Structure Of Bulk Metallic Glasses Research Articles

Superior mechanical properties of a Zr-based bulk metallic glass via laser powder bed fusion process control

Additive manufacturing has made the fabrication of large-dimensioned bulk metallic glasses (BMGs) achievable; however, questions remain regarding how to control the processing parameters to obtain dense and fully amorphous BMGs with desirable mechanical properties. Here, laser powder bed fusion (LPBF) was used to produce dense and fully amorphous Zr59.3Cu28.8Nb1.5Al10.4 BMG samples from two different starting powders within a large processing window of laser powers and scanning speeds. X-ray diffraction (XRD) revealed that fully amorphous XRD patterns were obtained for samples with high relative densities (>99 %) when the LPBF energy density ranged from ∼20 J/mm3 up to ∼33 J/mm3 for coarse powder with low oxygen and up to ∼30 J/mm3 for fine powder with higher oxygen. For the fully XRD amorphous samples, strength and hardness increased with increasing energy density while the relaxation enthalpy and ductility decreased. Transmission electron microscopy revealed that the softer samples contain larger medium range order clusters within the amorphous matrix. With higher LPBF energy density, high relative density was still achieved along with devitrification and embrittlement. While lower energy densities below ∼20 J/mm3 could retain a fully XRD amorphous structure, such samples had relative densities <99 %. When comparing the two powders, the coarse powder with four times lower oxygen content gave better glass forming ability, compression ductility up to 6 % plastic strain, and fracture toughness up to ∼38 MPa√m. These findings demonstrate that it is possible to tailor the structure and mechanical properties of BMGs by tuning the LPBF process parameters within a wide processing window.

Special glass structures for first principles studies of bulk metallic glasses

The atomic-level structure of bulk metallic glasses is a key determinant of their properties. An accurate representation of amorphous systems in computational studies has traditionally required large supercells that are unfortunately computationally demanding to handle using the most accurate ab initio calculations. To address this, we propose to specifically design small-cell structures that best reproduce the local geometric descriptors (e.g., pairwise distances or bond angle distributions) of a large-cell simulation. We rely on molecular dynamics (MD) driven by empirical potentials to generate the target descriptors, while we use reverse Monte Carlo (RMC) methods to optimize the small-cell structure. The latter can then be used to determine mechanical and electronic properties using more accurate electronic structure calculations. The method is implemented in the Metallic Amorphous Structures Toolkit (MAST) software package.

Application of lever law in dual-cluster formulas model to interpret eutectic and bulk metallic glass

It is well known that the structure of bulk metallic glass has a close relationship with that of its crystalline eutectic phase, but how this is manifested on the atomic scale is not yet clear. The present paper further develops and optimizes dual-cluster formulas model to describe the eutectic and bulk metallic glass, as C1[(principal cluster)1(glue atom)1 or 3] + C2[(principal cluster)2(glue atom)1 or 3]. C1, C2 are the proportions governed by the lever law, and the two principal clusters which have the highest atomic close-packing efficiency and large degree of isolation are derived from the eutectic-related phases. Cu-Zr binary system are studied using this model and the results match well with the experiments. According to dual-cluster formulas and valence electrons, the correlation between deep eutectic and bulk metallic glass may be disclosed, and a detailed step towards composition design of bulk metallic glass is established as well.

Atom Probe Analysis of a Zr-based Bulk Metallic Glass

Abstract Zr-based bulk metallic glasses (BMGs) are amorphous alloys that can exhibit excellent mechanical properties, including high yield strength and fracture toughness. These properties are linked to local microstructural heterogeneities. Whether via microscopy-based techniques, synchrotron techniques, or calorimetric approaches, the amorphous structure of BMGs makes the characterisation of the details of these local structural and chemical heterogeneities extremely challenging. Our focus here is on atom probe tomography (APT), where considerable uncertainty remains in terms of how and when to apply this otherwise powerful technique to amorphous materials. This work reports a systematic evaluation of the experimental parameter space. We report results of BMG composition acquired against various APT operating parameters for Zr63.96Cu13.36Ni10.29Al11.04Nb1.25 (at. %). We demonstrate that a customised peak-based ranging approach yields satisfactory compositional accuracy with absolute errors of &amp;lt;1 at. %. Beyond composition, we have discussed the data quality in terms of attributes of the mass spectra: mass resolution, signal-to-thermal tail ratio, and overlapped peak ratio. We also assess the composition of the well-known clustered evaporation effects, common in APT data of BMGs. We conclude that these regions have negligible differences in composition from the surrounding “matrix” or bulk in these alloys.

Effect of sample size and cooling rate on the plastic deformation behavior of bulk metallic glasses: A comparative study

Understanding the differences between the cooling-rate and size effects on the plastic deformation behavior of bulk metallic glasses (BMGs) is important not only for uncovering the plastic deformation mechanisms but also for achieving better plasticity of BMGs. In the present work, both the cooling-rate and size effects of a Zr-based BMG were investigated by quasi-static compressive experiments. The findings show that the macroscopic plasticity of BMGs can be remarkably enhanced by increasing the casting cooling rate, but moderately improved by decreasing the sample size. The intrinsic atomic arrangement stemming from cooling rate is the primary factor which affects the plastic deformation behavior rather than the extrinsic size effect, and the size effect is also dependent on the intrinsic cooling rate. The findings provide more explanations for the different size effects of BMG communities at different conditions, and are also of significance for achieving better plasticity in BMGs and BMG structures.

Распределение микротвердости по поверхности металлического стекла на основе циркония, подвергнутого интенсивной пластической деформации кручением

Identifying the peculiarities of the transformation of the structure and properties of bulk metallic glass (BMG) under high-pressure torsion (HPT) is of great interest. It is known that under HPT, the degree of deformation differs from the center to the edge of a disk which leads to the non-uniformity of the structure of obtained specimens. The change in microhardness value indicates the direction of change in BMG structure under the HPT, and the microhardness distribution indicates the HPT-specimen non-uniformity. The aim of the study is to identify the HPT influence on the microhardness value and microhardness distribution over the surface of specimens of amorphous alloys using an example of Vit105Zr-based BMG (Zr52.5Cu17.9Ni14.6Al10Ti5). The authors studied the distribution of microhardness over the surface of Vit105 Zr-based bulk metallic glass (BMG) in the initial state, in the state after HPT at n=1 and n=5 rotations, and after relaxing annealing. The study shows that the initial Vit105 BMG is characterized by a small spread in microhardness values, which indicates the material's high homogeneity. By reducing the excessive free volume, relaxing annealing increases microhardness without a significant increase in the spread of its values. HPT leads to a decrease in the zirconium BMG microhardness, which indicates an increase in the excessive free volume, but, at the same time, increases the uneven microhardness distribution over the specimen, while the microhardness values in one half of the HPT sample (n=1) are higher than in the other one. It demonstrates that BMG specimen deformation during HPT is related to the specific loading mechanisms.

Structural rejuvenation in a Zr-based bulk metallic glass via electropulsing treatment

Structural rejuvenation is a fascinating issue of bulk metallic glasses (BMGs). In this paper, we use the electropulsing treatment (EPT) to rejuvenate the atomic packing in a Zr45(Cu4.5/5.5Ag1/5.5)48Al7 BMG, which remains amorphous after undergoing fast heating and cooling. It is found that the voltages less than 110 V have little rejuvenation effect on BMGs. However, the discharging at 130 V greatly reduces the density, modulus, and hardness while generating more excess free volume and enhancing the β-relaxation. The changes in properties of EPT samples are almost consistent with the variations of total content and average size of free volume. Our work provides an efficient way to alter the structure and properties of BMGs.

Study of micro indentation assisted deformation on HPT processed Zr62Cu22Al10Fe5Dy1 bulk metallic glass

In the present work, investigation on high pressure torsion (HPT) and accumulative HPT (ACC HPT) processed Zr62Cu22Al10Fe5Dy1 bulk metallic glass (BMG) is performed. BMG plate was cut into four pieces (disc) for HPT and ACC HPT processing. BMG specimens obtained after n=2, 10 and 11 revolutions of the HPT and ACC HPT are termed as BMG 2, BMG 10 and BMG 11, respectively. Alloys are found to retain the glassy phase before and after HPT process. Homogeneous amorphous structure of BMG initial is obtained with slight decrease in microhardness at different locations on the plate. Besides, significant reduction in hardness for BMG 2, BMG 10, and BMG 11 specimens is observed due to HPT processing from central to peripheral region of the disc. Present study shows that ACC HPT more intensively transform the structure of BMG in comparison to that of conventional HPT.

β-Relaxation and Crystallization Behaviors in a Pulse-Current-Thermoplastic-Formed La-Based Bulk Metallic Glass.

We use the pulse current thermoplastic forming technique based on joule heating to rejuvenate the atomic structure of a La62Al14Ag2.34Ni10.83Co10.83 bulk metallic glass (BMG). The pulse-formed sample exhibits more pronounced β-relaxation than the as-cast one due to the increased free volume. Instead, the sub-Tg annealing clearly weakens the β-relaxation and also makes it more isolated from the α-relaxation, showing contributions from free volume and preferred structure. However, both treatments exhibit little influence on the following α-relaxation and high temperature crystallization kinetics. Our results open an effective way to rejuvenate the structure of BMGs and provide an in-depth understanding of the relationship between structural relaxations and crystallization kinetics of BMGs.

A reduction of settlement probability of Chlorella vulgaris on photo-chemically active ceramics with hierarchical nano-structures

Biofouling is defined by the adsorption of biomolecules or microorganisms on technical surfaces, which are causing adverse effects on the functionality (decrease of ship speed) and safety (infections of implants) of quite a number of industrial products. Conventional, anti-biofouling in marine environments is done by coating a technical surface with highly poisonous tin-organic, which have already been banned for environmental protection. Therefore, the developing of biologically benign coatings becomes a long-term pursue for the industry. Here, we study the Chlorella vulgaris settlement on self-disinfecting titanium dioxide surfaces with three different micro-structures: a flat surface, a light harvesting surface with nano-structure and a hierarchical surface structure, spanning over 5 orders of magnitude (from 0.1 nm to 10 μm). These titanium dioxide surfaces were prepared by Ostwald ripening. This sample manufacturing process gains new catalytic properties as a self-cleaning effect, especially for the light harvesting surface with nano-structure (bulk metallic glass). Chlorella vulgaris dispersions were growing in glass flasks together with the different surface samples over the full time of the experiments. Therefore, this study was made as part of a laboratory scale test. It was found, that bulk metallic glass structures made by Ostwald ripening are showing the highest catalytic and at the same time the best self-cleaning effects. Additionally, the Chlorella vulgaris settlement probability was found to depend on the Wenzel roughness. The surfaces with a high Wenzel roughness were the ones with the lowest Chlorella vulgaris settlement. A semi-field test is proving the comparable antibiofouling performance of our surfaces with existing polymeric or sharkskin like structures on the timescale of one month.

A bi-phase core-shell structure of Mg-based bulk metallic glass for application in orthopedic fixation implants.

Mg-based bulk metallic glass (BMG) and its composites have been promising candidates for orthopedic fixation implants because of their biocompatibility, low degradation rate, and osteogenic potential. However, the amorphous state is affected by the cooling rate during the casting process. Solid, unstable structures combined with amorphous and crystalline structures are generated when an insufficient cooling rate is used. Here, we aimed to design and synthesize a novel core-shell structure comprising an amorphous shell and a crystalline core in order to overcome the material size limit imposed by the cooling rate effects. Our results show that the core-shell structure of Mg-based BMG does have a lower degradation rate and can maintain a more amorphous structure after six weeks of degradation. Moreover, the biocompatibility and osteogenic effects were similar between the core-shell and solid structures of Mg-based BMG. In conclusion, the core-shell structure of Mg-based BMG exhibits a lower degradation rate while still enhancing osteogenic potential in vitro. This core-shell structure of Mg-based BMG overcomes the cooling rate effects and provides a new structure for manufacturing Mg-based BMG.

Effects of Strontium incorporation to Mg-Zn-Ca biodegradable bulk metallic glass investigated by molecular dynamics simulation and density functional theory calculation

Molecular dynamics (MD) simulation and density functional theory (DFT) calculations were used to predict the material properties and explore the improvement on the surface corrosion resistance for the Mg66Zn30Ca3Sr1 bulk metallic glass (BMG). The Mg66Zn30Ca4 BMG was also investigated to realize the influence of the addition of Sr element on the material behaviors of Mg66Zn30Ca4. The Mg-Zn-Ca-Sr parameters of the next nearest-neighbor modified embedded atom method (2NN MEAM) potential were first determined by the guaranteed convergence particle swarm optimization (GCPSO) method based on the reference data from the density functional theory (DFT) calculation. Besides, using the 2NN MEAM parameters of the Mg-Zn-Ca-Sr system, the structures of Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1 were predicted by the simulated-annealing basin-hopping (SABH) method. The local atomic arrangements of the predicted BMG structures are almost the same as those measured in some related experiments from a comparison with the calculated and experimental X-ray diffraction (XRD) profiles. Furthermore, the HA index analysis shows that the fractions of icosahedra-like local structures are about 72.20% and 72.73% for Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1, respectively, indicating that these two BMG structures are entirely amorphous. The uniaxial tensile MD simulation was conducted to obtain the stress-strain relationship as well as the related mechanical properties of Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1. Consequently, the predicted Young’s moduli of both BMGs are about 46.4 GPa, which are very close to the experimental values of 48.8 ± 0.2 and 49.1 ± 0.1 GPa for Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1, respectively. However, the predicted strengths of Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1 are about 850 and 900 MPa, both are slightly higher than the measured experimental values about 747 ± 22 and 848 ± 21 MPa for Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1. Regarding the thermal properties, the predicted melting temperature of Mg66Zn30Ca3Sr1 by the square displacement (SD) profile is about 620 K, which is very close to the experimental melting temperature of about 613 K. The self-diffusion coefficients of Mg, Zn, Ca, and Sr elements were also calculated for temperatures near their melting points by means of the Einstein equation. The methodology can determine the diffusion barriers for different elements by utilizing these diffusion coefficients resulting in a fact that the diffusion barriers of Ca and Sr elements of Mg66Zn30Ca3Sr1 are relatively high. For the electronic properties predicted by the DFT calculation, the projected density of states (PDOS) profiles of surface Mg, Zn, Ca, and Sr elements clearly show that the addition of Sr into Mg66Zn30Ca4 effectively reduces the s and p orbital states of surface Mg and Zn elements near the Fermi level, particularly the p orbits, which suppresses the electron transfer as well as increases the surface corrosion resistance of Mg66Zn30Ca4. Consequently, this study has provided excellent 2NN MEAM parameters for the Mg, Zn, Ca, and Sr system by the GCPSO method to predict real BMG structures as well as by means of the DFT calculation to explore the electronic properties. Eventually, through our developed numerical processes the material properties of BMGs with different compositions can be predicted accurately for the new BMG design.

Fast rejuvenation in bulk metallic glass induced by ultrasonic vibration precompression

Tuning energy state of metastable bulk metallic glasses (BMGs) is well known significant for understanding their glass nature and controlling their performances. Using a newly developed ultrasonic vibration precompression (UVPC) method, fast rejuvenation is achieved in a Zr-based BMG in seconds. The rejuvenated BMG has more heterogeneous structure and largely improved plasticity. The underlying mechanism is revealed. During UVPC treatment, high frequency strain energy is converted into thermal/internal energy due to heterogeneous atomic packing structure of BMG. The combined effect, resulting from external applied elastic stress, internal converted heat and ultrasonic resonance of atoms, drives the loosely packed atoms to a higher energy basin and simultaneously introduces more free volumes in the structure of the BMG. Therefore, the UVPCed BMG has more stored energy and more flow units to initiate shear bands. The findings in this work provide an effective method to modulate the energy state of BMGs and shed insights into comprehensive understanding of energy-structure-properties relationship of amorphous materials.

Surface integrity investigation of a Zr&lt;SUB align="right"&gt;52.5Ti&lt;SUB align="right"&gt;5Cu&lt;SUB align="right"&gt;17.9Ni&lt;SUB align="right"&gt;14.6Al&lt;SUB align="right"&gt;10 metallic glass after grinding

Crystalline alloys, widely used in the industry, are a topic of important research regarding their integrity resistance towards machining. Such investigations are less intense when dealing with relatively newer and less popular materials such as bulk metallic glasses (BMGs). This is even truer when specific machining techniques, such as grinding, are involved. As BMG are increasingly promoted to diverse industrial areas, the study of the impact of grinding on their microstructural integrity is necessary. Indeed, the amorphous structure of BMG, at the origin of their remarkable properties, is metastable such that any exterior solicitation (heating, cooling, loading, etc.) can induce structural modifications, for instance local crystallisation, and therefore degrade or at least modify their characteristics. Using quick and straightforward characterisation techniques such as SEM imaging, optical microscopy and X-ray diffraction, the microstructural changes occurring in the BMG after severe grinding passes could be identified. It is shown that crystallisation can be detected below the grinded surface even for relatively small depth of cut. The results obtained on this peculiar composition help to propose a more generic methodology to safely use grinding on BMG.

A Study of Structure and Morphology of Bulk Metallic Glasses after Laser Beam Treatment

Recently, the Zr-based bulk metallic glasses (BMGs) have attracted attention because of their good glass-forming ability and excellent mechanical properties. The amorphous structure is thermodynamically unstable, and it tends to change into an equilibrium crystalline state. The objective of this work is to test the influence of the laser beam on the surface structure and morphology of BMGs with zirconium matrix. The material for the examination was Zr55Cu30Ni5Al10 alloy obtained by high-pressure copper mold casting method in the form of a plate in specified dimensions. In this paper, irradiation with single impulse and laser beam welding emitted in a pulsed mode which consists of a series of overlapping point interactions were used. Different parameters of the laser beam were employed. Microscopic observations proved that after laser treatment amorphous-crystalline structure appears in the heat-affected zone (HAZ). High-resolution transmission electron microscopy analysis of irradiated metallic glass showed that small amount of crystalline Zr2Ni-type phase was produced in the HAZ, whereas in the fusion zone and parent material amorphous structure appears. What is more, it has been observed that the higher laser beam frequency and energy the more weld penetration takes the shape of an ellipse.

Achieving stable plastic flows in a Zr-based bulk metallic glass under tailored mixed-mode (I/II) loading conditions

The achievement of stable plastic flows plays a key role for practical structural applications of bulk metallic glasses (BMGs). In this work, by designing complex stress fields through tailored double-side notches, stable plastic flows in a Zr-based BMG were achieved under mixed-mode (I/II) loading conditions. The deformation behavior of the notched BMG specimens, including the shear-banding and fracture behaviors, was examined and correlated to the designed complex stress fields. The findings have shown that the stable plastic flows were mainly caused by the introducing of mode-II component under the mixed-mode (I/II) loading conditions. Increase of the mode-II component can result in not only more stable plastic flows, but also larger bearing loads. The present findings are of significance in uncovering the plastic deformation mechanisms of BMGs under mixed-mode loading conditions and in designing BMG structures with better mechanical performance.

Three-Dimensional Unit Cell Study of a Porous Bulk Metallic Glass Under Various Stress States

Porous bulk metallic glasses (BMGs) exhibit an excellent combination of superior mechanical properties such as high strength, high resilience, large malleability, and energy absorption capacity. However, a mechanistic understanding of their response under diverse states of stress encountered in practical load-bearing applications is lacking in the literature. In this work, this gap is addressed by performing three-dimensional finite element simulations of porous BMGs subjected to a wide range of tensile and compressive states of stress. A unit cell approach is adopted to investigate the mechanical behavior of a porous BMG having 3% porosity. A parametric study of the effect of stress triaxialities T = 0, ±1/3, ±1, ±2, ±3, and ±∞, which correspond to stress states ranging from pure deviatoric stress to pure hydrostatic stress under tension and compression, is conducted. Apart from the influence of T, the effects of friction parameter, strain-softening parameter and Poisson’s ratio on the mechanics of deformation of porous BMGs are also elucidated. The results are discussed in terms of the simulated stress-strain curves, pore volume fraction evolution, strain to failure, and development of plastic deformation near the pore. The present results have important implications for the design of porous BMG structures.

Role of tensile elastostatic loading on atomic structure and mechanical properties of Zr55Cu30Ni5Al10 bulk metallic glass

In this work, the pre-tensile elastostatic loading with low stress values (5%–20% σy) was used to rejuvenate Zr55Cu30Ni5Al10 bulk metallic glass (BMG). The atomic pair density function analysis shows that the rejuvenation evolution is accompanied with a significant disordering of short and medium atomic scales in the BMG structure. The results also indicate that the decreasing rate of annihilation in CuZr pairs under rejuvenation is higher than ZrZr and ZrAl pairs. In general, it is suggested that the interaction of stress domains under elastostatic loading is the main reason for disordering during the rejuvenation evolution. The compression test reveals that a brittle-to-ductile transition is happened from as-cast sample to the rejuvenated ones. Moreover, it is found that the serrations in the plastic region of stress-strain curves of highly-rejuvenated samples show a self-organized critical state. As a whole, with increase in pre-tensile elastostatic loading value, the structural rejuvenation enhances and the homogeneous plasticity improves.

Development and characterization of titanium-based bulk metallic glasses

The amorphous structure of bulk metallic glasses results in outstanding properties, which are able to outshine the properties of common metallic biomaterials. In particular, Ti-based bulk metallic glasses are interesting candidates for the use as biomaterial or as structural material in lightweight applications. However, the usage of known Ti-based bulk metallic glasses with casting thicknesses of several millimeters is strongly limited due to the large content of rare, expensive and harmful elements. In this work, we design Ni-free Ti-based bulk metallic glasses with a low Cu-content, being suitable for the application as biomaterial. On the basis of the binary Ti-Ni system, novel glass forming alloys are developed by the addition of sulfur. In order to increase the glass forming ability and to reduce the Ni-content, Ti was substituted with Zr and Ni with Cu. Finally, it is possible to produce synchrotron X-ray amorphous samples with a diameter of 3 mm for the Ni-free alloy Ti40Zr35Cu17S8. Furthermore, it is shown that industrial grade raw materials can be used for the synthesis of amorphous samples of this alloy without diminishing the glass forming ability or the mechanical properties. Characterization by mechanical and electrochemical tests proves the superiority of the novel alloys over commercial amorphous and crystalline alloys. All in all, a new class of Ni-free Ti-based bulk metallic glasses with unprecedented properties is developed, being highly attractive for a widespread use.

Dynamic characterization of Cu–Zr binary bulk metallic glasses: A molecular dynamics study

In the present study, a molecular dynamics simulation employing embedded atom method potential is performed to investigate the formation and characterization of CuZr bulk metallic glasses (BMGs). To elucidate the effect of component concentration of three samples of BMGs including Cu25Zr75, Cu50Zr50, and Cu75Zr25 that are formed by melt quenching. The local structure of BMGs is analyzed by means of radial distribution function and local atomic number density, ρ. The mechanical behavior of three compositions is investigated using uniaxial compressive loading at a constant strain rate. It is revealed from the results that yield strength increases with increasing Cu concentration. Thermal expansion of CuZr BMGs is examined and variation in length and volume is measured. The analysis revealed that Cu25Zr75, and Cu50Zr50 exhibited the typical expansion behavior while Cu75Zr25 showed an anomalous behavior.