configurational entropy. These correlations can be simulated using a first principles hybrid Monte Carlo/molecular dynamics method. Applying the cluster variation formulation to these correlation functions yields quantitative values of the actual mixing entropy. Analyzing these simulations across a range of temperatures using the multiple histogram technique yields the temperature-dependent free energy and total entropy. We can thus separately quantify vibrational and configurational entropy. In this talk I will apply these methods to the refractory high entropy alloys MoNbTaW, CrMoNbV, HfNbTaZr and NbTiVZr and also to AlxCoCrFeNi. Trends in relative entropy will be interpreted according to their interatomic interactions.

We present observations on the targeted and site specific segregation of solute atoms to dislocations and internal interfaces [1] followed by complexion formation [1] and / or nanoscaled, spatially confined reverse transformation of these decorated regions [2-4]. This concept enables us to synthesize self-organized, nanostructured martensite-austenite and alpha-beta Titanium bulk laminates via a simple segregation plus reversion transformation heat treatment. We apply this approach to various types of compositionally lean Fe-Mn, Fe-C and Ti-Mo alloys in conjunction with specific heat treatment concepts for designing ductile martensitic materials by defect-specific segregation plus confined reversion transformation [1-4]. These transformed defect regions in high strength martensitic steels and Ti alloys act as self-organized laminate barriers against dislocation motion and as soft compliance layers, i.e. as mechanical buffer zones impeding crack propagation among lath martensite lamellae. Martensite materials can thus be rendered into ductile materials, substantially deviating from the inverse stress-strain relation.

1. Kuzmina, M., Herbig, M., Ponge, D., Sandlöbes, S. & Raabe, D. 2015, "Linear complexions: Confined chemical and structural states at dislocations", Science, vol. 349, no. 6252, pp. 1080-1083.
2. Raabe, D., Herbig, M., Sandlöbes, S., Li, Y., Tytko, D., Kuzmina, M., Ponge, D. & Choi, P.-. 2014, "Grain boundary segregation engineering in metallic alloys: A pathway to the design of interfaces", Current Opinion in Solid State and Materials Science, vol. 18, no. 4, pp. 253-261.
3. Raabe, D., Sandlöbes, S., Millán, J., Ponge, D., Assadi, H., Herbig, M. & Choi, P.-. 2013, "Segregation engineering enables nanoscale martensite to austenite phase transformation at grain boundaries: A pathway to ductile martensite", Acta Materialia, vol. 61, no. 16, pp. 6132-6152.
4. Kuzmina, M., Ponge, D. & Raabe, D. 2015, "Grain boundary segregation engineering and austenite reversion turn embrittlement into toughness: Example of a 9 wt.% medium Mn steel",Acta Materialia, vol. 86, pp. 182-192.

Novel chip-based fast differential calorimetry (FDSC) is able to determine thermophysical properties at rates of several 10⁵ K/s. Thus we can measure complete time-temperature-transformation diagrams in the undercooled liquid range upon heating and cooling, even for moderate glass formers, and determine critical heating and cooling rates [1]. Further, by investigating a metallic system with sluggish transformation kinetics (metallic glass) with an ultrafast detection technique, we have been able to present evidence for a solid-solid phase transition via metastable melting for an Au-based glass [2]. In this talk, I will describe the corresponding transformation kinetics and discuss the underlying thermodynamics. The concept can be extended to binary metallic glass-forming liquids where various metastable phases form depending on the corresponding rates of heating and cooling. Consequently, this transformation mechanism appears to be a feature of many metallic systems and generate further insight into phase transition theory.

[1] S. Pogatscher, P. J. Uggowitzer, J. F. Löffler, ‘In-situ probing of metallic glass formation and crystallization upon heating and cooling via fast differential scanning calorimetry’, Appl. Phys. Lett. 104, 251908 (2014).
[2] S. Pogatscher, D. Leutenegger, J. Schawe, P. J. Uggowitzer, J. F. Löffler, ‘Solid-solid phase transitions via melting in metals’, Nature Comm. 7:11113, doi:10.1038/ncomms11113 (2016).

Yielding magnetically switchable strains of several percent, ferromagnetic
shape memory alloys have attracted tremendous interest during the past years
for use in contact-less actuators in engineering and biomedical applications.
Energtic ion beams provide a powerful tool to precisely control materials
properties, including phase, the martensite-austenite transformation and
magnetic behavior. After synthesis, application of laser induces shock waves
provides a convenient way of switching phase and aligning martensite variants
- that can be combined in a complementary way with thermally induced phase
changes and magnetically induces reorientation of twin variants. Employing
atomistic computer simulations, we unveil the underlying physics, which is
dominated by changes in short range order / defect generation and
preferential alignment of the martensitic unit cell along the shock wave
direction.

[1] A. Arabi-Hashemi and S.G. Mayr, Phys. Rev. Lett. 109, 195704.
[2] A.J. Bischoff, A. Arabi-Hashemi, M. Ehrhardt, P. Lorentz, K. Zimmer and S.G.
Mayr, Appl. Phys. Lett. 108, 151901

Graded nanocrystalline materials with spatial grain size gradients, such as surface nanocrystalline materials and multilayered structures, are promising to achieve otherwise competing mechanical properties, but are susceptible to microstructure evolution due to the microstructural gradient and the high density of grain boundaries in them. While alloying is known to be able to stabilize nanocrystalline grains, alloying effect on kinetics in an imposed spatial grain size gradient is not well understood. This talk will present our modeling of microstructure evolution in graded nanocrystalline alloys with spatial grain size gradients. Our study provides quantitative understanding of the concomitant evolution of solute distribution and the spatial grain size gradient.

The studies on dynamics in glassy materials are particularly challenging because of their strongly disordered atomic nature. In the picture of potential energy landscape (PEL), the evolution of inherent structure (IS) energy can be attributed to the competitions between activations and relaxations. In particular, whether glasses will undergo aging or rejuvenation depends on whether the activation barrier is smaller or larger than the relaxation energy. We prepared 6 metallic glass samples with different thermal histories (from 10¹³K/s to 10⁹ K/s), and further explored their activation barrier and relaxation energy spectra through advanced atomistic sampling technique. The spectra are found sensitive to the systems’ IS energy. Stemmed from such dependence, a self-consistent equation in describing IS energy evolution has been derived. Without any empirical parameter, this equation can well explain the equilibrium line of supercooled liquid and the cooling curves at different rates directly obtained by MD studies.

Recent advancement in bulk metallic glasses (BMGs), whose properties are usually superior to their crystalline counterparts, has stimulated great interest in their production and application. While a great deal of effort has been devoted to this field, the fabrication of BMGs with large cross sections has remained an alchemist’s dream because of the limited glass forming ability (GFA) of these materials. The small molten pool deposition characteristic with point by point in laser rapid prototyping (LRP) makes this technology be able to be used to prepare BMGs without the limitation of critical cooling rate and critical casting diameter. This can further promote the application and development of BMGs. In the present study, the microstructural evolution during pulsed laser deposition of Zr-based metallic glass was particularly characterized using the state-of-the-art facilities. The underlying deformation mechanisms related to the improvement of strength and ductility were systematically investigated by focusing on the interaction between nanocrystallines/ductile dendrites and deformation units, i.e. shear bands, dislocations or twins. The improved nanoindentation creep resistance and strain hardening behaviors were attributed to both the initiation of the deformation bands or twins in the dendrites and the suppression of the highly localized shear deformation in the nanocrystallines-included amorphous matrix. Based on the present model and experiment results, LRP may be a promising way to fabricate high-performance BMG composites without size and shape limit.

This talk will describe the results of recent computer simulation studies investigating the effect of applied pressure on the structure, stability and properties of model metallic glass systems. For the Cu-Zr system, we present results investigating glasses prepared through quenching of liquids with varying imposed pressures. The distinct configurational states that are achieved by such a process act to tune the local atomic environments, including both topological and chemical short-range order. The resulting glasses are shown to be less energetically stable, while displaying a higher atomic density, and a significant increase of icosahedral short-range order. Such observations are contrary to the current understanding of the atomic structure in MGs, and demonstrate the importance of accounting for changes in chemical short-range-order in predicting the effects of pressure in the preparation of the metallic glass structures. The work is complemented by studies of glasses prepared through normal melt quenching at ambient pressure, which are subsequently exposed to applied pressures. In these simulations we examine the effect of pressure on structure, elastic, relaxation, electronic and vibrational properties. Overall, the computer simulation studies reviewed in this work compliment existing experimental observations and provide a rich picture of the effects that pressure can have on the structure and properties of metallic glasses. This work is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Science and Engineering Division, under Contract No. DE-AC02-05CH11231.

Hydrogen-induced phase separation of Cu-Zr binary amorphous alloys during hydrogen gas charging at elevated temperature was demonstrated: the homogeneous binary alloy was decomposed into pure Cu and Zr-hydride by absorbing hydrogen into the structure. The decomposition, which is attributed to the opposed affinity to hydrogen of Cu and Zr, took place in nanometer scale. The structure after hydrogen absorption was analyzed using x-ray diffraction, ultra-small angle neutron scattering, and electron microscopy. Calorimetric study was carried out to reveal the transition of Zr-hydride from tetragonal to cubic phase. Also, the effect of hydrogen pressure and alloy chemistry on the phase formation was discussed.

Multicomponent alloys for high temperature applications often exhibit intriguing and complex oxidation behavior. Oxidation can be viewed as a sequence of localized phase transformations fed by a continual supply of oxygen from the environment and cation species from the substrate. These phase transformations couple diffusion with structural changes. Both chemical and mechanical driving forces play a crucial role in selecting the direction of an oxidation process. In this talk we will describe our efforts at developing a first-principles underpinning of oxidation using multi-component alloys based on Ni, Co and Ti as examples. A key ingredient in any treatment of oxidation is a self-consistent thermodynamic description of all metal and oxide phases. High temperatures introduce challenges arising from an increased importance of entropy. First-principles approaches must therefore rely on statistical mechanics techniques to account for configurational and vibrational excitations. At very high temperatures, traditional approaches relying on the harmonic approximation to treat vibrational excitations break down and methods that account for anharmonicity must be used. In fact many high temperature solids are dynamically unstable at low temperature and become entropically stabilized by anharmonic excitations. The complexity of high temperature thermodynamic descriptions is compounded by the importance of configurational entropy in multicomponent alloys and oxides.

Amorphous ferromagnetic alloy with the composition Fe56Co24Nb4B13Si2Cu1 was obtained by rapid quenching from the melt. Samples cut from the ribbons were annealed at 450, 550, 650 and 750 C in a vacuum furnace. 57Fe Mossbauer spectroscopy was used to identify the phases formed based on the refined values of the hyperfine parameters. The as-quenched specimen was analyzed with a hyperfine magnetic field distribution and corresponded to an in-plane orientation of the magnetic moment directions. The sample annealed at 450 C was found to be in a nanocrystalline state due to observation of the (FeCo)-Si alloy with the DO3 structure. The balance of the composition was represented by a metalloid-enriched amorphous grain boundary phase. In contradistinction to this, the samples annealed at 550-750 C were totally crystallized, but the new phases formed were alpha-(FeCo), (FeCo)2(BSi) and (FeCo)3(BSi). These findings suggest that nanocrystallization is obtained only at select processing temperatures. A new set of Mossbauer spectra was obtained by recording simultaneously the intensity transmitted by a sandwich of the sample with the stainless steel etalon, based on the dual absorber method recently introduced by us. The values of the recoilless fraction can be derived from the relative spectral areas. The f factor value dropped from 0.6 to 0.37 for the sample annealed at 450 C, consistent with the onset of nanocrystallization in the system. For the completely crystallized specimens, the f factor maintained values close to 0.5. This indicates that the presence of quenched-in stresses may play a role in the ability of samples to undergo recoilless emission and absorption of gamma rays.

As is known, the formation of metallic glasses is always a non-equilibrium process, due to the severe limitation of kinetic conditions, complicated intermetallic compounds are not able to nucleate and grow. The competing phase against the metallic glass is therefore the solid solution frequently of a simple structure. Consequently, the issue related to the glass formation is converted into comparing the relative stability of the solid solution to its amorphous counterpart, which was decided by the atomic interaction. Thus, it is a good choice to predict the glass formation compositions from interatomic potential, and tow ternary system has been well predicted.
A realistic interatomic potential is constructed for the Al-Mg-Ca [1] system under the smoothed and long-range second-moment approximation of tight-binding formalism, and then applied in Monte Carlo simulations to compare relative stability of crystalline solid solution versus its disordered counterpart using solid solution models. Simulation results not only predict a pentagonal region, within which the Al-Mg-Ca metallic glass formation is energetically favored and the region is defined as glass formation region (GFR), but also determine the amorphization driving force (ADF), i.e. the energy difference between the solid solution and disordered state. The ADF is proposed to be correlated positively with the glass forming ability (GFA), suggesting that the larger the ADF is, the easier the amorphous alloy is to be produced or the more stable the amorphous alloy is. The predictions are fairly consistent with the experimental results reported so far.
With the aid of ab initio calculations, the realistic interatomic potential of Al-Ca-Cu [2] system is constructed and then applied to molecular dynamics simulation to investigate the formation of Al-Ca-Cu metallic glasses. The simulations predict a hexagonal composition region, i.e., glass-formation region, within which an amorphous alloy is energetically favored to form. Moreover, the energy difference between the solid solution and it amorphous counterpart, defined as the amorphization driving force, is also calculated. The calculation shows that the alloys locating the sub-region of Al_xCa_100-x-yCu_y, (x=25-35; y=25-40), have larger amorphization driving forces. It suggests that the amorphous alloys in the sub-region are more obtainable or stable than others in the Al-Ca-Cu system. The simulation and calculation help understanding the metallic glasses formation and provide some guidance in designing the composition the Al-Ca-Cu glass alloys.

1.Zhao, S. Li, J. H. Liu, J. B. Li, S. N. Liu, B. X. RSC Advances, 2015, 113, 93623
2. Zhao, S. Li, J. H. Liu, B. X. Journal of Materials Science, 2016,14, 6600

Identifying and characterizing the properties of metallic glass alloys is traditionally an arduous process, requiring the serial casting of numerous compositions in an attempt to identify the “best” glass former. While empirical rules exist to guide the identification of possible glass forming compositions, these rules provide little insight into the critical cooling rate required, the thermal stability of the resulting glass, or other properties of interest. Recently, our group has developed a high-throughput, laser-deposition-based fabrication approach that enables the efficient evaluation of a large multicomponent composition range in a single fabrication step. In this presentation, we will discuss our studies of Cu-Zr, Cu-Zr-Ti and Zr-Al-Ni alloy systems. Continuous, compositionally graded deposits were rapidly screened for glass formation by a simple optical microscopy-based observation of the surface topography of the deposited material. Regions with very smooth as-deposited surfaces were confirmed to be amorphous via TEM. By varying the laser processing parameters during deposition, and thereby controlling the cooling rate of the melt, the relationship between composition and the critical cooling rate required for glass formation could be experimentally deduced within a limited number of experiments. Furthermore, using the same laser deposition technique to create discrete composition libraries, we applied nanoindentation methods to quickly establish the relationship between composition and mechanical properties over large regions of composition space. Trends in the indentation hardness and modulus with composition provide further insight into the underlying glass structure and suggest an efficient means for identifying glasses with optimal combinations of glass forming ability, elastic modulus, and ductility.

The dynamical behaviour of atoms and their short-range order are crucial for understanding the glass-forming ability of dense metallic liquids. To investigate the short-range order (SRO) we measured partial structure factors for the melts of Zr-Ni and Nb-Ni by combining electrostatic levitation with neutron diffraction and isotopic substitution. Zr₃₆Ni₆₄, Zr₅₀Ni₅₀, Zr₆₄Ni₃₆ and Nb₄₀Ni₆₀ alloy melts have been studied as a function of temperature, which all exhibit a pronounced chemical SRO [1, 2].
Mode coupling theory (MCT) allows us to establish structure-dynamics relations for transport coefficients such as diffusion coefficients, using static partial structure factors as an input [3].
The self-diffusion coefficients derived by MCT show the same trends with respect to composition and temperature dependence compared to experimentally determined self-diffusion coefficients measured by quasi-elastic neutron scattering as well as radiotracer measurements [4, 5].
For analyzing the impact of chemical SRO on diffusion, we performed also MCT calculations on a chemically randomly ordered hard-sphere model, which predict a composition-independent ratio of diffusion coefficients by a factor of 1.9(1.5) between Zr(Nb) and Ni.
However, self-diffusion coefficients calculated by MCT using the measured partial structure factors show that diffusion is partly coupled, exhibiting a distinct composition dependence of the ratio of diffusion coefficients. We will discuss this effect in terms of variations of the chemical SRO and of the fraction of different atomic nearest-neighbours in the melt.

[1] D. Holland-Moritz, S. Stüber, H. Hartmann, T. Unruh, T. Hansen, A. Meyer, Physical Review B 79, 064204 (2009)
[2] D. Holland-Moritz, F. Yang, J. Gegner, T. Hansen, M. D. Ruiz-Martín, A. Meyer, Journal of Applied Physics 115, 203509 (2014)
[3] Th. Voigtmann, A. Meyer, D. Holland-Moritz, S. Stüber, T. Hansen, T. Unruh, EPL 82, 66001 (2008)
[4] D. Holland-Moritz, S. Stüber, H. Hartmann, T. Unruh, A. Meyer, Journal of Physics: Conference Series 144, 012119 (2009)
[5] S.W. Basuki et al., to be published (2016)

3D Metallic glass architectures are fabricated through different thermoplastic forming (TPF)-based additive manufacturing methods. A wide variety of metallic glass architectures with different relative densities and topologies covering a broad range of length scales on microstructural feature sizes can be fabricated. Through manipulating the processing protocols and the architectural features design, metallic glass architectures with benchmark mechanical properties such as very high elasticity and elastic energy storage can be realized. The combination of metallic glass inherent properties, and introduced versatile additive manufacturing fabrication methods opens a new window for structural and functional applications of metallic glasses.

In a series of works the author and colleagues studied the surface structure and properties of bulk metallic glasses (BMGs) by scanning probe microscopy and other methods in metallic state as well as with native and artificial surface oxides. The existence of the atomic clusters (~1 nm) was found the metallic glassy phase [1]. The existence of these clusters leading to hard and soft zones is likely responsible for the changes in properties observed on cryogenic cycling treatment of BMGs [²]. Moreover, the formation of a native oxide layer on the surface of bulk metallic glasses was found to influence significantly the nanoscale tribological properties and mechanical behavior of the BMGs used in micro and nanodevices [3]. We also conducted a combined state-of-the-art experimental technique study of the atomic structure, oxidations states and electrical conductivity of the native surface oxides formed at ambient conditions on the Cu-Zr-Al [4] and Ni-Nb [5] BMGs by aberration-corrected scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS) and conductive atomic force microscopy (AFM). This allowed shedding light on the atomic structure, metal oxidation state, growth behavior and nanoscale electrical properties of the surface oxides. The conductive AFM measurements reveal that the electrical conductivity of the native oxide layer transits from the initially metallic to a nonlinear one after exposure in air, and finally it becomes an insulator. These findings represent a significant step forward in the knowledge of amorphous surface oxides on BMGs and open up the possibility of fabricating nanoscale electrical devices based on BMGs with controllable conductivity.
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4. D.V. Louzguine-Luzgin, C.L. Chen, L.Y. Lin, Z.C. Wang, S.V. Ketov, M.J. Miyama, A.S. Trifonov, A.V. Lubenchenko and Y. Ikuhara, “Bulk metallic glassy surface native oxide: Its atomic structure, growth rate and electrical properties”, Acta Materialia, 97, (2015), 282–290.
5. A. S. Trifonov, A. V. Lubenchenko, V. I. Polkin, A. B. Pavolotsky, S. V. Ketov and D. V. Louzguine-Luzgin, “Difference in charge transport properties of Ni-Nb thin films with native and artificial oxide”, Journal of Applied Physics 117, (2015) 125704.

Plastic deformation of large-sized metallic glasses (MGs) is highly localized into shear band under uniaxial stress. Associated with this, the deformation behaviour of metallic glasses is characterized with 2% elastic limit and very little plastic strain. They usually failure in a catastrophical manner at room temperature. If the root cause of the brittle failure, i.e. the shear banding can be suppressed, the deformation behaviour of metallic glasses would be expected to be quite different at room temperature. In our talk, we will show that bulk metallic glasses are capable plastic deformation to large tensile extension and the fracture is ductile in nature with all the signature characteristics of ductile metals. The mechanism for such a difference will also be discussed.

Glasses play an important role in technology as a result of their macroscopic homogeneity (e.g., the clarity of window glass) and the ability to tune properties through composition changes. A problem with liquid-cooled glasses is they exhibit marginal kinetic stability and slowly evolve towards lower energy glasses and crystalline states. We have used physical vapor deposition and the mobility of glassy surfaces to prepare what are likely the most stable glasses on the planet. Our materials have the properties expected for “million-year-old” glasses, including high density, low enthalpy, and high mechanical moduli. Surprisingly, these glasses “melt” like crystals, with a constant velocity transformation front. In addition, these efficiently packed glasses suppress the amplitude of the beta relaxation process and also the number of tunneling two-level systems.

The interesting properties of vapor-deposited glasses all arise from the mobility of glass surfaces. During deposition, molecules near the free surface have the opportunity to sample many different packing arrangements. This leads to highly equilibrated glasses, as measured by enthalpy and density. It also leads to oriented glasses, as characterized by ellipsometry and spectroscopy. Most recently, we have shown that vapor-deposited glasses of liquid crystals can achieve a range of ordered microstructures not accessible at equilibrium and that highly aligned smectic layers can be produced directly by deposition even on substrates where such high levels of organization cannot be achieved by thermal annealing. These developments present significant opportunities to expand our understanding of amorphous packing and to design new anisotropic solids for applications such as organic electronics.

Series of instrumented nano-indentations were performed at room temperature to analyze shear bands in a Mg65Cu12.5Ni12.5(Ce75La25)10 metallic glass. Two statistical investigations were performed. First, depth variation at constant load was used to measure activation volume controlling shear band formation. The series revealed a large scattering of data not following a normal distribution and spanning from 100 A°³ to 800 A°³. A model is proposed to define a relevant and unique value of the activation volume of about 60 A°³. Second, serrations size in the loading part were measured and a Poisson law was fitted to their distribution. In particular, the statistical analysis contains structural length which scales with activation volume determined in the constant loading investigation.

Thin film metallic glasses (TFMGs) have attracted recently considerable interest as they are able to improve the mechanical properties of the substrate they cover. The microstructure of these TFMGs can be tailored to obtain nanostructured amorphous thin films. These thin films present an important interest for catalytic and biomedical applications. However, the true mechanism of formation of these amorphous nanostructures still needs to be investigated.
In the present study, we deposited Au-based and Zr-based thin film metallic glasses on Si wafers under different conditions. The deposition occurred under different working pressure of Ar ranging from 4 µbar to 100 µbar. All thin films were confirmed to be amorphous by XRD measurements. However, we found important differences in the microstructure of the thin films. They were homogeneous for low working pressure, nanoglobular for intermediate working pressure and nanocolumnar for high working pressure. The mechanical properties of these thin films were measured by nanoindentation. The shear bands around Vickers indenter imprints were also observed under SEM, to understand the deformation behavior of the different thin films. The influence of the deposition time and substrate initial roughness were afterward studied, with deposition time ranging from 5 min to 1 h. The size and the high of the nanoglobules increased with deposition time. We could also observe a transition from a homogenous thin film to a nanoglobular thin film by increasing the deposition time at an intermediate working pressure. Finally, an increase of the substrate initial roughness allowed the nanoglobular microstructure to be formed at lower working pressure.

A glass is defined by its temperature and its fictive temperature. Within the material class of bulk metallic glasses (BMGs), a wide range of fracture behavior has been reported, ranging from ideal brittle to exceptionally tough. We have recently developed a method to precisely and repeatedly determine fracture toughness. Here we will use this method to study the effect of fictive temperature on the fracture toughness of model BMGs. Fictive temperature is varied through aging and controlled cooling strategies. We also employ processing protocols, which allow us to untangle effects of Debye-Grueneisen contraction and fictive temperature effect. Fracture toughness measurements are accompanied by structural characterization using synchrotron x-ray diffraction, elastic constants measurements using speed of sound measurements, as well as thermal analysis.

We observed that the widely utilized ratio of shear modulus to bulk modulus for predicting ductility in BMGs is insufficient to explain the range behaviors in BMGs, especially as a response to fictive temperature change. We propose a mechanical fragility, which describe the change in fracture toughness over fictive temperature.

Introduction of transformation induced plasticity(TRIP) effect by adding metastable austenite particles to amorphous matrix have been shown as the successful method leading to work hardening in bulk metallic glass composite(BMGC). Although, TRIP effects are well known phenomenon in conventional materials such as austenitic steels, the deformation mechanisms in the BMG composite should be sorely different and need to be investigated.
In the present study, we investigated the TRIP behavior of Cu-based BMG composite, containing NiTi austenite phase as a 2nd phase. The martensitic transformation of 2nd phase and resultant load bearing, stress/strain partitioning were analyzed by the integrated approach of in-situ synchrotron radiation during compression test. Additionally, DFT-MD simulation was carried out to study shear banding of amorphous matrix in a sub-nm scale. Finally, delocalized shear banding occurred at amorphous matrix, which can be a critical advantage of TRIP assisted BMG composites.

Magnesium-based bulk metallic glasses (BMGs) exhibit high specific strengths, yet are prone to brittle failure and tend to exhibit low thermal stability. In this talk we will present recent developments for a range of new magnesium-precious metal-based BMGs, some of which exhibit high glass transition temperatures (>200 °C), and the highest compressive plastic (1.6%) and total (3.8%) strain of any Mg-based glass reported to date.
These alloys were designed for high ductility utilising atomic bond-band theory and a topological efficient packing model. Here it is postulated that the degree of electronic charge transfer between the constituent elements needs to be minimized in order to decrease directional bonding and thus facilitate plastic strain.

Despite their wide technological importance, the influences of the microstructure of galvanized coatings on their corrosion behaviors is poorly studied, at least in part due to the coatings’ intricate, nano-scaled features that require advanced instrument for examination. The present study employs modern microscopy techniques to systematically analyze the microstructure and nano-scaled features of galvanized coatings and their passivation layers, and establishes a microstructure-corrosion property relationship framework for them. Three representative sets of alkaline non-cyanide based galvanized coatings produced under different conditions are studied, using field emission scanning electron microscopy (FE-SEM), X-ray diffractometry (XRD), focused ion beam (FIB), and transmission electron microscopy (TEM). The corrosion properties of the coatings are then evaluated in light of the grain structure, crystallographic texture, and internal porosity, and general principles for improved coating performance are identified.

Iron and magnesium represent a very attractive materials association in order to combine strength and lightweight. The main difficulty in fabricating Fe–Mg alloys is that iron and magnesium have extremely low mutual solubility and do not form intermetallic compounds, so it is not possible to use phase transformations or precipitation to generate nano/microstructures and the lack of interaction between Fe and Mg makes difficult to produce reliable exogenous composites.

In a recent paper, we have outlined a non-conventional metallurgical route that uses spark plasma sintering to simultaneously decompose and consolidate a precursor bearing Fe and Mg intimately bonded at the atomic scale in a complex hydride.

The nanostructured hybrids of iron and magnesium thus obtained, where the size distribution spans from few nanometers to few hundred of nanometers, exhibit superior specific properties and fill holes in the materials-property space for light structure.

In this presentation, we will utilize three dimensional and multi-modal correlative imaging techniques which allow for a detailed understanding of the internal structure through length scales. By characterizing the microstructural and compositional distributions of the nano-networks of iron and magnesium at different scales, we will illustrate the value of these approaches to gain a better understanding of the effect of microstructure on properties.

Recent experiments on flat multi-component glass surfaces have suggested that a particular surface’s propensity to absorb water may play a critical role in how that surface accumulates and dissipates electrical charge. It is believed that a key driver for glass surface-water reactivity may be structural defect concentration(s) at the surface, which can be largely influenced by bulk composition. To further explore these hypotheses, a series of CAS glasses were modeled using classical Molecular Dynamics (MD) with the primary goal of understanding how glass composition impacts structural defect concentrations (NBO, under-coordinated Si and/or Al, etc.) in the upper layers (~5Å) of the surface. Concurrently, CAS glass surfaces with the same compositions were prepared in the laboratory and analyzed for charge response at variable humidity using a Rolling Sphere Test (RST) and a newly developed metrology for contact charging phenomena called an Electrostatic Gauge (ESG). Molecular water interaction with the CAS surfaces was studied using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The results of these experiments, along with the corresponding MD calculations, show that glass bulk chemistry and resulting surface defect states (most likely NBO) represent crucial driving factors in how glasses behave when placed in glass-metal contact systems.

Pure Cu and Cu-Al alloys with varying stacking fault energies (SFEs) were processed by surface mechanical attrition treatment (SMAT) at room temperature (RT) and liquid nitrogen temperature (LNT) respectively. Gradient structures (GS)with grain size increasing gradually from the top surface to the central region were generated on the surface layer of the sample. LNT SMAT samples exhibited higher strength but lower ductility compared to the RT SMAT counterparts.Lowering the SMAT processing temperature leads to an increase of the deformation twins and dislocation densities. In addition, a thicker GS layer with finer grains was obtained in the LNT SMAT-processed sample. The results indicated that LNT SMAT process can effectively suppress the dynamic recovery and recrystallization and produce ultrahigh strength of pure Cu and Cu-Al alloys without seriously sacrificing the ductility.

Optimal mechanical property combinations often require microstructures with phases or zones of different mechanical characters. However, phase separation is not always an option for all alloy systems. Here in this work, we explore the benefits of the intrinsic heterogeneities developed during cold-working, to design microstructures with bi-modal grain size distributions and site-preferential nano-precipitation, which exhibit good combination of mechanical properties. This design strategy is successfully validated on two distinctly different alloy systems: a β-Ti gum metal and an austenitic high-Mn lightweight steel. All microstructures (as-processed and deformed) are characterized employing various in-situ/post-mortem scanning electron microscopy, transmission electron microscopy and X-ray diffraction based techniques. In both of the two systems, the target microstructures are achieved by controlling the recrystallization progress in the holding stage and subsequently triggering nano-precipitation in the cooling stage. Mechanical properties are evaluated employing digital image correlation at both macro- and micro-scales, revealing for both of the studied alloys, mechanical properties which overcome the inverse strength-ductility relationships of conventional microstructures.

Graphene nano-ribbons and nano-sheets are produced in the crystal lattice of metals by a process we call “electrocharging assisted process” (EAP) at high temperatures. In this process the metal is heated above its melting temperature together with particles of activated carbon. The mixture is stirred, and a high current is applied between a graphite electrode inserted into the metal and the crucible containing the mixture. The current is believed to induce ionization of the carbon from the particles followed by polymerization with other C ions and bonding to metal atoms forming ribbons and chains. Raman scattering, X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) of the resulting materials, called covetics, reveal the presence of graphitic carbon with mostly sp2 bonding and small amounts of sp3 bonding. Transmission electron microscopy (TEM), in combination with EELS spectrum imaging, indicate that the graphitic regions form graphene nanoribbons and nanosheets which have a preferred orientation with the lattice of the metal.^1,2. The graphitic nanostructures give rise to an increase in the ultimate tensile strength ³.and changes in the electrical conductivity of the metal. For copper with carbon the conductivity first decreases and then increases with carbon content. Films of copper covetic deposited by e-beam evaporation and pulsed laser deposition (PLD) using bulk copper covetic as target show higher transmittance than pure copper films of the same thickness and higher resistance to oxidation under ambient conditions.⁴ Density functional theory and first principles calculations of the phonon density of states indicate strong C-metal covalent bonding at edges of graphene nanoribbons and sheets. The C-metal bonds are Raman active and produce weak signals in the Raman spectra. We will present structural characterization and electrical measurements of bulk Cu, Al and Ag covetics as well as transmittance and resistance Cu covetic films grown by e-beam deposition and PLD.
* Funded by ONR Award No N000141410042, ANL/DOE subcontract No. 6F-30062 and DARPA/ARL under Grant No. W911NF-13-1-0058.
¹ L. G. Salamanca-Riba, R. A. Isaacs, M. C. LeMieux, J. Wan, K. Gaskell, Y. Jiang, M. Wuttig, A. N. Mansour, S. N. Rashkeev, M. M. Kuklja, P. Y. Zavalij, J. R. Santiago, and L. Hu, Advanced Functional Materials 25, 4768 (2015).
² H. M. I. Jaim, R. A. Isaacs, S. N. Rashkeev, M. Kuklja, D. P. Cole, M. C. LeMieux, I. Jasiuk, S. Nilufar, and, L. G., and Salamanca-Riba, Carbon 107, 56 (2016).
³ D. R. Forrest, I. Jasiuk, L. Brown, J. P., A. Mansour, and L. Salamanca-Riba, in Nanotech 2012 (Santa Clara, CA, June 18-21, 2012, 2012).
⁴ R. A. Isaacs, H. Zhu, C. Preston, A. Mansour, M. LeMieux, P. Y. Zavalij, H. M. I. Jaim, O. Rabin, L. Hu, and L. G. Salamanca-Riba, Applied Physics Letters 106, 93108 (2015).

Nanoimprint, mechanical injection and dealloying methods have been used to make metallic glass in nanometer scale. But nanoimprinting is limited in the metallic glass with large super-cooled region and strong oxidation resistance; Injection is impeded by the high template temperature; and dealloying process is widely used but limited by its electrochemistry nature to tune compositions. Therefore, there is still a need to for a simple and cheap method to make thin nanomaterials. With the help from eletrospinning, cosputtering and thermal evaporation, a new kind of transparent conducting electrode is made from metallic glass that exhibits both superior eletrochemical performances and remarkable mechanical flexibility under both stretching and bending stresses. In this research, the nanotrough is composed of a 100 nm free-standing soft magnetic CoFeTaB metallic glass substrate and a 5 nm L1₀ phase FePt (111) layer grew on it. We demonstrate the practical suitability of our nanotrough by fabricating a flexible transparent electrode and compare it with hydrothermal nanoparticles in oxygen reduction reaction performance.

Bulk metallic glasses (BMGs) and nanocrystalline metals (NMs) have been investigated extensively due to their superior mechanical properties, most notably, their high strength and high elastic limit. Despite these excellent mechanical properties, the practical application of such materials has been limited by their low ductility at room temperature and their poor microstructural stability at elevated temperatures. Thus, there is a clear need for a metallic material system which can overcome the performance shortcomings of BMGs and NMs.
Recently, we discovered novel Cu-based and Ni-based metal-intermetallic nanostructured composites (MINC) which exhibit high yield strengths (over 1.7 GPa), high compressive ductility (over 20%) and superior microstructural stability at a elevated temperature. Rapid solidification produces a unique ultra-fine microstructure that contains a large volume fraction (over 60%) of a strong intermetallic compound phase, which is responsible for the high strengths measured in our MINCs. Mechanical and microstructural characterizations reveal that there is substantial accumulation of localized shear displacement at the metal/intermetallic phase boundaries. This indicates that plasticity is controlled mainly by the structure and distribution of phase boundaries. In this work, we investigated how the cooling rate during solidification process affects the structure and distribution of phase boundary as well as plastic deformation mechanisms. We confirmed that as the cooling rate becomes faster, MINCs become much more ductile while most materials becomes brittle. We explain this counter-intuitive behavior in terms of the smoothness and connectivity of phase boundary in ultra-fine grained structure. Advanced electron microscopy was used to understand the relationship between cooling rate, phase boundary and ductility. Our results will offer the methodologies to improve the mechanical properties of metal-intermetallic nanostructured composites.

The incorporation of carbon nanostructures into aluminum alloys, such as Al6061 and Al7075, has the potential to further improve the mechanical, electrical and anti-corrosion properties of these alloys. We report on a novel method to incorporate up to 10.0 wt% carbon into the crystal structure of Al 6061 alloys to form a new material “Al Covetics”. In this method, a DC current (up to 100A) is applied to molten Al metal containing activated carbon particles in an enclosed inert (with Ar flowing) environment. The current facilitates ionization of the carbon atoms and their bonding to each other, forming graphitic chains and layers along preferential directions of the Al lattice.
X-ray photoelectron spectroscopy (XPS) depth profiles determine that at least 3at% carbon is present in Covetics sample. The carbon peak decomposition in XPS from the covetics indicates sp² and sp³ bonding from graphitic structures and carbide bonding from Al-C bonds. In contrast, XPS from the Al6061 source material exhibits a carbon peak for which decomposition only shows hydrocarbons and contamination from Ar sputtering. Raman scattering supports that sp² bonding of carbon is found all over the bulk of the Covetics. Especially, Raman mapping of the G and D peaks of graphitic carbon prove the role of the current in ensuring that the carbons not only remain but also spread evenly into the matrix of the metal by electro-static force.
Strong carbon signals are also observed everywhere in Covetics with Energy Dispersive X-ray Spectroscopy (EDS) mapping; while only weak hydrocarbon signals are detected in original Al6061 alloys.
A weak spot pattern that does not correspond to Al is observed in electron diffraction patterns, in addition to spots due to the Al 6061 host, which demonstrates an evidence of 3D epitaxy of carbon in Al matrix. In Electron Energy Loss Spectroscopy (EELS), a sharp C-K edge is obtained at 284eV in the spectra from Al6061 Covetics, which further confirms the presence of sp² bonding in Covetics.
X-Ray Diffraction (XRD) peak fitting reveals that lattice constant and crystal size of Covetics are smaller than pure Al6061, which may result from lattice distortions due to carbon insertion. Generally speaking, our fabrication method is efficient for making Al6061 Covetics with better distribution of carbons in Al matrix. In addition, the dependence of the mechanical, electrical and structural properties of Al Covetics on carbon content from 3 to 10 wt. % will be presented.

Supported by ONR grant N000141410042 and ANL/DOEANL/DOE contract 6F-30062.

Novel high-carbon alloys called covetics are known to improve upon the mechanical and electrical properties of several common metals and alloys. Electrical current applied during covetics processing ionizes carbon atoms and causes nanoscale graphitic ribbons and chains to form within the metal lattice. Despite the potential of Al covetics to improve desirable properties of widely used Al alloys, the atomic-scale mechanism is not fully understood.

We perform Raman spectral deconvolution and mapping to characterize the nature of the bonding and spatial distribution of carbon in Al covetics. Raman spectra are subjected to nonlinear iterative fitting and principal component analysis to identify their major and intermediate peaks. Prominent G and D peaks throughout the covetics provide evidence of sp² carbon bonding which are not observed in the parent alloys. Relative intensities of the G and D peaks reveal pronounced spatial variation in both the degree of graphitic order and the nano-graphite crystallite size. Large regions of the covetics exhibit greater order than the activated carbon precursor material used in fabrication. Additionally, we find significant shifts in the G peak positions to higher wavenumbers compared to those of pure graphite, which may indicate decreased interatomic distance and strain in the graphitic structures within the covetic lattice.

X-ray Photoelectron Spectroscopy (XPS) peak deconvolution confirms the presence of sp² bonding as well as carbide Al-C bonding in the covetics. A characteristic sp² carbon edge is observed with Electron Energy Loss Spectroscopy (EELS), further confirming the Raman results. Potential evidence of Al-C bonding in Raman spectra will also be discussed.

Supported by ONR grant N000141410042 and ANL/DOEANL/DOE contract 6F-30062.

Nanocrystalline diamonds are of great interest because of their high mechanical strength and thermal stability. Fundamentally, there are two categories of nanocrystalline diamonds; one is diamond film prepared by chemical vapor deposition (CVD) and the other is bulk diamond prepared using a high-pressure and high-temperature (HPHT) method.
We prepared boron-doped nano-polycrystalline diamond (B-NPD) by HPHT, without any graphitic component or other non-diamond components such as boron carbide. B-NPD was directly converted from boron-doped graphite into diamond by HPHT. B-NPD was synthesized with boron concentrations up to 1x10²¹ cm^-3; the concentration was measured by secondary ion mass spectrometry. The absence of graphitic components or sp² as observed from the X-ray diffraction and Raman spectroscopic measurements confirmed that the diamond grains are strongly held by sp³ type covalent bonds. The temperature dependences of the electrical conductivity showed typical semiconducting behavior indicating ideal doping of the acceptor. The work function, which is related to tribo-charging, of sintered B-NPD measured by ultraviolet photoelectron spectroscopy was found to be 3.9-4.0 eV. The tribo-plasma damage, which is observed in conventional diamonds, was not observed to a great extent for boron concentrations in the range of 1x10¹⁸ - 1x10²⁰ cm^-3. The performance of cutting tool prototypes made of B-NPD for the machining of difficult-to-cut materials for diamond such as fused quartz (SiO₂) and polycarbonate surpassed that of conventional diamond because of the tribo-plasma suppressing effect. In addition to being an electron acceptor in diamond, we have found that boron also plays the role of a solid lubricant by forming oxides on the surface of B-NPD.

Focused laser spike (FLaSk) thermocapillary dewetting has shown high potential for thin film patterning of polymer and metallic materials. The extreme thermal forces generated by FLaSk lead to dewetting behavior determined only by melt viscosity, with polymers forming trench-ridge structures and metals dewetting both as Rayleigh droplets and trench-ridge structures. By selection of different exposure conditions, heating rate, time, temperature, and shear force can all be independently controlled. The convenience of tuning thermal field grants FLaSk the capability to do kinetic studies for materials whose viscoelastic properties are difficult to otherwise assess. In this way, we can probe whether a thin film of a particular metallic alloy behaves in a molecular melt-like or supercooled melt-like fashion under different conditions. Combining this approach with combinatorically sputtered libraries allows for high-throughput kinetic studies of a large range of compositions, which can then be analyzed through optical or atomic force microscopy to probe the response to a particular thermal and shear history. The key to this combined approach is that a single FLaSk “experiment,” consisting of a point-exposure with a controlled set of parameters, can occur on a sub-mm or even sub-micron region independently from other FLaSk experiments. Therefore, a mm-patch of compositional space can contain hundreds to millions of experimental conditions. By using a calibrated substrate, the thermal conditions of the different exposure profiles can be well known. Conducting a wide variety of exposures at different heating conditions provides information for mapping the time-temperature-transformation-diagram of that portion of the compositional space. This positions FLaSk as a tool for the rapid metrology and optimization of bulk metallic glass compositions.

The magnesium phosphate containing fluorophosphates glass, aMg(PO₃)₂-bBaF₂-cCaF₂ (a+b+c=1), samples were synthesized by a conventional method. The powdered starting materials were barium fluoride BaF₂, calcium fluoride CaF₂ and magnesium phosphate Mg(PO₃)₂. Fluorophosphate glass is a glass with spectial optical properties such as high Abbe value, transmittance in near UV and IR regions and low nonlinear refractive index(n₂). All these properties are influenced by glass structure and impurity effects.
The structural model of fluoride phosphate glasses can be described as P₂(F,O)₇-octahedral chains bonding by mono and diphosphate groups and cations. Several techniques have been used to study the structure of fluorophosphates glasses, but to our knowledge, no study has as yet used Raman spectroscopy, which is an excellent technique for the study of glass matrix structure.
In this article, we describe the results obtained by raman spectroscopy on a series of mg(PO₃)₂ containing fluorophosphates glasses, to which have then been added successively calcium and barium fluorides.
Transmittance of samples were investigated by UV/Vis spectroscopy. Measurements of the linear refractive index were performed using the prism coupler technique. optical transmission was measured using spectrophotometers operating from the midinfrared to the near ultraviolet. Some spectral parameters of the fluorophosphates glasses were calculated according to Judd-Ofelt (JO) theory.

The microstructure of different glass ceramic materials, obtained by thermal processing of vitrified products synthesized from tannery waste, was investigated using electron microscopy (TEM) techniques. Preliminary structural characterization was conducted by X-Ray Diffraction (XRD) while morphologies and compositions of the materials at the mesoscopic scale were attained using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDXS).
A first series of materials were synthesized by sintering chromium rich ashes of tannery waste with low grade soda lime glass powder. Three different mixtures were prepared with proportions 30/70, 40/60 and 50/50 of Cr-ash over glass powder and sintered at 600°C, 800°C, 1000°C and 1200°C [1]. Depending on the temperature, crystalline phases with chromium and non-chromium content were solidified. At the low temperatures the resulting products were opaque ceramics with granular morphology and high porosity. At the high temperatures they consisted of a vitreous matrix with dispersed crystalline phases having the morphology of typical glass-ceramics.
A second series of materials were synthesized using chromium ash (10wt%-20wt%) and SiO₂, Na₂O and CaO as vitrifying agents, in different relative proportions considering the low solubility of chromium inside the silicate melts [2]. Only the product with the lowest Cr-ash proportion was X-Ray amorphous with no indication of crystallites. HRTEM observations verified that the product retained the amorphous character even at the nanoscale since no nanostructured crystallites were detected. In all other vitrified products Eskolaite (Cr₂O₃) crystallites of hexagonal shape were grown in the melt. Devitrification of the as-casted products resulted to various crystalline phases dispersed within the vitreous matrix, depending on the initial batch composition. Eskolaite crystallites were not affected by the thermal processing and Devitrite (Na₂Ca₃Si₆O₁₆), Combeite (Na₄Ca₄Si₆O₁₈) and Wollastonite (CaSiO₃) crystallites were grown.
[1] S. Varitis, P. Kavouras, G. Vourlias, E. Pavlidou,Th. Karakostas, Ph. Komninou, Int Jour of Chem. Nucl. Mat. Metall. Eng. Vol:9, No:6, 2015
[2] S. Varitis, E. Pavlidou, P. Kavouras, G. Vourlias, K. Chrissafis, A. Xenidis, Th. Karakostas, Journal of Thermal Analysis and Calorimetry (2015) 121:203–208

Titanium (Ti) has proved to be an excellent candidate for bone implants due to its good biocompatibility, however, an ideal orthopedic implant would also need to ensure the appropriate integration of the implant into the patient’s body. The application of titanium dioxide (TiO₂) coatings on the Ti-implant surface is one of the best ways to enhance the interactions between the material and the biological environment at the interface. Moreover, the photoinduced phenomenon occurring on TiO₂ surfaces has been studied for potential bactericidal effect. Atom layer deposition (ALD) has been shown to be a promising technique for the synthesis of highly-adherent, highly crystalline, thin coatings made of TiO₂, providing favorable nanoroughness and higher exposed surface area, as well as greater hydrophilicity. Therefore, the aim of this study was to investigate the effect of nanocrystal TiO₂ coatings on human osteoblast adhesion, proliferation and differentiation, as well as its antibacterial properties.

During the process of coating TiO₂ onto Ti substrates, TiO₂ goes from amorphous to crystalline with the increase of temperature from 120 °C to 190 °C confirmed by Scanning electron microscopy (SEM) and Atomic force microscopy (AFM). The surface roughness values obtained by AFM showed increased surface roughness as expected. Water contact angle test showed a reduction of contact angle values on TiO₂-coated surfaces compared with untreated ones. It is well known that biological events are influenced by surface chemistry, surface topography (roughness), and surface wettability as well. In vitro results indicated that osteoblast adhesion and proliferation were enhanced significantly on TiO₂-coated substrates after culture of 4 hour and 7 days, respectively. More results about the osteoblast differentiation and mineralization will be presented. It can be expected that applying ALD technique to synthesize nanocrystal TiO₂ coatings on Ti implant would result in enhanced osteogenesis and a tighter bone-to-implant contact in vivo. Additionally, this TiO₂ coating has potential to reduce the risk of Ti implant associated infections.

Acknowledgements:
The authors would like to thank Northeastern University for funding and Ultratech for ALD technical support.

TiO₂ has been widely used as a nucleation agent forming precursor nuclei such as Al₂Ti₂O₇ to provide nucleation sites to a mother glass. Recently, glass-ceramics bearing pure TiO₂ quantum dots (TiO₂ QDs) have been reported only in a couple of papers. To our knowledge, however, detailed nucleation mechanism of TiO₂ QDs in a glass matrix has not been suggested and also their photocatalytic properties have not been reported.
Herein, we report the crystallization kinetics and photocatalytic properties of anatase TiO₂ QD glasses. The transparent TiO₂ QD glasses were prepared via the general melt-quenching method and crystallization heat treatments of 35BaO-xTiO₂-110B₂O₃ (in mol) (x = 20, 25, and 30) glasses. XRD and HRTEM with FFT analyses were performed to reveal the formation of high-density anatase TiO₂ QDs in glass matrices. The average size of precipitated TiO₂ QDs were measured to be from 10 to 20 nm in accordance with TiO₂ contents. Through the DSC scan analyses, the crystallization behavior of TiO₂ QDs was verified in each glass based on non-isothermal kinetics. According to the increase in TiO₂ content in mother glasses, the peak temperature of TiO₂ crystallization decreases, whereas the activation energy for TiO₂ crystallization increases. A possible mechanism for this behavior was proposed based on Raman spectrum results. It was speculated that the significant increase in the number of non-bridging oxygens with TiO₂ content substantial influence on the crystallization kinetics of anatase TiO₂ QDs in our glasses.
The energy band gap of precipitated TiO₂ was measured to be 3.2 eV from the clear UV absorption edges at ~387 nm. The TiO₂ QD glasses exhibited almost 0% UV light transmittance and 60-75% visible light transmittance. The photocatalytic properties of TiO₂ QD glasses were evaluated using the decomposition of methylene blue. Around 70% of methylene blue solution was decomposed within 180 min. Here, we for the first time demonstrate the development of UV blocking and photocatalytic TiO₂ QD glasses.

Ever since quasicrystals were first discovered, they have been found to possess many unusual and useful properties. A long standing problem, however, significantly impedes their practical usage: steady-state plastic deformation has only been found at high temperatures or under confining hydrostatic pressures. At low and intermediate temperatures, they are very brittle and suffer from low ductility and formability. Consequently, their plastic deformation mechanisms in these this temperature range are still not clear. Here, we systematically study the deformation behavior of icosahedral Al-Pd-Mn and decagonal Al-Ni-Co quasicrystals using a micro-thermomechanical technique over a range of temperatures (25-500 °C), strain rates, and sample sizes with accompanying microstructural analysis. We demonstrate three temperature regimes for the quasicrystal plasticity. At room temperature, cracking controls deformation. Between 100-300 °C, dislocation activities control the plastic deformation, exhibiting serrated flows and a constant flow stress. In the higher temperature range, 400-500 °C, diffusion enhances the plasticity showing homogenous deformation. The micrometer-sized quasicrystals exhibit both high strengths of ~2.5-3.5 GPa and enhanced ductility of over 15% strain between 100-500 °C. Deformation of small-scale QCs reveals a unique mechanism to with crystalline and amorphous metals, which is peculiar to quasicrystals rather than regular crystals and amorphous solids.

To overcome the limitation concerning catastrophic failure of bulk metallic glasses (BMGs), the concept of developing heterogeneous microstructure with in-situ formed secondary phases has been used. We recently developed novel TiCuNiSiSn BMG composites with super-elastic secondary phase, which exhibit a remarkable work-hardenability. Because mechanical properties of super-elastic alloys highly depend on martensitic phase transformation temperature, compositional dependence of martensitic phase transformation has been reported in various super-elastic alloy systems. However, the role of martensitic phase transformation temperature is not fully understood for in-situ formed BMG composites with super-elastic secondary phase. Thus, in the present study, we systematically discuss how to control the characteristics of the seuperelastic phases in amorphous matrix and optimize their mechanical properties by modulating martensitic phase transformation temperature and volume fraction of super-elastic secondary phase in BMG composites. This provides fruitful guideline for the tailor-made composite design with unique phase transformable secondary phase.

Modulating of the energy states and control of the micro-structure of metallic glasses is significant in understanding the nature of glasses and controlling their properties. In this talk, we show that a high energy state can be achieved and preserved in bulk MGs by using high pressure annealing, which is a controllable method to continuously alter the energy states of MGs. Contrary to the conventional annealing at ambient pressure, in which the internal energy of metallic glasses decreases with increasing annealing temperature, the proposed high energy state induced by high pressure annealing can be enhanced with increasing of annealing temperature. Using double aberration corrected scanning transmission electron microscopy, it is revealed that the unique high energy state, which is attributed to coupling effect of high pressure and high temperature, originates from the structural heterogeneity that contains “negative flow units” with a higher atomic packing density compared to that of the elastic matrix of metallic glasses. This results could be interpreted by using the potential energy landscape theory. The application of high pressure annealing is an effective way to lead the metallic glasses into a higher energy state and may assist in understanding the microstructural origin of the rejuvenation in metallic glasses.

It has been decade-long and enduring efforts to decipher the structural mechanism of plasticity in metallic glasses; however, it still remains a challenge to directly reveal the structural change, if any, that precedes and dominants plastic flow in them. Here, by using the dynamic atomic force microscope as an “imaging” as well as a “forcing” tool, we unfold a real-time sequence of structural evolution occurring on the surface of an Au-Si metallic glass. In sharp contrast to the common notion that plasticity comes along with mechanical softening in bulk metallic glasses, our experimental results directly reveal three types of nano-sized surface regions, which undergo plasticity but exhibit different characters of structural evolution following the local plasticity events, including stochastic structural rearrangement, unusual local relaxation and rejuvenation. As such, yielding on the metallic-glass surface manifests as a dynamic equilibrium between local relaxation and rejuvenation as opposed to shear instability in bulk metallic-glasses. Our finding demonstrates that plasticity on the metallic glass surface bears much resemblance of that of colloidal gels, of which nonlinear rheology rather than shear instability governs the constitutive behavior of plasticity.

Atomistic simulations of transient testing in nanocrystalline Al

Zhen Sun^1,2, Maxime Dupraz¹, C. Brandl³, Helena Van Swygenhoven<sup>1,2

1</sup>Swiss Light Source (SLS), Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
²NXMM laboratory, IMX, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
³Institute for Applied Materials, Karlsruhe Institute of Technology, D-76344, Eggenstein Leopoldshafen, Germany

Author contact: [email protected]


Molecular dynamics has been wildly used to explore the deformation mechanisms in nanocrystalline metals. Because of the high strain rate effect, the method does not allow to reveal the rate limiting deformation mechanism responsible for the constant flow stress typically observed in simulations and in experiments. The constant flow stress reached during uniaxial deformation reflects a quasi-stationary balance between dislocation slip and grain boundary (GB) accommodation mechanisms. Experimentally, stress reduction tests have been carried out to suppress dislocation slip and bring recovery mechanisms into the foreground [1]. When combined with in situ X-ray diffraction it was shown that at intermediate stress drops, dislocation slip can be re-activated after a period dominated by GB accommodation mechanisms.

To get a deeper insight in the interplay between dislocation slip and GB accommodation, similar transient tests have been carried out using molecular dynamics performed on a nanocrystalline Al sample with an average grain size of 10 nm. After deforming the sample with 10⁸ /s, stress drops with ratios between 0.9 and 0.3 are carried out and the sample is allowed to creep up to 2.3ns at much lower strain rates (~ 10⁶ /s). During this creep period, important changes in the grain boundary structure occur via mechanisms such as GB dislocation climb and GB migration. After structural changes dislocation nucleation and slip are re-activated. Besides confirming the interpretation of the experimental in situ tests, our simulations at low strain rates reveal deformation mechanisms that have not been observed during constant strain simulations at 10⁸ /s, such as dislocation-dislocation interaction mechanisms leaving point defects in grain interior and forwards and backwards motion of grain boundaries. Furthermore it is shown that the activated slip systems also depend on the strain rate, which will be discussed in terms of the Schmid factor.


REFERENCES

[1] Z. Sun, S. Van Petegem, A. Cervellino, K. Durst, W. Blume, H. Van Swygenhoven, Volume 91(2015)91



Keywords: molecular dynamics, nanocrystalline, deformation, dislocations

Analogous to the well-known polymorphism in crystalline materials, the polyamorphic transition in glassy matters is also a ubiquitous and intriguing phenomenon in the natural world, such as the glassy water (amorphous ice) can exit in two distinct forms: low- and high-density amorphous states, under different pressures. Although glass-glass transitions (GGTs) seems difficult to occur in metallic glasses due to their densely-packed atomic structures, nevertheless, it was recently reported that high pressure did induce the GGT in some special glassy systems such as lanthanide-based and Ca-Al amorphous alloys. In this talk, we will report our discovery of a non-pressure-induced GGT in a metallic glass. In particular, the following information will be delivered:
(1) How the GGT occurred during reheating under ambient pressure condition,
(2) what happened during the GGT from standpoint of atomic packing structures,
(3) What the origins responsible for such unprecedented phenomenon are and
(4) Differences between the present and the previously reported GGTs in metallic glasses.

The extrinsic size reduction of metallic glass into the nanoscale is a unique route for enhancing plasticity of metallic glasses with avoiding the general propensity of strength-ductility trade-off. Despite the accumulated researches on mechanical behaviors of nanoscale metallic glass, however, the homogeneous deformation of a nanoscale metallic glass with the increased strength compared to its bulk yield strength is still controversial due to its veiled origin and a lack of understandings on its characteristics. Here, we elucidate the origin and flow characteristics of homogeneous deformation of nanoscale metallic glasses from the viewpoint of viscous flow, based on nanocompression tests of metallic glass nanoparticles and a deformation map for the alloy. The quantitative viscosity analysis of homogeneously deformable nanoscale metallic glasses reveals that the non-Newtonian homogeneous flow occurs under a sufficiently low viscosity state through a whole sample volume as supercooled liquid state. Motivated by this finding, we construct a deformation map reflecting the stress-induced viscosity variation (or “mechanically-induced” glass transition) and the sample size effect enabling the detectable homogeneous flow without shear banding at room temperature. This provides fruitful interpretations from a novel perspective on the homogeneous deformation of metallic glass at room temperature.

Alloying allows the properties of a material to be tuned to a specific application, and advanced materials for a wide variety of applications are often alloys. Radiation-induced point defects can modify and degrade material properties. In an alloy, defect energies are sensitive to the occupations of nearby atomic sites and thus vary with location in the alloy, which leads to a distribution of defect properties. When radiation-induced defects diffuse from their initially non-equilibrium locations, this distribution becomes time-dependent. Furthermore, the defects can become trapped in energetically favorable regions of the alloy leading to a diffusion rate that slows dramatically with time. Density Functional Theory (DFT) allows the accurate determination of ground state and transition state energies for a defect in a particular local environment in the alloy but requires thousands of processing hours for each such calculation. Kinetic Monte-Carlo (KMC) can be used to model defect diffusion and the changing distribution of defect properties but requires energy evaluations for millions or billions of local environments. We have used the Cluster Expansion (CE) formalism to “glue” together these seemingly incompatible methods in order to model defect diffusion in alloys. In the CE approach, the occupation of each alloy site is represented by an Ising-like variable, and products of these variables are used to expand quantities of interest. Once a CE is fit to a training set of DFT energies, it allows very rapid evaluation of the energy for an arbitrary configuration, while maintaining the accuracy of the underlying DFT calculations. These energy evaluations are then used to drive our KMC simulations. We will demonstrate the application of our DFT/MC/KMC approach to model thermal and carrier-induced diffusion of intrinsic point defects in III-V alloys.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.

Structural heterogeneities play a critical role in determining the ductility and plastic deformation mechanisms in metallic glasses and their composites. In this work, modulus mapping with a nanoindenter probe is used to detect elastic heterogeneities in a Zr-based monolithic glass under different processing conditions, as well as a series of Ti-based BMG-crystalline composites. The monolithic glass was processed using conventional casting, isothermal annealing below T_g, and laser melting. The as-cast samples displayed a networked arrangement of elastically stiff and compliant regions that was strongly correlated with the direction of the temperature gradient during quenching from the melt. Upon annealing, the statistical distribution of the measured modulus values narrowed while bifurcating into two distinct peaks, which may be caused by a combination of structural relaxation and chemical segregation during annealing. The laser-pulsed samples showed more pronounced heterogeneous elastic microstructure with increasing laser power, demonstrating that the heterogeneities are also dependent on cooling rate. While the widths of the modulus distributions varied with processing, maps of the Zr-based glass under all conditions yielded similar sizes and spacing of the elastic heterogeneities, on the order of 100 nm. Nanoindentation arrays performed on the as-cast sample, with an indent spacing of 3 μm, showed a high degree of spatial variability of the hardness and modulus at the larger length scale as well. The combined results from the modulus mapping and indent arrays suggest the existence of a hierarchy of heterogeneities in the metallic glass. Bulk metallic glass-crystalline composites are intrinsically heterogeneous due to the presence of the glass and crystalline phases. However, modulus mapping again showed a further level of heterogeneity in the glass phase, indicating hierarchical heterogeneities in the composites as well as the monolithic glass. The size of the heterogeneities was found to be of similar magnitude as those in the monolithic glass. Further quantifying these heterogeneities is important to the understanding of how deformation is transferred between the crystalline and amorphous phases. Moreover, the hierarchical heterogeneous structure observed in both the monolithic metallic glass and the composites may provide valuable insight for understanding the microstructural origins of ductility or brittleness in these materials.

Bulk metallic glasses (BMGs) and nanocrystalline metals (NMs) have been investigated extensively due to their superior mechanical properties, most notably, their high strength and high elastic limit. Despite these excellent mechanical properties, the practical application of such materials has been limited by their low ductility at room temperature and their poor microstructural stability at elevated temperatures. Thus, there is a clear need for a metallic material system which can overcome the performance shortcomings of BMGs and NMs. It is extremely difficult to design a structural material that exhibits high strength, high ductility, and good high temperature stability. Generally, achieving superior performance in any one of these characteristics comes at the expense of the others. In this work, we report the discovery of novel Cu-based metal-intermetallic nanostructured composites (MINC), which exhibit high yield strengths (over 1.7 GPa), high compressive ductility (over 20%) and superior microstructural stability at a temperature above that of the glass transition temperature for Cu-based BMGs.
The presence of the narrow eutectic region between ductile metal (Cu) and strong intermetallic compounds allows us to achieve a composite having a large volume fraction of intermetallic compounds with minimal use of Zr, an expensive secondary alloying element. Rapid solidification produces a unique ultra-fine microstructure that contains a large volume fraction (over 60%) of a strong intermetallic compound phase, which is responsible for the high strengths measured in our MINCs. Mechanical and microstructural characterizations reveal that there is substantial accumulation of localized shear displacement at the metal/intermetallic phase boundaries, as well as micro-crack blunting at a ductile Cu metal phase, which accounts for the extensive ductility observed in these materials. Thus, plasticity of our MINCs is controlled dominantly by the distribution and structure of phase boundaries, not by conventional dislocation plasticity. We also confirmed that our MINCs have superior thermal stability at temperatures above the glass transition temperatures of Cu-based BMGs. Transmission electron microscopy showed negligible change in the microstructure of our material after 24 hours annealing at 500 ^oC, while the grain size of typical Cu alloys becomes nearly four times larger after only 15 minutes time at the same annealing temperature. The thermally stable Cu-Zr intermetallic compound serves as a diffusion barrier, which inhibits grain growth of the Cu phase. We believe that the high strength, high ductility and superior high temperature stability of the Cu-based MINCs presented in this work make these material very promising candidates for engineering materials in a wide variety of structural applications, such as in the aerospace, military, automobile, and building construction fields.

The present work uses a potential energy landscape exploration algorithm and molecular dynamics to investigate localized structural excitations in model amorphous systems that do not involve a driven material instability. The PEL exploration methods reveal extended chain like structural excitations, the energy of which is controlled predominantly by the elasticity of the surrounding amorphous matrix. This static work is complemented by very long molecular dynamics simulation runs spanning four orders of magnitude in temperature ramping time-scales. The simulations demonstrate significant local structural excitations as a function of increasing temperature which are compatible with those found by the static exploration methods, the kinetics of which can be quantitatively understood in terms of a power-law distribution of barrier energies.

The density of a metallic glass is a convenient indicator of its structural state. It is well known, for example, that increasing the cooling rate during solidification results in a glass of lower density. On the other hand, density can be increased by structural relaxation by annealing. The different structural states implied by these differences in density can be characterized in terms of free volume, which is commonly used as an order parameter in models of deformation behavior. Although it is widely assumed that shear-induced dilatation reduces the density of metallic glasses during plastic deformation, it has recently been suggested that the presence of hydrostatic tensile stresses during deformation promotes diffusion-assisted structural relaxation, resulting in effective strain hardening and delaying catastrophic failure by shear banding (Z. T. Wang, et al., Phys. Rev. Lett. 111, 135504 (2013)).
Here, we present quantitative measurements of the density of Zr-based metallic glasses during tensile deformation at room temperature. The samples were deeply notched to introduce significant hydrostatic components of stress. By imaging the samples with a high energy (80 keV) x-ray beam projected onto an area detector, we were able to make spatially-resolved measurements of the x-ray attenuation and therefore of the density of the material. We observe that the density evolves during deformation, dropping by as much as 2% during tensile loading but rapidly recovering if the sample is held at constant displacement under load. This behavior is observed not only for nominally elastic deformation but also when the deformation is clearly plastic. The decrease in density is larger than can be explained on the basis of elastic deformation alone, indicating a significant contribution from plastic dilatation. The spatial distribution of density changes during the test, becoming more uniform across the sample during plastic deformation. These observations suggest that in the presence of a significant hydrostatic tensile stress metallic glasses exhibit a transition from nominally elastic behavior at low stresses to behavior indicative of a non-Newtonian fluid at high stresses.

Selection of secondary phase is a key parameter in designing bulk metallic glass composites (BMGCs) to overcome the brittleness and improve their mechanical properties. Thus, various concepts of developing composite microstructure using in-situ/ex-situ formed secondary phases have been actively studied. Recent advancements in BMGCs come at the intersection between two different material areas of shape memory alloys and metallic glasses, whereby the shape memory alloy has been integrated as the phase, effectively using the concept of transformation-induced plasticity. By partially crystallizing/adding the BMG into a composite with shape memory 2nd phase, extensive work hardening can be achieved, which greatly enhances the potential structural applications for BMGCs. In the present study, we systematically investigated the compositional dependence of martensitic phase transformation temperature in work-hardenable secondary phase of superelastic BMGC based on the heterogeneous nucleation theory and internal friction. Indeed, the dependency can be utilized for tailoring work-hardenability of superelastic BMGCs, which open up a new direction for developing extremely sustainable BMGCs in service.

Metallic glass formation is equivalent with the avoidance of crystallization during solidification. The glass forming ability is quantified by the critical cooling rate, the lowest cooling rate, which avoids crystallization. These rates are remarkably low, ranging 1-100 K/s for bulk metallic glasses, which are up to 12 orders of magnitude lower than required rates for pure metals. However, despite these low rates which suggest experimentally convenient time scales for processing, a robust fabrication that results in desirable properties that can be repeatedly be achieved in complex shapes of even the most processable bulk metallic glass formers remains a challenge.
In this talk, I will investigate the origin for this challenge and quantify the effects that deteriorate the mechanical properties on the way from a bulk metallic glass forming melt to a complexly shaped final article with desirable properties. Specifically, the effect of (partial) crystallization, cooling rate, flaws, and chemical composition will be quantified by fracture toughness measurements. This work revealed that thermoplastic-based processing methods are most promising to fabricate complex bulk metallic glass articles with consistently high performance.

We have used a suite of experimental techniques to investigate the structure origin of a suspected polyamorphous phase transition in Pd-Ni-P metallic glasses, including in-situ synchrotron diffraction, small angle neutron scattering coupled with simultaneous differential scanning calorimetry, and high-resolution transmission electron microscopy (TEM). No crystalline peaks were detected from in-situ synchrotron diffraction measurements, and high-resolution TEM confirmed that the local structures remained amorphous until crystallization is reached. Furthermore, simultaneous small angle neutron scattering and differential scanning calorimetry measurements ruled out the possibility of a phase separation. Taken together, the experimental evidence supports the scenario of an amorphous-to-amorphous phase transition. The results of our study offer microscopic insights on polymorphous phase transition and a possible link to the excellent glass-forming ability of Pd-Ni-P BMGs.

An ability to alter the intermetallic phases that form at the interfaces between two metals is very desirable for controlling the microstructure and properties of alloyed materials. DSC and TEM experiments in the Ni/Al multilayer system have suggested that the formation of certain intermetallic phases can be suppressed by a sharp composition gradient across the Ni/Al interface, giving other intermetallic phases the opportunity to form instead. The suppression is thought to occur because the composition gradient across the interface affects both the thermodynamics and the kinetics of the nucleation of intermetallic phases. Given that composition gradients are common when intermetallic phases form from solutions, the goal of this study is to gain a fundamental understanding of the composition gradient effect in nucleation, and incorporate this effect in classical nucleation theory (CNT) for broader applications.

We studied the effect of a composition gradient on the nucleation of intermetallic phases from amorphous solutions in the Ni/Al system using molecular dynamics simulations and a semi-empirical EAM potential. Phase stability was examined over a wide range of Ni/Al compositions. Cooling and heating simulations were performed to observe the formation of the NiAl intermetallic phase and to determine crystallization temperatures. We observed that crystallization was deferred by the presence of a composition gradient, namely, crystallization occurred at lower temperatures (for cooling) or at higher temperatures (for heating). Crystal growth at various gradients and temperature conditions was also examined to evaluate the growth kinetics and the final composition of the intermetallic. Important thermodynamic quantities including the melting point, interfacial free energy and critical nucleus size were calculated to estimate the thermodynamic driving force as a function of concentration gradient. These calculations were used to validate and develop a CNT formulation that incorporates the composition gradient effect, originally proposed by Desré and Yavari (1990) [1].

[1] Desré, P.J. and A.R. Yavari, Suppression of crystal nucleation in amorphous layers with sharp concentration gradients. Physical Review Letters, 1990. 64(13): p. 1533-1536.

Amorphous solids, which lack crystal structure, find wide application from consumer goods to photovoltaics, but issues quantifying disorder have stymied reliable mechanical constitutive laws for these materials. Quantitatively predicting strain localization, a limiting failure process in high-strength metallic glasses and other amorphous materials, requires adequately capturing fluctuations in material structure and their interactions via the material’s mechanical response. I will discuss two advances in this regard. The first is an effort to quantify local yield stress measured by direct local probing of shear stress thresholds and the plastic rearrangements observed during remote loading in shear. This purely local measure shows a higher predictive power for identifying sites of plastic activity when compared with more conventional structural properties. Most importantly, the sites of low local yield stress thus defined are shown to be persistent, remaining predictive of deformation events even after fifty or more such plastic rearrangements. This direct and non-perturbative approach gives access to relevant transition pathways that control the stability of amorphous solids. Our results reinforce the relevance of modeling plasticity in amorphous solids based on a gradually evolving population of discrete and local zones pre-existing in the structure. The second advance is a direct cross-comparison of molecular dynamics simulations and continuum calculations. Comparisons are achieved by coarse-graining molecular dynamics data using an effective-temperature formalism. Direct cross-comparison allows us to test and validate these constitutive theories.

Non-equilibrium molecular dynamics simulations were used to model the thermal–stiffening mechanism that was recently reported in a polymer nanocomposite system consisting of poly(methyl methacrylate) covered silica nanoparticles dispersed in poly(ethylene oxide) matrix (Senses, Isherwood, Akcora “Reversible Thermal Stiffening in Polymer Nanocomposites,” ACS Appl. Mater. Interfaces 2015, 7, 14682). The storage modulus of this nanocomposite system decreases with increasing temperature, however, in the vicinity of the glass transition temperature of PMMA, the storage modulus starts increasing and approaches to that of neat PMMA. The observed thermal–stiffening effect is reversible and repeatable. The molecular scale mechanisms responsible for this unique behavior are being investigated via equilibrium and non-equilibrium molecular dynamics simulations. The simulations consist of a nanocomposite system that contains high-Tg chains grafted onto spherical nanoparticles, which are dispersed within a soft (low-Tg) polymer matrix. We employ both constant shear-rate and oscillatory shear deformations to investigate viscoelastic behavior as a function of temperature. In a blend system containing only high-Tg and low-Tg polymer chains, a significant thermal–stiffening was observed due to the coupled relaxation and dynamics of both polymeric phases. The effects of shear rate, interaction strength between phases and the corresponding structural changes and dynamics leading to reversible stiffening are studied and are corroborated with experimental findings.

This material is based upon work supported by the National Science Foundation under Grant No. CMMI-1538730.

Molecular dynamics simulations of strain localization in disordered solids are carried out using different materials systems and interatomic potentials including Cu64Zr36 modeled using the embedded-atom method, Si modeled using the Stillinger-Weber potential and binary Lennard-Jones systems. Quench schedules and strain rates are varied. Marked similarities in plastic behavior are found between systems. Shear Transformation Zone (STZ) theory is used to analyze systematic differences between these systems in an effort to develop a generalized constitutive framework for plasticity in glasses. An effective temperature, a local coarse-grained measure of the degree of disorder, is inferred from potential energy.

Although intensive research on the deformation and fracture mechanisms has been performed on metallic glasses, the fundamental mechanisms governing the mechanical behaviour as well as the recently observed mechanical size effects in this class of materials are still not fully understood. Recently, amorphous Zr₆₅Ni₃₅ (%at) freestanding thin film MGs (TFMGs) deposited by magnetron sputtering [1] have been deformed using a on-chip technique based on MEMS design principles [2,3]. The results have shown that the ductility of the films is highly enhanced compared to bulk MGs and other TFMGs in the literature, and that the plastic deformation occurs homogenously, i.e., without the observation of mature shear bands until fracture. In order to unravel the origin of these remarkable mechanical properties, the films have been investigated in-depth using advanced transmission electron microscopy (TEM).
Quantitative nanobeam electron diffraction (NBED) revealed clear disruption of the local atomic order with increasing deformation due to the activation of Shear Transformation Zones (STZs). Furthermore, high resolution annular dark field scanning TEM (HAADF-STEM) and electron energy loss spectroscopy (EELS) revealed a heterogeneous microstructure with Ni-rich and Zr-rich regions exhibiting different atomic densities with characteristic length of 2-3 nm. Such behaviour can be attributed to the sputter deposition process involving very high cooling rates compared to bulk MGs. These results raise several fundamental questions that will be addressed: Does the nucleation of the STZs preferentially occur in regions with specific enriched chemical composition and atomic density? How will this affect the interaction between the STZs? How such features can be used to explain the exceptional high plastic deformation levels, the absence of shear bands and the delayed fracture in the Zr₆₅Ni₃₅ TFMGs used in the present work?

References
[1] M. Ghidelli, S. Gravier, J.-J. Blandin, P. Djemia, F. Mompiou, G. Abadias, J.-P. Raskin, T. Pardoen. Acta Materialia 90 (2015) 232
[2] H. Idrissi, B. Wang, M.S. Colla, J.P. Raskin, D. Schryvers, T. Pardoen. Advanced Materials. 23 (2011) 2119
[3] M.S. Colla, B. Amin-Ahmadi, H. Idrissi, L. Malet, S. Godet, J.P. Raskin, D. Schryvers, T. Pardoen. Nature communications. 6 (2015) 5922

A number of years ago, it was predicted [1] and subsequently demonstrated [2] that the magnetocaloric effect of a material could be enhanced when the material’s independent magnetic spins were replaced by groups of magnetic spins. At that time, a paramagnetic garnet, Gd₅Ga₃O₁₂, was modified by the addition of Fe to transform the garnet into a superparamagnet. This magnetic state transformation occurred because the Fe atoms enabled a magnetic superexchange interaction between the Gd spins. As a consequence, the magnetocaloric effect of the material increased by a factor of 4-5. As a test of how generic this effect is, we have investigated a classic spin glass material, Cr-Fe [3], quenched from high temperature solid solution and sequentially aged at low temperature to grow larger magnetic clusters in the material. Here, a review of the origins of the enhancement will be presented along with a report on the response of the magnetocaloric effect in such a material as the cluster size is sequentially changed.

[1] R.D. McMichael, R.D. Shull, L.J. Swartzendruber, L.H. Bennett, and R.E. Watson, J. Mag. & Magn. Mat. 111, No. 1-2, 29 (1992).
[2] R.D. McMichael, J.J. Ritter, and R.D. Shull, J. Appl. Phys. 73(10), 6946 (1993).
[3] R.D. Shull and P.A. Beck, Magnetism and Magnetic Materials-1974 AIP Conf. Proc., vol. 24, 95 (1975).

Neodymium-based magnets are among the most important permanent magnetic materials due to their high BH product. Future improvements to the performance of these magnets requires a detailed understanding of the origin of their temperature dependent properties and interfaces responsible for nucleated reversal mechanisms. We present atomistic spin dynamics simulations of bulk Nd₂Fe₁₄B explicitly modeling the correct crystallographic structure, localized Fe and Nd moments, pairwise Heisenberg exchange and localized 2^nd and 4^th order magnetocrystalline anisotropies in this complex material. Using a combination of Langevin spin dynamics[1] and Constrained Monte Carlo[2] methods we simulate the temperature dependent magnetic properties including the sublattice magnetization, Curie temperature, spin reorientation transition and anisotropy fields. Using careful parameterization and extending the spin temperature rescaling (STR) method[3] to multi-sublattice materials, where the rescaling is different for different elements, we find quantitative agreement between the calculated temperature dependent properties and experimental results for a large single crystal. We apply our model to simulate the hysteretic behavior of a single domain sample, equivalent to a single micromagnetic macrocell, showing exceptionally high coercivity. Compared to the usual easy-axis Stoner-Wohlfarth hysteresis cycle which has a square loop, at low temperatures the simulated loops exhibit reduced squareness due to the easy cone anisotropy. The simulated temperature dependence of the canting angle of the easy cone anisotropy compares well with experimental data, where the natural thermal fluctuations in the simulation reproduce the spin reorientation transition due to competing second and fourth order anisotropies with their different temperature dependencies.
[1] R F L Evans et al, J. Phys.: Condens. Matter 26 103202 (2014)
[2] P. Asselin et al, Phys. Rev. B. 82, 054415 (2010)
[3] R. F. L. Evans, U. Atxitia, and R. W. Chantrell, Phys. Rev. B 91, 144425 (2015)

Quantitatively correlating the amorphous structure in metallic glasses with their physical properties has been a long-sought goal. Here we introduce “flexibility volume” as a universal indicator of the structural state, to correlate with properties on both atomic and macroscopic levels. The flexibility volume is assessed via atomic vibrations that probe local configurational space and interaction between neighboring atoms, and is defined to be measurable both computationally and experimentally. We show that this indicator deterministically predicts the shear modulus, which is at the heart of key properties of metallic glasses, and correlates strongly with atomic packing topology as well as the activation energy for thermally activated relaxation and the propensity for stress-driven shear transformations. The concrete structure-property correlations discovered are robust and prognostic for all metallic glass compositions, processing conditions and length scales. All these advantages advocate flexibility volume as a replacement of the widely cited but ambiguous “free volume” structural parameter in understanding the properties of metallic glasses.

The current work presents a protean twin thickness contour zone map that illustrates how the nucleation and the mobility of twin boundaries affects the twin thickness of sputtered films. The twin thickness contour zone map can be used as a versatile guide to synthesize fully nanotwinned films with tailored twin thicknesses in materials with a wide range of stacking fault energies between 6 mJ/m² to 60 mJ/m². Additionally, the formation of twin boundaries in high stacking fault energy metals (SFE > 125 mJ/m²) in thick films (> 10 μm) is presented. We report the observation of twin boundaries that are inclined with respect to the film growth direction across the entire thickness of the films. The formation of these inclined twin boundaries results in localized changes in the texture of the columnar grains. The experimental observations provide an explanation on the formation of twin boundaries during the synthesis of the films, and emphasize that, in addition to stacking fault energy restrictions, high grain boundary mobility is a limiting factor on the nucleation of twin boundaries.

[Introduction] Bulk metallic glasses (BMGs) have attracted a lot of interests because of their superior mechanical properties due to their unique long-range disordered microstructures. However, the room-temperature brittleness is one main disadvantage for the potential applications of BMGs. To overcome this limited ductility, fabricating BMG matrix composites (BMGMCs) can be effective. Normally, there are two ways to fabricate the BMGMCs: in situ way and ex situ way. The in situ BMGMCs always show better mechanical properties than ex situ ones because of stronger bonding strength between the matrix and secondary phase. However, the in situ process is difficult to design. In this study, in situ NiTi (shape-memory-alloy) have been successfully introduced into a Mg-based BMG matrix by a novel process and the microstructures have been optimized in order to further improve the mechanical properties.
[Experimental] The compositions of the BMGMCs were (Mg_0.69Ni_0.15Gd_0.10Ag_0.06)₉₅Ti₅ (at.%) —Composition A and (Mg_0.69Ni_0.15Gd_0.10Ag_0.06)₈₆Ni₄Ti₁₀ (at.%) —Composition B. The Ni-Ti-Gd pre-alloys were prepared either by arc-melting —SC (slow cooling) pre-alloy or by copper mold casting after arc-melting to produce rod-shaped pre-alloys —RC (rapid cooling) pre-alloy. Different sizes of RC pre-alloys, 8, 5 and 3 mm in diameter, were prepared to investigate the cooling rate effects on the size of NiTi dispersoids. Thus the naming rule is, e.g., 5 mm sized RC pre-alloy with composition A is denoted as A-RC5 pre-alloy. Then the pre-alloys were inductively melted with Mg and Ag pieces. Finally rod-shaped BMGMCs with 2 mm in diameter and rectangular-shaped BMGMCs (3×3×25 mm³) were fabricated by the copper mold casting technique. The microstructures and mechanical properties were investigated by XRD, SEM, Intron and Shimadzu mechanical testing machines.
[Results] The in situ B2-NiTi reinforced Mg-based BMGMCs have been successfully fabricated by the novel process. The A-SC BMGMCs shows improved fracture strength, ~906 MPa, and high plastic strain, ~6.7%, compared with its monolithic counterpart. The stress-induced martensitic transformation of B2-NiTi phase is confirmed and contributes to the mechanical properties. Furthermore, the size and inter-particle spacing of NiTi dispersoids have been optimized by controlling the cooling rate of precursor (A-RC) and adjusting the composition (B-RC), respectively. The best plasticity obtained is ~23%, highest among all kinds of in situ Mg-based BMGMCs to date. The reason is considered to be that the optimized average size and inter-particle spacing of NiTi dispersoids are very close to the processing zone size of the matrix, which can further stabilize the shear bands to develop into cracks and improving the interactions between the matrix and the dispersoids. The novel in situ shape-memory-alloy reinforced BMGMCs are believed to make a breakthrough in designing work-hardenable plastic BMGMCs.

Amorphous metal thin films (AMTFs) could lead to disruptive changes in the internal architecture design of a thermal inkjet (TIJ) printer. Ink injection in a TIJ printer is accomplished in a harsh operating environment. This means that a thin film resistive heater must be mechanically robust and chemically inert at an operating temperature in excess of 340 °C [1]. Due to the lack of grains and grain boundaries, AMTFs inherently provide advantageous mechanical properties and enhanced chemical stability, which is highly desirable for a TIJ printer application. However, the metastable nature of an AMTF, i.e., its tendency to crystallize, is a serious liability. The focus of the work discussed in this presentation is development of new refractory AMTFs (Ta_xM_ySi_z, M = Ni, Mo, and W) that are suitable for use in a TIJ printer. Thin films prepared via sputter deposition from a multicomponent metal target are found to be amorphous, to have ultra-smooth surfaces (<0.5 nm), and to have resistivities typical of an amorphous metal (~200 μΩ-cm). Optimally performing thin films, deposited from a Ta₃₀W₃₀Si₄₀ target, retain their ‘as-deposited’ properties and remain amorphous up to an annealing temperature of 1000 °C. Additionally, these Ta_xM_ySi_z thin films show enhanced oxidation stability compared to the sputtered ß-Ta thin films commonly used in a TIJ printer. With thermal stability to ~1000 °C and enhanced oxidation performance, Ta₃₀W₃₀Si₄₀ thin films appear to be excellent candidates for TIJ printer applications.

1. R.R. Allen, G. Dispoto, E. Hanson, J.D. Meyer, and N. Moroney: Inkjet, in Color Desktop Printer Technology, edited by N. Ohta, M. Rosen. (Taylor and Francis Group, Boca Raton, FL, 2006), pp. 119.

The dominant presence of grain boundaries in nanocrystalline materials present a difficult stabilization problem, as these materials can be highly unstable against grain growth. Alloying can provide stability when the solute species exhibit a strong preference for grain boundary segregation and thus energetically prefer nanocrystalline states over bulk states. In this talk, we explore alloy selection criteria for stabilizing nanocrystalline materials, developed both analytically and through thermodynamic Monte Carlo simulations, and evaluate the relative stability of the nanocrystalline state against grain growth, phase separation into a solvent- and solute-rich solutions, as well as the formation of known ordered compounds. Such Monte Carlo simulations provide a more detailed understanding of the thermodynamics of nanocrystalline alloys, and how temperature and composition can be used to tune the nanostructures produced upon alloying.

Thermoplastic molding is well known method for making small scale structures for size-effect studies and functional applications in metallic glasses. However, the fabrication of high aspect ratio nanostructures is challenging due to enhanced capillary pressure and mold friction. Template free wire drawing has been suggested as a possible solution to overcome the high pressure requirement for making high aspect ratio structures. However, existing wire drawing technique is sequential and therefore, not suitable for high-throughput needs such as catalysts, sensors, and electrodes. In the present work, we report a novel fabrication technique to make an array of micro- and nanostructures with aspect ratios approaching carbon nanostructures. By controlling the processing parameters, we are able to mass produce variety of metallic glass nanostructures such as tips, rods, wires, and tubes. These nanostructures can also be transferred on other rigid (Si, glass, steel) or flexible (SU8 or PMMA) substrates. We extend the technique for high-throughput fabrication and testing of metallic glass nano-wires to characterize the size-effects. Effects of processing conditions and testing temperature on critical sample size for homogeneous flow are investigated.

Crystallization in multicomponent alloys is a complex phenomenon in which quantitative theories are lacking and many fundamental questions remain unanswered. Bulk metallic glass (MG) forming systems are an ideal model system to study crystallization due to their multicomponent characteristics and the ability to select a convenient, experimentally accessible time scale of observation through crystallization from the supercooled liquid state. We have recently shown that MGs can be molded into nanoscale rods¹, which are suited for transmission electron microscopy (TEM) observations.
Here, by using in situ TEM as a direct investigation tool, we show that the crystallization mechanism departs from the established theory at the atomic resolution. More specifically, we observe non-monotonic crystallization kinetics for nanoscale MG samples². We show that, in addition to the size effect, the processing conditions of MG nanorods affect the crystallization kinetics strongly. We reveal structural-coupled growth evidenced by stochastic growth rates, which suggests accelerated growth rate by local structure order. Moreover, we unveil the microscopic origin of the asymmetry between critical heating and cooling rates in MGs, by applying different thermal processes on MG nanorods.
Our in situ TEM crystallization study on MG nanorods highlights the departure from the established theory. Technologically, our findings offer central knowledge on processing of MGs to avoid crystallization during nanomolding.

1 Kumar, G., Tang, H. X. & Schroers, J. Nanomoulding with amorphous metals. Nature 457, 868-872 (2009).
2 Sohn, S. et al. Nanoscale size effects in crystallization of metallic glass nanorods. Nature Communications 6, 8157 (2015).

Metallic glasses (MG) are known to have low thermal stability, which currently limits their applications. Recently, novel organic glasses with increased thermodynamic and kinetic stabilities, as well as with superior properties, such as higher elastic modulus and hardness, higher glass transition temperature and lower enthalpy than the melt quenched glasses, have been prepared by physical vapor deposition. These organic vapor deposited glasses have been referred to as being ultrastable. Here, we explore the possibility that physical vapor deposition can be used to create more stable metallic glasses. Specifically, we have carried out molecular dynamics (MD) simulations of vapor deposition of CuZrAl glass and found that vapor-deposited MGs have a higher density and a lower potential and inherent structure energies than the ordinary glasses prepared by quenching the liquid CuZrAl systems. Moreover, the deposition rate plays a critical role in the stability of the vapor-deposited MGs. The optimal substrate temperature has been predicted to be 0.75*T_g, where T_g is the glass transition temperature. This finding is in good agreement with the experimental values, which are in the range of 0.80-0.85*T_g. The structures of the vapor-deposited glasses were analyzed using Voronoi analysis. The short (SRO) and medium range orders (MRO) of those MGs were compared with the SRO and MRO of the ordinary MGs. In vapor deposited MGs, the total number of icosahedral clusters is higher, as compared to the ordinary MGs. Surprisingly, we have also found that the vapor deposited glasses have a lower degree of chemical mixing, which effect is attributed to the presence of surfaces. We found that in order to obtain MGs from melt-quench technique with similarly low energy as in the vapor deposition process, the cooling rate during quenching would have to be many orders of magnitude smaller than currently accessible to MD simulations.

Ongoing work has CWRU has focused on the effects of systematic changes in stress state, test temperature, and strain rate on the flow and fracture behavior of metallic glasses. The presentation will first review the effects of changes in stress state (e.g. superimposed pressure, notches, tension vs. compression, etc.) on the flow/fracture behavior, followed by similar studies conducted at temperatures approaching Tg. This will be followed by an examination of the effects of changes in strain rate/loading rate on the flow and fracture behavior at temperatures up to Tg on tests conducted under both displacement-controlled and load-controlled conditions. The presentation will include an examination of the effects of changes in sample size on plasticity in such systems on samples prepared via different techniques. If time permits, a short review of similar studies on BMG composites will be presented.

Research supported by ARO-W911NF-12-1-0022, DTRA-HTDRA-11-1-0064, DARPA-SAM, ONR, and AFOSR

Distinct structural and chemical heterogeneities of metallic glasses have recently been extensively reported in the literature. These spatial heterogeneities, suggested by simulations and experiments, have been considered as “microstructure” to qualitatively explain various properties of metallic glasses. Nevertheless, the nature and origin of the spatial heterogeneity in the disordered materials have not been well understood. In this talk, I will introduce our recent work on experimental characterization of the spatial heterogeneity of metallic glasses by combining amplitude-modulation atomic force microscopy and angstrom beam electron diffraction. Evident variation in phase-lag distribution unveils the viscoelastic origin of the nano-scale spatial heterogeneity. Importantly, the evolution of spatial heterogeneity intrinsically correlates with sub-T_g beta-relaxation and fragility of glass-forming liquids. The characteristic relaxation time and activation energy of heterogeneity evolution are in accord with those of excess enthalpy release by beta-relaxation. Local structure analysis indicates that the spatial heterogeneity originates from the competition of structural/chemical ordering and geometric frustration during liquid-to-glass transition.

A glass’s property depends on its chemistry and its fictive temperature. For bulk metallic glasses (BMGs) a wide range of behavior in terms of their fracture toughness has been reported. However, measuring fracture toughness in BMGs has been challenging which has resulted into large scatter in the reported results. We have recently developed a method to characterize fracture toughness of BMGs accurately and repeatably. Here, we use this method to measure fracture toughness in a consistent manner for a various composition of BMGs. Results are accompanied by thermal analysis, elastic constant data, hardness, ultimate strength, and are discussed the effect of composition on the mechanical property of BMGs.

Silicate glasses are widely used in applications such as LCD displays, touchscreens for hand held devices, and car windows for which scratch and wear resistance are important. It is well known that the deformation under a point contact (e.g. an indentation or scratch test) in such materials is very complicated and includes elastic deformation as well as permanent deformation by shear, densification and fracture. While the mechanisms of elastic deformation and fracture are well known, and some models for the atomic movements involved in densification have been presented, the bond breaking and reformation processes and their relationship to glass structure remain largely unknown. We have used tectosilicate calcium aluminosilicate, magnesium aluminosilicate, and calcium galliosilicate glass systems to systematically evaluate the effects of glass composition and structure on resistance to plastic deformation. All of these systems show a non-monotonic variation in hardness, but a monotonic decrease in modulus with increasing SiO₂ content. The modulus correlates well with a modified topological constraint model, while the non-monotonic hardness is attributed to a plastic deformation mechanism switch from shear to densification. Variations in resistance to shear deformation are correlated with changes in glass speciation (densities of non-bridging oxygen and higher-coordinated Al and Ga), as determined by NMR. We describe a unit deformation mechanism model as a first step in understanding the determinants of the plastic strength of silicate glasses and compare results with plastic deformation mechanisms in metallic glasses.

Studying the evolution of melt structure of metallic glasses as we approach their glass transition temperature is essential to understand glass formation. Known for their excellent glass forming ability, Cu-Zr binary metallic glasses are very well studied in the literature. Here, we have focused on Cu-Zr-Ti ternary metallic glasses in order to extend the current understanding of metallic glasses to more complex and eventually more realistic glasses. We have used molecular dynamics simulations to characterize the changes in the structure and dynamics of (Cu₅₀Zr₅₀)_1-xTi_x (0 < x < 1) melt with the quenching. We observe that compositions with x < 0.5 form glassy solid and with x > 0.6 form crystalline solid for the same quench rates which agrees well with the experimental findings.

Here, we have explored the factors that govern if a melt will form a glassy or crystalline solid. We will show that the overall density and dynamics doesn’t have significant effect on solidification path the melt will follow. We will focus on the short and medium range order of the melt and their diffusion as a function of composition and temperature to highlight the differences in the melt behavior of the amorphous glasses from their crystalline form.

Al-based metallic glasses (MG) are known to have very good mechanical properties and better corrosion resistance and formability as compared to their crystalline counterparts. Rapidly quenched Al-based MGs are usually characterized by a primary crystallization reaction upon heating, which produces a high density of Al nanocrystals and leads to enhancement of the mechanical properties of these alloys. However, the effects of the solute concentration on nucleation kinetics and the atomic level mechanisms that control nucleation are not well understood yet in Al-based MGs. To address these problems we studied the isothermal nucleation kinetics in Al-Sm metallic glass at 0.0 – 3.0 at.% Sm concentration (x_Sm) using a combined molecular dynamics (MD) simulations and phase field approach. The time-temperature-transformation curves are obtained from simulations. We find that the average delay time of Al nucleation increases exponentially with x_Sm, while the estimated critical cooling rate necessary to avoid crystallization decreases exponentially with x_Sm. We extracted the nucleation kinetic barrier (Q) based on the classical nucleation theory and we identify physical mechanisms responsible for this energy barrier. Atomistically informed phase field model is used to determine the distribution of precipitates as a function of Sm concentration and temperature in our systems.

Mechanical alloying of immiscible elements through severe plastic deformation was shown in the past to lead to formation of homogeneous mixing in crystalline and amorphous phases. We describe results from molecular dynamics simulations of SPD of dilute immiscible Cu-X alloy systems. Depending on properties of the immiscible solute atom, SPD leads to spontaneous formation of large scale structures, with phase separation between crystalline solid solution and an amorphous B rich phase. We find that the B rich phase is unique and so the relative number of atoms within this phase can be described using a simple lever rule. This, together with the observed increase in B concentration within the crystalline copper region, suggests that indeed the driven system follows a unique phase diagram differing significantly from the equilibrium phase diagram. Calculations are reported for systems with varying heat of mixing, namely Cu-V, Cu-Nb and Cu-Ta demonstrating increasing range of mixed crystalline-amorphous stabilization. The mechanisms controlling the amorphous phase stabilization as well as the spontaneous self-organization are discussed with possible relevant experimental scenarios.

The critical cooling rate, R_C, is the most direct measure of the glass forming ability (GFA) of metallic glass forming alloys. Despite its importance, R_C has only been determined for very few alloys. This is due to experimental challenges. Here we report a high-throughput method to measure R_C for a large number of alloys. Combinatorial alloy libraries of variable composition were fabricated using magnetron co-sputtering. The libraries were then subjected to laser scans of variable power dosage and scan speeds, followed by characterization using optical microscopy and x-ray diffraction. In conjunction with computer simulations based on laser parameters, the glass-forming region in the compositional space could be determined as a function of both composition and cooling rate. This technique could be used to quantify GFA for alloy libraries as well as construct the time temperature transformation (TTT) diagrams, therefore greatly accelerating the discovery and optimization of metallic glass compositions.

Fe-based soft magnetic metallic glasses have drawn substantial attention due to their high permeability, low coercivity (H_c) and high conductivity which contribute significantly to low core losses[1]. As energy exhaustion and environment problems have threatened human sustainable development, novel high efficiency and energy saving materials, such as Fe-based soft magnetic metallic glasses are highly desired which have extensive applications of transformers, motors, sensors and other magnetic devices[2]. However, inherent low saturation flux density (B_s) and/or low glass forming ability (GFA) of commercial Fe-based soft magnetic metallic glasses has hindered their further applications[3]. In order to obtain fully glassy state that ensuring excellent soft magnetic properties of Fe-based alloys, certain amount of metalloid elements (B, C, Si, P) and other glass forming elements (Al, Ti, Ga, Nb, Mo, etc) are essential constituent. Whereas, those non-magnetic elements addition will diminish the B_s[4] and all the Fe-based soft magnetic metallic glasses with GFA up to 1 mm, show B_s no more than 1.65 T[5, 6]. The basic way to raise B_s is increasing the Fe content, but which makes the alloy deviate from the eutectic point then deteriorating GFA[7]. Consequently, developing Fe-based soft magnetic metallic glasses with both high GFA and high B_s bears profound significance.
According to Pauling-Slater curve, FeCo-based crystalline alloys show higher B_s than Fe-based counterparts, and thus Co was applied to partially replace Fe of Fe-Mo-P-C-B-Si system in this work. Moreover, introducing a new element (Co) in alloy systems could increase GFA based on Inoue experimental principle for GFA. On the other hand, fluxing purify method was used to dislodge excessive oxygen dissolve in the alloy to avoid heterogeneous nuclear then promoting GFA. Finally, Fe-Co-Mo-P-C-B-Si alloy was obtained with GFA up to 1 mm and B_s better than 1.7 T, which is the highest B_s of Fe-based bulk metallic glasses reported so far. The H_c was also as low as 4.5 A/m. Higher B_s and lower H_c of soft magnetic alloys conduce to smaller size of their devices and save more energy resource. In addition, the 3050 MPa compression strength and better plasticity up to 1.2% also endow the alloys a better service performance. All these make them a promising candidate for commercial applications as magnetic functional and structural materials.
[1] D. Azuma, R. Hasegawa, IEEE Trans. Magn., 47 (2011) 3460-3462.
[2] G. Herzer, Acta Materialia, 61 (2013) 718-734.
[3] C. Suryanarayana, A. Inoue, International Materials Reviews, 58 (2013) 131-166.
[4] A.D. Wang, H. Men, C.T. Chang, Journal of Alloys and Compounds, 656 (2016) 729-734.
[5] J.H. Zhang, C.T. Chang, A.D. Wang, B.L. Shen, Journal of Non-Crystalline Solids, 358 (2012) 1443-1446.
[6] J.-F. Li, X. Liu, K.F. Yao, Journal of Magnetism and Magnetic Materials, 386 (2015) 107-110.
[7] S. Meng, H. Ling, Q. Li, J. Zhang, Scripta Materialia, 81 (2014) 24-27.

Recently, structural rejuvenation has been intensively studied in the metallic glass research fields. Many rejuvenation methods (e.g. plastic deformation, elastostatic loading, ion irradiation and so on) have been proposed so far. Among the methods, we have focused on the thermal approach, so called the recovery annealing technique, to attain the rejuvenation. This is just the thermal way to the rejuvenation, which can be discussed easily in comparison with those based on the mechanical ones. Our previous studies indicate that the rejuvenation is surely occurred in the Zr-based metallic glass after the recovery annealing. However, the important question, “Can we rejuvenate a metallic glass again and again by the recovery annealing?”, is still uncertain. Since the method is based on annealing in the supercooled liquid region (~1.05 T_g, T_g: glass transition temperature), the concern about the crystallization during the process seems to be inevitable. Therefore it is of great importance to clarify the rejuvenation mechanism and its correlation with the crystallization behavior of recovery annealed metallic glass.
In this study, Zr₅₀Cu₄₀Al₁₀ metallic glasses with different thermal histories (i.e. as-cast, relaxed, recovery annealed) were prepared and then we investigated the rejuvenation mechanism and the crystallization behavior using DMA, DSC and TEM. The DMA results reveal that the rejuvenation (i.e. unstable process) is certainly occurred and the β-relaxation region is mainly related to the recovery. On the other hand, the DSC and TEM results show the possibilities of the origin of initial crystallization (i.e. stable process) in the amorphous matrix.
Until now, two opposite processes of the rejuvenation and the crystallization are recognized independently, but they can be occurred simultaneously. Our present results open up new possibilities for developing the structural research for the random atomic configuration systems.

Exchange-spring hard/soft phase nanocomposite magnets have extremely high energy density owing to the inter-phase exchange coupling. The precondition for the effective exchange coupling is the nanoscaling of the soft phase grains that are homogenously distributed in the hard-phase matrix. Recently we found that by severe plastic deformation, the soft phase α-Fe grains in a SmCo/Fe composite system with the Fe(Co) concentration up to 35% can be “self-nanoscaled” to ~10 nm from the original particle size of 10 µm. The 10³ size reduction is a result of a brittle/ductile composite deformation that leads to the elongation of the ductile component into noodle-like grains. The “noodles” then breakdown into nanoscale grains with further deformation. Subsequent warm compaction processing of the deformed nanocomposite particles produces fully dense bulk magnets with retained nanoscale morphology of the hard/soft phase composites. The energy density of the composite magnets is as double high as the single-phase counterparts.

Nanocrystalline diamond (NCD) possesses outstanding mechanical, optical and electrical properties. The fabrication of NCD coating on foreign substrates is in fact applied today on an industrial level; however, besides CVD coated cemented carbide tools, only few applications based on NCD coatings have been successfully established in the market. A new and advanced trend in NCD coatings, is the tailoring of the materials properties by specific grain boundary and grain size engineering. Thus, it is possible to tailor and to optimize the diamond coating properties towards the underneath substrate and the corresponding application. For example, decreasing the average grain size below 10nm, yield to a significant reduction in Young’s modulus towards 700GPa and an increase in fracture strength in the range of 3-5GPa. This allows for a better adhesion of diamond films on Cemented carbide (Young’s’ modulus of 400-600GPa) and additionally reduces the risk of diamond film delamination or chipping under elevated mechanical stress. In addition, plasma machining allows a precise post finishing and precise structuring in the sub-micrometer range, even of thin NCD coatings (<1µm) on foreign substrates. This enables promising new market applications, not only limited to niche markets. In this presentation we will give examples of successful industrial application of NCD diamond films in different emerging market segments. An overview about the NCD properties, the historical technical progress, the manufacturing- and post structuring process and the industrialization and upscaling strategies will be given. As examples for industrial applications, we will present diamond coated plasma sharpened blades for the converting industry, CVD coated plasma sharpened tools for carbon fiber reinforced plastic machining, plasma sharpened scalpels for ophthalmic surgery and long-life razor blades for shaving. Additional applications such as lubrication-free micromechanical watch parts and NCD-coated precision spheres for industrial metrology CMM probes will be discussed. The presentation will also cover aspects of manufacturing costs, upscaling effects and future trends in nanocrystalline CVD diamond coatings.

Processing of bulk metallic glasses (BMGs) is limited by the temperature interval in which the alloys are in their supercooled liquid range (SCLR). The commonly short period in the SCLR can be considered the processing window, i.e. the time before crystallization occurs. Processing of BMGs is frequently carried out from the molten state maintaining high cooling rates to quickly pass the SCLR and freeze the amorphous structure, e.g. by copper-mold casting. However, detailed information about how the high cooling rate affects the melt flow behavior during casting – and vice versa – is lacking.
We present the prerequisites to monitor quantitative cooling curves by high-speed infrared thermography. Using low-T_g Au-based BMGs, we discuss the cooling curves for fully amorphous, partly amorphous and crystallized regions of cast parts. Comparing the respective cooling behavior with TTT and CCT diagrams measured by fast differential scanning calorimetry (FDSC) reveals good agreement of the data. The proposed technique thus makes it possible to study crystallization during casting in dependence of the processing history (e.g. overheating and casting temperature), and process parameters, mold material, and mold temperature.

At the core of designing high specific strength nanocrystalline metals lies stabilization of the nanostructure against thermal and mechanical instabilities. Thermodynamic nanostructure stability models have predicted that equilibrium nanocrystalline states can be achieved through the introduction of solute that segregates to the grain boundaries, in turn reducing or even eliminating the energetic penalty of these interfaces. However, augmenting the grain boundary state also influences the mechanical behavior especially in nanocrystalline metals, which are dominated by grain boundary mediated plasticity. In this presentation, the design of stable nanocrystalline aluminum alloys with strengths among the strongest lightweight metals will be described. Lattice Monte Carlo simulations parameterized via the regular nanocrystalline solution (RNS) model were employed to map nanocrystalline-stable states in binary aluminum alloys. The thermal stability of stable and unstable alloy configurations were directly compared and understood in the context of nanostructure stability maps for aluminum binaries. Nanocrystalline Al-Mg alloys were selected to isolate the effect of grain boundary doping on hardness and strength. Alloying and grain size refinement were simultaneously accomplished through high-energy mechanical milling to produce a range of nanocrystalline Al alloys containing 1 – 7.5 at. % Mg with a constant grain size of 24 nm determined via x-ray diffraction. Low homologous temperature heat treatments were employed to tailor segregation state, which was quantified through a combination of energy dispersive x-ray spectroscopy and x-ray diffraction. The strongest alloy with a hardness of 4.56 GPa exhibited global and grain boundary compositions of 7 and 30 at.% Mg, respectively. This enhanced hardness relative to pure nanocrystalline Al of comparable grain size was dominated by grain boundary segregation strengthening, which scaled with the interfacial excess driven by the reduction in the grain boundary formation energy upon segregation. The yield strength from microcompression experiments on this particular alloy was 865 MPa, resulting in a specific strength of 329 kN-m/kg.

When a constant load is applied to a crystalline metallic material at room temperature, no permanent plastic deformation or structure change could take place if the load is below the yield strength. However, metallic glasses does not follow this role, in which static load, even below the yield strength, could cause significant change in structure and mechanical properties. To clarify this issue, in the present study, elastostatic compression test on Zr_60.14Cu_22.31Fe_4.85Al_9.7Ag₃ bulk metallic glass (BMG) was performed at stress level of 90% of the yield strength for different holding times (24-120 hs). The structural evolution of the BMG and mechanical properties at different loading stages was then systematically examined by differential scanning calorimetry, density measurement and, instrumental nanoindentation and quasi-static compressive tests. It was found that homogeneous deformation with 0.48% plastic strain and without shear banding was achieved after holding for 120 days. The pre-loaded BMG shows decreased relaxation enthalpy and increased hardness and density with increasing the loading times, indicating stress-induced relaxation and hardening. The phenomena are interpreted in terms of atomic level stress theory and the coalescence of negative and positive free volume in BMGs.

One of the central tasks in materials science is to systematically build a ‘phase diagram’--the correlation amongst composition, structure, processing parameters and properties of materials. Conventional way of phase diagram construction, feathered by synthesizing and characterizing one sample at a time, is costly, time-consuming, unsystematic, and, given these disadvantages, insufficient for the current era of accelerating technology. In contrast, combinatorial materials chip technology, featuring high-throughput synthesis and the characterization of materials libraries containing 10²–10⁴ samples on one small substrate, demonstrated great potential to significantly accelerate the process. In this study, we attempted to establish isothermal section of a ternary phase diagram with combinatorial materials chip preparation technique, combined with the quick, accurate and systematic compositional and structural characterization by synchrotron radiation and hierarchical clustering analysis.
The multi-layered thin film precursors covering the entire composition range of Fe-Co-Ni ternary system was prepared using a high throughput combinatorial ion beam deposition system (HTC-IBD) with a continuously moving shutter. The composition of each sample (pixel) is determined based on the relative thickness of the three component layer on the pixel. The overall thickness of the film stack is 100 nm. The as-deposited chip was sealed into an evacuated quartz tube and crystallized isothermally in a muffle furnace at the designated temperature for 2 hours.
Micro-beam X-ray diffraction was performed pixel-by-pixel on beamline 11-ID-D of APS，Argonne National Laboratory. The diffracted signal was collected using a Pilatus 2M area detector by Dectris Inc. at a rate of 1 s/pattern. A total number of over 1000 X-ray diffraction patterns were automatically processed, phase-identified and categorized by hierarchical clustering analysis using a self-developed computer software CombiView. The isothermal section of the Fe-Co-Ni ternary phase diagram constructed by the combinatorial chip approach in this study is consistence with the phase diagram in the ASM Alloy Phase Diagram Database^TM.

The selective electrochemical/chemical dissolution of less noble components of an alloy constitutes the process known as de-alloying.Both crystalline [1-4] and amorphous alloys can be selectively etched giving nanoporous structures with different morphology [4-8]. Although the mechanism of de-alloying has been studied extensively for crystalline materials [4], the mechanism of ligament development by de-alloying amorphous alloys is unknown yet. Here we report on the de-alloying mechanism of the Au₄₀Cu₂₈Ag₇Pd₅Si₂₀alloy in the form of amorphous ribbons, starting from the very first steps of the process until full development of ligaments and pores by using a combination of microscopy techniques[8]. To improve the understanding of microstructures further, relaxed, partially, and fully crystallized ribbons, which were fabricated by isothermal annealing at temperatures close to the glass transition, Tg, were also de-alloyed. The results show how the alloy phase constitution affects the final structure.

Liquid metal dealloying (LMD) is a promising technique to form a variety of composites and freestanding porous structures from a broad range of metals. In LMD the structure is formed by spontaneous pattern formation during selective dissolution of one or more component of an alloy in a liquid melt. LMD from binary alloys can produce several distinct morphologies, spanning non-connected droplets to fully bicontinuous porous networks. In order to engineer complex structures with multiple desired properties, LMD must be understood in a more nuanced way, encompassing the details of surface evolution during dealloying and interactions in multicomponent alloy systems. In this work, we present advances in LMD that provide control over phase nucleation and crystallographic texturing in dealloyed composites.
First, we investigate adding minority elements to the dealloying system to modify the dealloying behavior. As a model system, we study the effects of a small amount of Si added to the initial binary alloy in the existing refractory metal-Cu LMD system. This change causes the final structure to contain dispersed refractory silicide phases with various morphologies, which form during dealloying as dissolved Si in the liquid phase reacts with the metals present.
Second, we study the dealloying of silicon alloys. Due to surface energy interactions during dealloying, we control the morphology of the ligaments in the material while simultaneously varying their surface characteristics.
Together, these techniques demonstrate general pathways to the fabrication of more complex structures by LMD. Taking advantage of the diffusion-limited nature of the dealloying process allows control of the nucleation and growth of multiple phases during a single processing step. Control of the surfaces produced by dealloying provides another additional technique to engineer the functionality of the dealloyed material.

Interfacial forces between crystals that depend on their mutual crystallographic alignment enable diverse multiscale phenomena such as crystal growth by oriented aggregation, Schiller layer formation, adhesion and friction anisotropy, and grain boundary structuring in polycrystals. While the attraction or repulsion of two crystallites is based on a set of forces that are generally well understood aspects of these forces that are sensitive to lattice alignment have been more difficult to probe. For example, in oriented aggregation, where crystal growth entails self-assembly from nanocrystals, the existence of torque-generating forces that align approaching particles to enable adhesion have been implied more often than proven. Only in recent in situ liquid-cell transmission electron microscopy (TEM) measurements was it shown that the rotation of ferrihydrite particles accelerates when they are nearly co-aligned, while still separated by a nanometer of solution. Likewise, for dipolar nanoparticles such as Pt₃Fe, anisotropic attachment forming chains-by-ends has been directly observed in situ. Conceptually, the types of interfacial forces that can be sensitive to lattice direction include Coulombic, van der Waals, solvation, and ion correlation forces. Although the theoretical underpinnings of these anisotropic forces are well established, techniques that can isolate and measure their magnitudes for a given pair of interacting oriented crystal faces have generally been limited to use of macroscopic yet atomically flat single crystals. Here we demonstrate direction-specific interaction force between nanocrystals, including TiO₂-TiO₂ and ZnO-ZnO, under precise control of distance, mutual orientation, and surface hydration extent. The force measurement was performed using atomic force microscope (AFM) and in-situ TEM. The results indicated that at tens of nanometers of separation the attraction is weak and shows no dependence on azimuthal alignment nor surface hydration. In attractive contact, the force strongly depends on azimuthal alignment, and systematically decreases as adsorbed water density increases. Measured forces are in comprehensive agreement with predictions from Lifshitz theory, and the role of intervening water emerges from potentials of mean force molecular dynamics simulations. The findings contribute to the understanding of torque-generating forces between particles interacting in solution and grains in materials.

Ca-based bulk metallic glasses (BMGs) have the lowest density compared to any other BMG group, and elastic and bulk moduli comparable to human bones which make them potential candidate for biomedical applications. They have low characteristic temperatures e.g., T_g, just above 100°C. It is crucial to investigate their crystallization behavior at high temperature and under stress to explore the stability of amorphous structure and to determine their suitability for potential applications. So far very few studies are conducted on their crystallization behaviors and there is no in-situ study. In this study, three Ca_75-xMg₂₅Cu_x (x=18, 27 and 35) BMGs with different glass forming abilities are considered. Effect of increasing Cu concentration on the as-cast amorphous structure is explored. Crystallization kinetics of Ca₄₀Mg₂₅Cu₃₅ BMG are studied under isothermal annealing above the glass transition temperature with and without load in-situ by using synchrotron high-energy x-ray diffraction. Under isothermal annealing conditions, crystallization process started right from the beginning of the experiment and its rate was quite rapid. However when compressive load was applied simultaneously while isothermally heating at the same temperature, no change in the atomic order was observed during the first ten minutes and after that there was only atomic rearrangement without any noticeable crystallization. It is suggested that stress restricted the atomic mobility, which suppressed crystallization.

Multi-principal element alloys (MPEAs), also known as compositionally complex alloys (CCAs) or high-entropy alloys (HEAs), have attracted much attention in recent years. Most studies on these alloys set off mainly to exploit the configurational entropy based single phase stabilization concept, yet, the resultant mechanical properties are often not substantially improved compared to more traditional low-entropy alloys such as steels or Ti alloys. Recently we developed a novel metastability-engineering strategy and the obtained transformation-induced plasticity (TRIP) CCAs (HEAs) show combined increase in both, strength and ductility. Here, we present how we further tuned the phase stability of the alloys by addition of interstitial carbon into the dual-phase structure. The approach, leading to a new class of interstitial CCAs, enables the synthesis of alloys that show, both TRIP and twinning-induced plasticity (TWIP) effects upon deformation. The novel interstitial CCA also contains a very low volume fraction of nano-carbides with elemental composition near to M₂₃C₆ (M: Cr, Mn, Fe, Co) according to atom probe tomography (APT) measurements. Owing to the multiple strengthening mechanisms active, the tensile strength of the grain-refined interstitial CCA is approximately twice that of the corresponding single-phase equiatomic CoCrFeMnNi alloy, while their elongation values under tensile load are identical.