Bulk Metallic Glass Formation Research Articles

Turing pattern and chemical medium-range order of metallic glasses

The formation of bulk metallic glass requires the constituent elements to have a negative heat of mixing but has no restrictions on its magnitude. An understanding of this issue is lacking due to the absence of a valid method for describing chemical ordering of metallic glasses. For example, the radial distribution function is ineffective in identifying the elemental preferences of packed atoms. Here, we show that using molecular-dynamics simulation, the chemical medium-range ordering of liquid alloys can be evaluated from persistent homology. This inherently arising chemical medium-range order in metallic glasses is exclusively regulated by the activation and inhibition of the constituent components, making the topology of metallic glasses a Turing pattern. The connecting schemes of atoms of the same species form three distinct regions, reflecting different correlations at the short and medium length scales, while the difference in the schemes corresponds to chemical ordering. By changing the elemental types, it is demonstrated that the chemical medium-range order strongly depends on the relative depth of the interatomic-potential wells. The study separates metallic glasses from crystals under the condition of negative heat of mixing by emphasizing their fundamental difference in interatomic potentials.

Enhancing Mechanical and Biocorrosion Response of a MgZnCa Bulk Metallic Glass through Variation in Spark Plasma Sintering Time

Development of metallic glasses is hindered by the difficulties in manufacturing bulk parts large enough for practical applications. Spark plasma sintering (SPS) has emerged as an effective consolidation technique in the formation of bulk metallic glasses (BMGs) from melt-spun ribbons. In this study, Mg65Zn30Ca5 melt-spun ribbons were sintered at prolonged sintering times (15 min to 180 min) via SPS under a pressure of 90 MPa and at a temperature of 150 °C (which is below the crystallization temperature), to provide an insight into the influence of sintering time on the consolidation, structural, and biodegradation behavior of Mg-BMGs. Scanning Electron Microscopy was used to characterize the microstructure of the surface, while the presence of the amorphous phase was characterized using X-ray diffraction and Electron Backscatter Diffraction. Pellets 10 mm in diameter and height with near-net amorphous structure were synthesized at 150 °C with a sintering time of 90 min, resulting in densification as high as 98.2% with minimal crystallization. Sintering at extended durations above 90 min achieved higher densification and resulted in a significant amount of local and partial devitrification. Mechanical properties were characterized via compression and microhardness testing. Compression results show that increased sintering time led to better structural integrity and mechanical properties. Notably, SPS150_90 displayed ultimate compressive strength (220 MPa) that matches that of the cortical bone (205 MPa). Corrosion properties were characterized via potentiodynamic polarization with Phosphate Buffered Solution (PBS). The results suggest that the sintered samples have significantly better corrosion resistance compared to the crystalline form. Overall, SPS150_90 was observed to have a good balance between corrosion properties (10× better corrosion resistance to as-cast alloy) and mechanical properties.

High-Throughput Synthesis and Characterization Screening of Mg-Cu-Y Metallic Glass

Bulk metallic glasses can exhibit novel material properties for engineering scale components, but the experimental discovery of new alloy compositions is time intensive and thwarts the rate of discovery. This study presents an experimental, high-throughput methodology to increase the speed of discovery for potential bulk metallic glass alloys. A well-documented system, Mg-Cu-Y, was used as a model system. A laser additive manufacturing technique, directed energy deposition, was used for the in situ alloying of elemental powders to synthesize discrete compositions in the ternary system. The laser processing technique can supply the necessary cooling rates of 103–104 Ks−1 for bulk metallic glass formation. The in situ alloying enables the rapid synthesis of compositional libraries with larger sample sizes and discrete compositions than are provided by combinatorial thin films. Approximately 1000 discrete compositions can be synthesized in a day. Surface smoothness, as discerned by optical reflectivity, can suggest glass-forming alloys. X-ray diffraction coupled with energy dispersive X-ray spectroscopy can further refine amorphous alloy signatures and compositions. Transmission electron microscopy confirms amorphous samples. The tiered rate of amorphous alloy synthesis and characterization can survey a large compositional space and permits a glass-forming range to be identified within one week, making the process at least three orders of magnitude faster than other discrete composition techniques such as arc-melting or melt-spinning.

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

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

Designing bulk metallic glasses materials with higher reduced glass transition temperature via machine learning

Bulk metallic glasses (BMGs) are considered as one of the most promising candidates in the field of alloy materials due to their outstandingly high yield strengths and large elastic limits at ambient conditions compared to their crystalline counterparts. The formation of BMGs is determined by reduced glass transition temperature (Trg) that represents the glass forming ability (GFA). Experimental search for BMGs with higher Trg is a challenging task due to the vast chemical space and lack of predictive guidelines. Here, we demonstrate a machine learning approach to guide experimental synthesis in search of BMGs with higher Trg. The relationships between the Trg and chemical compositions of BMGs were investigated by using three different machine learning algorithms including support vector regression (SVR), back propagation artificial neural network (BPANN) and gradient boosting regression (GBR). The best predictions from SVR, BPANN, and GBR models were recommended for experimental tests. Based on the leave-one-out cross-validation (LOOCV), the correlation coefficient (R) and root mean squared error (RMSE) between predicted and experimental Trg were 0.894 and 0.0116, respectively. Furthermore, virtual screening based on machine learning results was adopted to design BMGs with higher Trg. The average relative error of Trg from newly synthesized BMGs is 1.00%, which is satisfactory compared to that of the known experimental samples (3.91%). Among the newly synthesized BMGs, Mg0.65Ag0.19Cu0.06Gd0.1 owns the highest Trg of 0.6658, exceeding the highest Trg (0.6576) in the reported experiments before.

Atomic transport in amorphous metals

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

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

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

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

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

Formation and properties of biocompatible Ti-based bulk metallic glasses in the Ti–Cu–Zr–Fe–Sn–Si–Ag system

The Ni-free Ti-based bulk metallic glasses (BMGs) with high glass-forming ability (GFA) can be synthesized in the (Ti, Cu, Zr)92.5Fe2.5Sn2Si1Ag2 alloy system over a wide composition range by copper-mold casting. Especially, the Ti48Cu37Zr7.5Fe2.5Sn2Si1Ag2 glassy alloy can form the BMG rod with a diameter up to 6 mm and showed a high compressive strength of ~ 2.05 GPa, elastic modulus of ~ 100 GPa, and microhardness of ~ 571 Hv. The Ti-based BMGs presented good bio-corrosion resistance in a simulated physiological environment attributed to the formation of Ti-, Zr-, Si-, and Fe-enriched passive films. The biocompatibility of the Ti-based glassy alloys was systematically investigated by both indirect cytotoxicity test and direct cell-culture experiments. The Ti-based BMG extracts had no cytotoxic effects on the proliferation of the cells. The good cell adhesion, proliferation, differentiation, and mineralization of bone-forming MC3T3-E1 pre-osteoblasts on the BMGs were parallel to those on the Ti–6Al–4V alloy. These results manifested that the present Ti-based BMGs are promising candidates as biomaterials.

Tuning Glass Formation and Mechanical Properties of ZrCoAl(Nb) Bulk Metallic Glass with Nb Microalloying Process

In the present study, the effect of Nb addition on mechanical behavior and glass formation in (Zr56Co28Al16)100−xNb(x=0,2,4) bulk metallic glasses was studied. The differential scanning calorimetry (DSC) was used to investigate thermal properties of samples. Further, mechanical behavior of prepared samples was analyzed using quasi-static compressive tests. The results showed that 2 at.% minor addition of Nb enhances the glass forming ability (GFA), but larger amount of Nb (4%) deteriorates GFA. The X-ray diffraction patterns also confirmed that the structures of whole of studied compositions are amorphous. In addition, the compressive test results revealed that 2% addition of Nb significantly improves compressive ultimate strength (up to 2.5 GPa) and increases the plastic strain from 1.1% in the composition having no Nb content to ~ 6%. The observation of shear band branching and also the increase in the density of vein-like patterns in the fracture surface (studied by scanning electron microscopy) also confirm the improvement of plasticity in (Zr56Co28Al16)98Nb2 sample. Our finding highlights the effect of minor alloying on the formation and mechanical properties of bulk metallic glasses.

A New High‐Entropy Alloy of Al–Fe–Co–Ni–Cu Possessing Single Face‐Centered Cubic Crystal Structure and Excellent Mechanical Properties at Room Temperature

A new powder combination of Al10(FeCoNiCu)90 is selected and verified for the potential to form solid solutions or else bulk metallic glasses (BMGs) in the alloy based on Guo et al.'s conditions and mismatch entropy criterion. The powder combination satisfies the above criteria to crystallize in solid‐solution phases and inhibits the formation of BMGs in the alloy. The alloy is fabricated by mechanical alloying of the powder mixture followed by sintering. A homogeneous powder mixture is obtained after ball milling for 80 h. The 80 h‐milled powder is crystallized in two face‐centered cubic (FCC) phases. The lattice parameters of the FCC1 and FCC2 phases are 3.60 Å and 5.21 Å, respectively. The microstructure of the alloy ingot reveals spinodal decomposition. The alloy ingot crystallizes in a single‐FCC structure. The lattice parameter of the corresponding FCC structure is determined as 3.66 Å. The compression test of the alloy ingot at room temperature reveals that the 0.2% offset yield stress of the high‐entropy alloy (HEA) is as high as 308 MPa. The compressive stress at 40% strain of the HEA is 1406 MPa. The overall Vickers hardness and density of the alloy ingot are 160 ± 14 HV0.5 and 7.7 g cm−3, respectively.

Effects of minor Si addition on structural heterogeneity and glass formation of GdDyErCoAl high-entropy bulk metallic glass

Glass-forming ability (GFA) of the Gd20Dy20Er20Co20Al20 high-entropy bulk metallic glass (HE-BMG) was improved through microalloying Si element. The critical diameter of this rare-earth-based HE-BMG increases from 1.5 to 8.5 mm with 0.5 at.% Si addition without any deterioration of magnetocaloric property. Compared with the Si-free sample, the BMG with 0.5 at.% Si addition exhibits low driving force for crystallization, strong supercooled liquid behavior and sluggish crystallization kinetics, whereas excessive Si addition leads to reverse variations. Furthermore, microalloying Si can enhance nanoscale heterogeneity of elemental distribution, and thus facilitates the formation of local crystal-like structures. The connectivity and competition between icosahedra-like clusters and crystal-like structures promote the formation of stable network structure which frustrates crystallization. As Si content is excessive, icosahedra-like cluster is consumed by the further grown crystal-like structure, leading to the destruction of network and decreasing GFA. This work not only elucidates the underlying mechanism of the GFA enhanced by microalloying, but also sheds light on improving the GFA of HE-BMGs through altering nanoscale elemental distribution and structural heterogeneity.

Developing in-situ Zr-based bulk metallic glass composites from multi-cluster competition strategy

The mechanism of structural evolution and phase formation for in-situ bulk metallic glass composites (BMGCs) remains unclear and needs to be described via a comprehensive strategy. Herein, four chemical compositions (named as ZN10, ZN30, ZN50, and ZN70, respectively) were designed and used as base alloys for exploring the microstructures and mechanical properties from the thermodynamic and structural points of view. A pseudo-binary (ZrNb)-(CuNiAl) phase diagram was constructed to predict the relationship between structural evolutions and chemical compositions. Phase formation and composition distributions of bulk metallic glasses (BMGs) and in-situ BMGCs were carefully investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with an energy-dispersive spectrometer (EDS). Based on experimental results, the structural evolutions were analyzed and discussed in details, showing the evolution from the atomic-scale ordered clusters to micro-scale phase precipitation in the supercooled liquid. Furthermore, the compressive mechanical tests of as-cast samples were performed to unveil the correlation between microstructures and deformation behaviors of different alloys. It is believed that the proposed strategy and experimental results can deepen the understanding of the formation of BMGs and in-situ BMGCs associated with atomic-scale ordered structures, and provide new insights to design in-situ BMGCs from the structural perspective.

Generality of abnormal viscosity drop on cooling of CuZr alloy melts and its structural origin

It has been widely accepted that the viscosity of alloy melts increases monotonically with temperature decreasing after exceeding the liquidus temperature. However, a distinct viscosity-drop has been observed in several alloy melts in our previous works. To further understand this dynamic anomaly, we investigated the viscosity of CuZrAl and CuZrTiNi melts by gradually changing the compositions of alloy systems. The results suggest that when the amounts of doping elements (Al, Ti, Ni) become large enough, this abnormal viscosity-drop disappears. For CuZr binary alloys, the anomaly exists when the composition of Zr ranges between 27.3–66.7%, which nearly corresponds to the available content of Zr in CuZr bulk metallic glasses. By conducting molecular dynamics simulation, the mechanism that four icosahedron-like clusters with high fragility evolves with temperature in Cu62Zr38 melts has been recognized. In contrast to general aggregation or growth of clusters during cooling process, an unexpected tendency that more segregations are formed among these clusters and the clusters become further dispersed in liquids has been discovered around 1400 K; meanwhile, the viscosity-drop in our experiments begins simultaneously at this temperature. Below 1300 K, these icosahedron-like clusters aggregate abruptly again. Experimental results from differential scanning calorimeter (DSC) and high-resolution transmission electron microscope (HRTEM) further approve the influence of these fragile icosahedron-like clusters on crystallization processes. This finding uncovers the same structural origin that underlies both the formation of bulk metallic glasses and the dynamic anomaly in melts, and demonstrates the dominant role of fragile icosahedron-like clusters in the feature of liquid dynamics.

Impact of Sulfur on the melt dynamics of glass forming Ti75Ni25−xSx

Substitution of Nickel by Sulfur leads to bulk metallic glass formation in eutectic Ti75Ni25. In order to understand the improved glass forming ability, a detailed analysis of the melt dynamics was performed. We measured the density, the viscosity, and the self-diffusion of Ti75Ni25−xSx (x = 0, 5, 8) melts as a function of temperature. Moreover, the viscosity was measured under reduced gravity conditions to benchmark ground-based data. The self-diffusion coefficients were probed by incoherent, quasi-elastic neutron scattering. All measurements were combined with electrostatic or electromagnetic levitation, in order to process these highly reactive Titanium-based melts. From the measured density, the packing fraction was calculated as a function of temperature. In contrast to other bulk metallic glasses, the decrease in melt dynamics comes along with the decrease in packing density.

Effect of Minor Addition of Oxygen on Bulk Metallic Glass Formation of Binary Cu–Zr Alloys via Containerless Processing

The effect of oxygen content on the glass‐forming ability of binary Cu–Zr alloys is investigated by the in situ observation of the cooling process during the containerless solidification using an aerodynamic levitation technique. The oxygen content of ≈0.72 at% induces the crystallization of undercooled liquid Cu47Zr53 alloy, whereas that of ≈0.33 at% enhances the glass‐forming ability and leads to the formation of bulk metallic glass when compared with the alloy containing ≈0.20 at% of oxygen. The differential thermal analysis of the bulk metallic glass samples shows the extension of the undercooled liquid region with an increase in the oxygen content from 0.20 to 0.33 at% in the Cu47Zr53 alloy. Time‐resolved synchrotron X‐ray diffraction experiments reveal structure evolution of the Cu47Zr53 alloy from the undercooled liquid to glass or crystal. The effect of oxygen on the glass‐forming ability is qualitatively described in terms of reducing the critical cooling rate of the alloy.

Rheology of colloidal and metallic glass formers

Colloidal hard-sphere suspensions are convenient experimental models to understand soft matter, and also by analogy the structural-relaxation behavior of atomic or small-molecular fluids. We discuss this analogy for the flow and deformation behavior close to the glass transition. Based on a mapping of temperature to effective hard-sphere packing, the stress–strain curves of typical bulk metallic glass formers can be quantitatively compared with those of hard-sphere suspensions. Experiments on colloids give access to the microscopic structure under deformation on a single-particle level, providing insight into the yielding mechanisms that are likely also relevant for metallic glasses. We discuss the influence of higher-order angular signals in connection with non-affine particle rearrangements close to yielding. The results are qualitatively explained on the basis of the mode-coupling theory. We further illustrate the analogy of pre-strain dependence of the linear-elastic moduli using data on PS-PNiPAM suspensions.

Bulk metallic glass formation in the (Ti,Zr)–(Ni,Cu)–S system

New bulk glass-forming alloy compositions, exceeding a critical casting thickness of 1 mm, are developed in the (quasi-ternary) (Ti,Zr)–(Ni,Cu)–S system. The ternary eutectic composition Ti65.5Ni22.5Cu12 is stepwise modified through additions of S (0–8 at%) and Zr (0–22.5 at%) at the expense of Ni and Ti, respectively. By increasing the plate thickness of the casted samples from 500 µm to 1.25 mm, the primary precipitating phases are identified which is for the best glass-formers (e.g. Ti58Zr7.5Ni18.5Cu12S4) an icosahedral phase. In calorimetric experiments, several exothermic crystallization events are observed upon heating glassy samples. The first exothermic event, obscuring the glass transition, is attributed to the formation of the icosahedral phase. As the icosahedral phase forms upon heating and cooling for the best glass-formers, the origin of the increased glass-forming ability might be attributed to a pronounced icosahedral short-range order in the liquid state, impeding the formation of the stable crystalline phases.

Exploring the Phase Space of Multi-Principal-Element Alloys and Predicting the Formation of Bulk Metallic Glasses.

Multi-principal-element alloys share a set of thermodynamic and structural parameters that, in their range of adopted values, correlate to the tendency of the alloys to assume a solid solution, whether as a crystalline or an amorphous phase. Based on empirical correlations, this work presents a computational method for the prediction of possible glass-forming compositions for a chosen alloys system as well as the calculation of their critical cooling rates. The obtained results compare well to experimental data for Pd-Ni-P, micro-alloyed Pd-Ni-P, Cu-Mg-Ca, and Cu-Zr-Ti. Furthermore, a random-number-generator-based algorithm is employed to explore glass-forming candidate alloys with a minimum critical cooling rate, reducing the number of datapoints necessary to find suitable glass-forming compositions. A comparison with experimental results for the quaternary Ti-Zr-Cu-Ni system shows a promising overlap of calculation and experiment, implying that it is a reasonable method to find candidates for glass-forming alloys with a sufficiently low critical cooling rate to allow the formation of bulk metallic glasses.

On the correlation between glass forming ability (GFA) and soft magnetism of Ni-substituted Fe-based metallic glassy alloys

The present work aims to investigate the mechanism of glass formation and correlates it to the soft magnetism of Ni-substituted Fe-B-Nb alloys. The impact of ferrous Ni on the mechanism of glass formation and soft magnetism of Fe-based bulk metallic glasses was critically analyzed. By quantifying glass forming ability and soft magnetic characteristic for varying degrees of substitution, we observe a maximum in glass forming ability together with a minimum coercivity, which we suggest is due to an increased atomic packing density of the glassy phase. Interestingly, a monotonic increase of Curie temperature with increasing substitution of Ni was observed, which could be attributed to a reduction of an antiferromagnetic Fe-Fe interaction in the glassy iron-rich matrix. The overarching goal of this study is to explore the underlying mechanisms of enhanced glass forming ability, improved soft magnetic properties, and increased Tc of Ni-substituted Fe-based glassy alloys.