Search

A wide range of behaviors, including non-monotonic rejuvenation and relaxation, and the ability to qualitatively change the effect by varying the structural state of the glass was observed during thermal cycling of bulk metallic glasses. For this, we considered various bulk metallic glasses, Zr44Ti11Cu10Ni10Be25, Pd43Cu27Ni10P20, Pt57.5Cu14.7Ni5.3P22.5, and La55Al25Ni20, at various fictive temperatures to study the effect of thermal cycling on structure, thermal signature, and fracture toughness. For some BMGs and conditions considered here, thermal cycling results in a looser structure and an increase in fracture toughness. We found that for certain other BMGs and conditions, thermal cycling results in relaxation, reflected in a denser structure, and a decrease in fracture toughness. All these responses are non-monotonic and reveal a pronounced extremum with fracture toughness values of ± 50% of the original value, before approaching a value similar to the original value prior to thermal cycling. Such richness in response to thermal cycling suggests incompleteness of the previous picture where monotonically decreasing local stresses resulting in a homogenization of the structure with increasing cycle number. Our finding suggests that relative comparisons of various contributions including activation barriers for α-relaxation have to be considered which are also constantly changing, to decide if further cycling results in an increase or a decrease in fracture toughness. The fracture toughness’ response to thermal cycling can be correlated with the average atomic structures’ response to thermal cycling, while the thermal response does not exhibit an obvious correlation.

High Entropy Alloys are inherently complex and span a vast composition space, making their research and discovery challenging. Developing quantitative predictions of their phase selection requires a large quantity of consistently determined experimental data. Here, we use combinatorial methods to fabricate and characterize 2478 quinary alloys based on Al and transition metals. All data are publicly available at http://materialsatlasproject.org/ . Phase selection can be predicted for considered alloys when combining the content of FCC/BCC elements and the constituents’ atomic size difference. Mining our data reveals that High Entropy Alloys with increasing atomic size difference prefer BCC structure over FCC. This preference is typically overshadowed by other selection motifs, which dominate during close-to-equilibrium processing. Not suggested by the Hume-Rothery rules, this preference originates from the ability of the BCC structure to accommodate a large atomic size difference with lower strain energy penalty which can be practically only realized in High Entropy Alloys.

Discovering new metallic glasses, non-crystalline alloys with unique combinations of mechanical and chemical properties, is a challenging endeavor because it requires exploration of a vast composition space. High-throughput experiments have greatly enhanced the efficiency with which composition-dependent properties of potential glass-forming alloys can be measured, but phase identification remains a bottleneck because slow or expensive techniques such as table-top or synchrotron-based X-ray diffraction measurements are required. In this study, we developed machine learning (ML) models that can classify amorphous and crystalline phases of alloys using electrical resistivity as a primary descriptor. Artificial neural networks were constructed to correlate the electrical resistivities and the X-ray diffractograms of a broad range of combinatorially synthesized alloys. The ML models are found to classify amorphous/crystalline phases in both thin-film libraries and bulk alloys with high accuracy.

We describe a sensor for measuring the electrical resistance of a conducting thin-film material as a function of temperature and composition. The sensor has excellent sensitivity and can be used at temperatures as high as the melting temperature of the material of interest. The sensor is fabricated by applying a simple lift-off process to a thin film. By combining combinatorial sputtering to fabricate composition spreads with arrays of sensors, the phase transformation behavior of complex alloys can be mapped. We demonstrate this capabilities by using the sensor to determine the glass transition and crystallization temperatures of several PdSiCu-based metallic glasses. We found that in two glass-forming systems, PdCuSi and NiZr, the ratio of the resistance of the crystallized to as-deposited material is correlated with the glass-forming ability. The ability to readily determine glass forming ability, suggests that the sensor is a powerful tool for measuring the glass-forming ability in a high-throughput manner over large compositional spaces.

The influence of test sample geometry on the fracture toughness measurements of bulk metallic glasses (BMGs) is systematically quantified. Through a combination of thermoplastic forming based toughness measurements and numerical simulations, we investigated the effects of test sample thickness, width, and notch precision on the fracture toughness of BMGs. Considering these findings, together with BMGs' specific fabrication challenges and potential application geometries, we propose that sample geometry to measure fracture toughness of BMGs should be much smaller and practically achievable than suggested by standards for crystalline alloys.

We have investigated the crystallization kinetics of Cu50Zr50 metallic glass thin films using nanocalorimetry. The crystallization process is growth-controlled during heating and nucleation-controlled during cooling, resulting in different critical heating and cooling rates to suppress crystallization. Measurements over a wide range of scanning rates (13 K/s to 21,000 K/s) reveal that crystallization does not follow Arrhenius kinetics upon heating. Instead, the behavior on heating is well described by a fragility-based model of growth-controlled kinetics that takes into account breakdown of the Stokes-Einstein relationship. Upon cooling, the quench rate required to suppress crystallization of the melt is much higher than for bulk samples. This reduced asymmetry in critical heating and cooling rates compared to bulk materials suggests that crystallization of the thin-film metallic glass is controlled by heterogeneous nucleation.

The flaw tolerance of bulk metallic glasses (BMGs) is evaluated using a thermoplastic synthesis approach. We found that flaw tolerance quantified by the notch toughness decreases apparently with decreasing radius until a critical value. Below this critical value, measured notch toughness is independent of its radius, revealing a flaw tolerance behavior of BMGs. We explain such flaw tolerance by a critical plastic zone originating from the BMGs' inherent crack tip blunting capability. This zone defines a characteristic distance over which stable shear banding plastic process develops prior to fracture instability. The specific characteristic distance and crack blunting capability vary widely among BMGs, which rationalizes the vast variety in their fracture behavior and suggest specific flaw tolerance. Our finding is encouraging for BMGs' structural applications since flaws smaller than the critical value are increasingly difficult to avoid but are “indistinguishable” in their influence to fracture toughness.

3D Metallic glass structures (3DMGs) are fabricated through thermoplastic forming (TPF)-based patterning of MG sheets combined with a parallel joining technique. To demonstrate this capability and benchmark 3DMGs, we have fabricated honeycomb-like MG architectures covering a wide range of relative densities. 3DMGs exhibit high elasticity of up to 40% loading strain, high elastic energy storability, and high energy absorption which is superior compared to those made from other materials such as conventional metals and ceramics, based on our theoretical analysis. The combination of MG properties and introduced versatile fabrication method suggest the possibility of developing a wide range of 3DMGs with excellent performance for specific applications.

Ion irradiation of thermoplastically molded Pt57.5Cu14.3Ni5.7P22.5 metallic glass nanowires is used to study the relationship between glass structure and tensile behavior across a wide range of structural states. Starting with the as-molded state of the glass, ion fluence and irradiated volume fraction are systematically varied to rejuvenate the glass, and the resulting plastic behavior of the metallic glass nanowires probed by in situ mechanical testing in a scanning electron microscope. Whereas the as-molded nanowires exhibit high strength, brittle-like fracture and negligible inelastic deformation, ion-irradiated nanowires show tensile ductility and quasi-homogeneous plastic deformation. Signatures of changes to the glass structure owing to ion irradiation as obtained from electron diffraction are subtle, despite relatively large yield strength reductions of hundreds of megapascals relative to the as-molded condition. To reconcile changes in mechanical behavior with glass properties, we adapt previous models equating the released strain energy during shear banding to a transit through the glass transition temperature by incorporating the excess enthalpy associated with distinct structural states. Our model suggests that ion irradiation increases the fictive temperature of our glass by tens of degrees – the equivalent of many orders of magnitude change in cooling rate. We further show our analytical description of yield strength to quantitatively describe literature results showing a correlation between severe plastic deformation and hardness in a single glass system. Our results highlight not only the capacity for room temperature ductile plastic flow in nanoscaled metallic glasses, but also processing strategies capable of glass rejuvenation outside of the realm of traditional thermal treatments. 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Stochastic cellular structures are prevalent in nature and engineering materials alike. They are difficult to manipulate and study systematically and almost always contain imperfections. To design and characterize various degrees of imperfections in perfect periodic, stochastic and natural cellular structures, we fabricate a broad range of metallic glass cellular structures from perfectly periodic to highly stochastic by using a novel artificial microstructure approach based on thermoplastic replication of metallic glasses. For these cellular structures, precisely controlled imperfections are implemented and their effects on the mechanical response are evaluated. It is found that the mechanical performance of the periodic structures is generally superior to that of the stochastic structures. However, the stochastic structures experience a much higher tolerance to flaws than the periodic structure, especially in the plastic regime. The different flaw tolerance is explained by the stress distribution within the various structures, which leads to an overall "strain-hardening" behavior of the stochastic structure compared to a "strain-softening" behavior in the periodic structure. Our findings reveal how structure, "strain-hardening" and flaw tolerance are microscopically related in structural materials.

We present a thermoplastic deforming method to join metallic glasses in air. Mechanistically during straining of the interface the oxide layer breaks and pristine alloy flows towards the interface and forms a metallic bond. To demonstrate the effectiveness of this method we chose reactive Zr 35 Ti 30 Cu 7.5 Be 27.5 as an example bulk metallic glass system. A model is introduced which quantitatively predicts the bonding strength solely from the shear strength of the metallic glass, the initial surface roughness, and the applied strain. The ability to join even reactive metallic glasses in air on a timescale of the order of milliseconds to seconds at low pressure and temperature with predictable joint strength suggest a highly practical and economic method to join metallic glasses.

Embrittlement of Zr46.75Ti8.25Cu7.5Ni10Be27.5 and Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glasses (BMGs) is studied after annealing at temperatures below and above the glass transition temperature Tg for time scales comparable with structural relaxation and crystallization. The effect of annealing on the bending ductility, the isoconfigurational elastic constants, the structure and the thermal stability is examined. The embrittlement during sub-Tg annealing originates from structural relaxation and can be reversed by subsequently annealing for a short duration above Tg. The embrittlement kinetics correlate with the structural relaxation. However, only a fraction of relaxation time at a given temperature (

A method is introduced as a standard to characterize the formability, the maximum strain a bulk metallic glass (BMG) can undergo in its supercooled liquid state before it eventually crystallizes. When considering accuracy and practicality it was found that the maximum diameter to which a 0.1 cm 3 BMG sample can be formed during heating through the supercooled liquid temperature region under a constant load of 4500 N is best suited as a measure of formability. Among the ten different alloys considered, by far the highest formability was found for Pt 57.5 Cu 14.7 Ni 5.3 P 22.5 . More generally, the results suggest that fragile liquid behavior, large Poisson ratio, and a low glass transition temperature are attributes indicating good formability. Various parameters, as well as an analytical expression for the formability, are tested against the experimentally determined formability to assess the degree of correlation.

We report on differential scanning calorimetry (DSC) studies to characterize the thermodynamics, kinetics and crystallization processes of the bulk metallic glass-forming alloy Pt 57.3 Cu 14.6 Ni 5.3 P 22.8 . The heat capacity of the alloy is measured for the crystalline, glassy and supercooled liquid phases. The heating rate dependence of the glass transition is used to calculate the kinetic fragility. Crystallization kinetics are determined under isothermal conditions and used to construct a time–temperature-transformation (TTT) diagram. The experimentally determined crystallization kinetics are fit to calculate the activation energy for crystallization. Our results suggest that Pt 57.3 Cu 14.6 Ni 5.3 P 22.8 is neither a thermodynamically nor a kinetically stabilized glass former. Other contributions, including the activation energy for crystallization and the use of a flux are considered in the discussion to explain the good glass formability of this alloy.

The trends in glass formation among bulk metallic glass-forming alloys are investigated within the framework of viscosity and specific volume measurements. This investigation was carried out using four alloys (Zr41.2Ti13.8Cu12.5Ni10Be22.5, Zr57Cu15.4Ni12.6Al10Nb5, Zr52.5Cu17.9Ni14.6Al10Ti5 and Ni59.5Nb40.5) that have widely different glass forming abilities (GFAs). This study shows that the viscosity at the melting temperature is correlated with volume change upon crystallization in accordance with Cohen–Grest free-volume theory. The viscosity of the best glass former (Zr41.2Ti13.8Cu12.5Ni10Be22.5) is found to be an order of magnitude larger compared to the worst glass former (Ni59.5Nb40.5) at their respective liquidus temperatures. The other two alloys have intermediate values. The specific volume results also support the trend in GFA with the best glass former showing the smallest volume change upon crystallization and worst glass former showing the largest change. This study suggests that high viscosity and correspondingly small free volume of the liquid at the melting temperature contribute significantly to improve the GFA.

Crystal nucleation in metallic glass forming systems is explored by means of a non-classical model assuming a diffuse solid/liquid interface. The model predictions are demonstrated initially for an example system with general hypothetical properties. The predicted nucleation rate matches that obtained from the classical nucleation theory for shallow undercoolings, and deviates from it with increasing undercooling. For certain system properties, a physical spinodal is predicted, i.e. the calculated energy barrier to crystal nucleation vanishes at a finite critical undercooling. The numerical results for the example system are then fitted into a simple formula for nucleation frequency, in which the critical undercooling is taken as an adjustable parameter. This formula fits well into the nucleation data of the bulk metallic glass forming system Zr 41 Ti 14 Cu 12 Ni 10 Be 23 , when a critical undercooling of 440 K is taken. The existence of a physical spinodal in this system is shown to be consistent with various other experimental observations. It is also shown that the nucleation behaviour in such systems cannot be explained by classical nucleation theory, even if phase separation in the liquid is taken into account.

Crystallization of undercooled Pd 43 Ni 10 Cu 27 P 20 melts is studied in a differential scanning calorimeter. Isothermal experiments allow us for the first time to determine the entire crystallization kinetics of a metallic liquid as a function of time from the liquidus temperature to the glass transition temperature. The results are summarized in a time–temperature-transformation (TTT) diagram that reveals two time scales. One is given by the time to reach 1% of crystallized volume fraction and reflects the typical “nose” shape of the TTT-diagram. The other is the width of the crystallization event itself, which increases with decreasing temperature from 90 s at 793 K to 10,200 s at 623 K. Additional information about the crystallization process is gained by dividing the sample into about 300 particles that are processed simultaneously and crystallization of each individual particle can be detected. At high temperatures the onset of crystallization of individual particles are spread out over 1.5×10 5 s, whereas all particles crystallize simultaneously below the nose and the crystallization is not distinguishable from that of one large sample. The results suggest that the dominant crystallization mechanism changes in a very narrow temperature range from a nucleation-controlled process at high temperatures to a growth-controlled process at low temperatures.

Al–Cu–Fe, Al–Fe and Al–Cu–Co melts of different compositions were undercooled by containerless processing in an electromagnetic levitation facility. The phase selection during solidification from the undercooled melt was determined by direct measurements of the temperature changes during recalescence. Complementarily, the phase selection and microstructure development was studied by scanning- and transmission electron microscopy (SEM, TEM) and X-ray diffraction (XRD) on the as-solidified samples with the undercooling and the alloy composition as experimental parameters. For comparison, rapidly quenched samples of the same alloys were produced by splat-cooling and investigated by TEM and XRD. The undercooling results were analyzed within the framework of classical nucleation theory. The activation threshold for the nucleation was found to be small for the icosahedral quasicrystalline phase in Al–Cu–Fe, medium for the decagonal D-phase in Al–Cu–Co and crystalline phases with polytetrahedral symmetry elements (Al13Fe4 and Al5Fe2), but large for the cubic phase of Al50(CuCo)50 with non-polytetrahedral crystalline symmetry. These results are explained assuming of an icosahedral short-range order that prevails in the undercooled melt and gives rise to an interfacial energy decreasing with increasing degree of polytetrahedral order in the solid nucleus.