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The corrosion behavior of TiZrHfBeCu(Ni) high-entropy bulk metallic glasses (HE-BMGs) has been investigated. The TiZrHfBeCu(Ni) HE-BMGs exhibited high corrosion resistance in 3.5 wt. % NaCl solution because of accumulation of ZrO2 and TiO2 in the passive film. Ni promoted increases of the ZrO2, TiO2, and HfO2 contents and a decrease of the BeO content, which improved the HE-BMG corrosion behavior. Compared with Zr41.2Ti13.8Ni10Cu12.5Be22.5 BMG, the high-entropy effect of HE-BMGs can significantly reduce the atomic mobility, which inhibits outward migration of Cu, reduces the kinetics of the dissolution reaction, and inhibits inward erosion by Cl−, thereby improving the corrosion performance. [ABSTRACT FROM AUTHOR]

We investigated the effects of thermal cycling on the kinetics of glass transition and crystallization of a classic Zr-based BMG (Vit-1) by differential scanning calorimetry (DSC). Two different thermal cycling conditions with different cycles and holding times were used. Sub–sub-Tg isothermal annealing condition was also adopted for comparison. Continuous DSC tests were conducted to calculate the activation energies of glass transition and crystallization based on different equations. Isothermal DSC tests were also conducted to further study the crystallization mechanisms in views of local Avrami exponent. Based on the experimental results, the related mechanisms have also been discussed. [ABSTRACT FROM AUTHOR]

The kinetics of glass transition and crystallization of a novel Zr40Hf10Ti4Y1Al10Cu25Ni7Co2Fe1 bulk metallic glass (BMG) with high mixing entropy have been studied by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The continuous DSC curves show five stages of crystallization at lower heating rates (≤ 20 K/min). The activation energies of glass transition were determined by Moynihan and Kissinger methods, while the activation energies of crystallization were calculated utilizing Kissinger, Ozawa, and Boswell models. The crystalline phases corresponding to each crystallization step have been found out. The kinetic fragility of Zr40Hf10Ti4Y1Al10Cu25Ni7Co2Fe1 BMG has also been evaluated. Based on the isothermal DSC curves, the Avrami exponent, evaluated from the Johnson-Mehl-Avrami equation, has been analyzed in detail. The current study reveals that the crystallization behavior of Zr40Hf10Ti4Y1Al10Cu25Ni7Co2Fe1 BMG exhibits characteristics of both the high entropy BMGs and traditional BMGs with a single principal element, leading to its high glass-forming ability. [ABSTRACT FROM AUTHOR]

The influence of sub- T g pre-annealing on the kinetics of glass transition and crystallization of a lightweight Ti 41 Zr 25 Be 28 Fe 6 bulk metallic glass (BMG) has been investigated by isochronal and isothermal differential scanning calorimetry tests. It was found that sub- T g annealing obviously enlarges the supercooled liquid region of Ti 41 Zr 25 Be 28 Fe 6 BMG, indicating an enhanced thermal stability. The activation energies for glass transition and crystallization of the as-cast and pre-annealed samples have been calculated and compared. The pre-annealed samples possess smaller values of fragility parameter and longer incubation times upon isothermal heating compared with the as-cast sample, indicating a retarded crystallization process. The isothermal crystallization kinetics of the samples with different annealed states has been investigated in terms of the Johnson-Mehl-Avrami equation and local Avrami exponent. The current study reveals a clear sub- T g annealing effect on the kinetics of glass transition and crystallization of Ti 41 Zr 25 Be 28 Fe 6 glassy alloy and the possible mechanisms have also been proposed. [ABSTRACT FROM AUTHOR]

The room temperature creep behavior of a newly developed Ti 16.7 Zr 16.7 Hf 16.7 Cu 16.7 Ni 16.7 Be 16.7 high entropy bulk metallic glass was investigated by nanoindentation measurements with various loading rates in comparison with that of a typical conventional Vitreloy 1 bulk metallic glass. A generalized Kelvin model was adopted to describe the creep curves. It was found that for both the two studied alloys, the creep displacement and strain rate sensitivity increases with increasing loading rate, indicating a positive loading rate effect. Vitreloy 1 BMG exhibits a more obvious loading rate effect compared with Ti 16.7 Zr 16.7 Hf 16.7 Cu 16.7 Ni 16.7 Be 16.7 HE-BMG. The creep compliance and retardation spectra were also derived to study the creep mechanism. Compared with other developed high entropy alloys and bulk metallic glasses (e.g. Vitreloy 1 alloy), the studied HE-BMG possesses relatively small strain rate sensitivity values, indicating a good creep resistance. Based on the experimental results, the mechanisms of the unique creep resistance of Ti 16.7 Zr 16.7 Hf 16.7 Cu 16.7 Ni 16.7 Be 16.7 high entropy bulk metallic glass have also been explained. [ABSTRACT FROM AUTHOR]

The shock forming process of Cu 50 Zr 50 metallic glasses is investigated via the classical molecular dynamics method. The velocity and density profiles are presented to describe the shock wave propagation, and the displacement vector is used to trace the atomic flow. The shock forming process is accompanied by the evolution of the internal short-range order of the metallic glasses. Beside, the influence of piston velocity and mould size on the shock forming process is explored. It is found that the suitable shock pressure is in the range of 18–30 GPa, with which the filling rate could be higher than 85%. A larger fillet radius or a smaller mould opening size is advantageous to alleviate the residual stress concentration. The sample obtained by shock forming exhibits better plasticity compared with the as-cast sample, which could be explained by the change in the number of the dominated Voronoi polyhedra. [ABSTRACT FROM AUTHOR]

• Oxidation occurs with the formation of oxide clusters. • Local atomic volumes increase around the oxide clusters. • Oxides lead to dynamic slowdown and stabilizes the glass-forming liquid. Oxygen has long been a concern in metallic glass-forming liquids since a minor addition can have a significant influence on liquid properties and glass formation, especially by inducing crystallization. However, how exactly oxygen affects the atomic structure and liquid properties on atomic level remains unclear to date. Here, by using extensive reactive molecular dynamics simulations, we conduct a comparative study of the atomic structures and diffusion behavior in both Al-Ni and its oxygen-containing liquids. We find that the oxidation occurs with the formation of extremely stable oxide clusters with the bonding preference between Ni and O atoms. The presence of the oxide clusters results in an increase in free volumes around them. It is the covalent bonding as well as the local excess volumes around the oxides that lead to changes in diffusion coefficients. The diffusion coefficients follow the Arrhenius relation at high temperatures, but a dynamical crossover happens above the glass transition temperature. We show that the diffusivity still correlates well with the free volume as predicted by Cohen and Turnbull although the effect of oxide clusters is not considered. This fundamental discrepancy may challenge our current understanding of liquid properties. [ABSTRACT FROM AUTHOR]

• In this work, we report the finding of the long-sought after atomic mechanisms of oxygen effects on the atomic structure and brittleness of metallic glasses. • Oxygen is generally considered to have adverse effects on metallic glass formation and mechanical properties by embrittlement of the materials, but our work shows that in certain cases it could lead to improvement in ductility. • In this paper, we report the first atomistic simulation in investigating underlying microscopic mechanisms and atomic structure changes for the beneficial ductility improvement from oxygen addition as reported from experiments. • This finding may lead to a paradigm shift in development of the metallic (glassy) materials. • Our theoretical prediction is independently confirmed by an experimental group where the oxygen effects were examined without our knowledge in real metallic glasses. It is not common that the atomistic simulation can achieve this level of predictability. Metallic glass is a brittle solid macroscopically but has tremendous ductility in nano and mesoscopic scales. Oxygen is generally considered to be the major culprit in causing the brittleness by inducing crystallization. Since oxygen is omnipresent in the making and applications of metallic glasses, how to avoid, and even take advantage of oxygen, becomes materials scientists' wild dream. Here we demonstrate using atomistic simulation that the brittle solid can be turned ductile in a model Al-based metallic glass with oxygen addition. There are two major underlying mechanisms found here, one is to keep the oxides in small size and the other is to preserve the free volumes in the surroundings of the oxide clusters. The relatively loosely packed neighboring regions of the nanosized oxide clusters make the glass softer and thereby improves deformability and thus ductility. The finding challenges experimentalists to explore and discover new class of glasses that are not only oxygen tolerant but beneficial in mechanical properties. [ABSTRACT FROM AUTHOR]

The influence of vibrational loading on the deformation behavior of a Zr50Cu46Al4 metallic glass (MG) was characterized via molecular dynamics approaches. High-frequency (1 GHz) vibrational loading was imposed on the elastoplastic deformation stage during the uniaxial tension of the MG conducted at 50 K. It was found that imposing vibrational loading scarcely reduces the average deformation resistance. On the contrary, it results in a notable residual hardening effect after the vibrational loading is removed, which differs significantly from the previously reported acoustic softening mechanisms. Vibrational loading can increase the fraction of STZed atoms and enhance the shear localization degree, which is beneficial to the shear deformation of MGs. Meanwhile, the influence of vibrational loading on the local microstructure of MG is negligible. A plausible explanation of these phenomena is given by considering the accelerated aging of MG stemming from the β relaxation. [ABSTRACT FROM AUTHOR]

Making composites with crystalline inclusions can improve plasticity in brittle metallic glasses. In this study, we show a new metallic glass matrix composite designed with a gradient network architecture made of amorphous matrix regions and gradient crystalline network regions. Using molecular dynamics simulations, we validated theoretically the hypothesized concept of this new composite. Our results show that the unique microstructure leads to significant strain delocalization which can delay the generation of catastrophic shear bands. This is resulted from the new mechanism by both forming diffuse embryonic shear bands in the thin crystalline network interface and hindering or guiding their formation and movement in thick crystalline network regions with the presence of internal stresses generated from the gradient. This work demonstrates the synergistic effects induced by the heterogeneous gradient network design in the mechanical properties of metallic glass matrix composites and may open up exciting new possibilities. Macroscopic plasticity can be improved by spatially patterning heterogeneity into "microstructures" in the homogeneous structure-less amorphous metals. By further introducing gradients in the microstructure, we can elevate the improvement to a new level. Here, we show a new metallic glass matrix composite designed with a gradient network architecture made of amorphous matrix regions and gradient crystalline boundary network regions (a). The designed gradient structure can greatly enhance the plasticity as compared to the monolithic metallic glass (MG) and nanoglass (NG) (b). The new design of the MG composite creates a hierarchical heterostructure in the spatial pattern that can effectively cause delocalization of local shear (c) and the significant delay of brittle fracture. [Display omitted] [ABSTRACT FROM AUTHOR]

Controlled activation of flow units and in-situ characterization of mechanical properties in metallic glasses are facing challenges thus far. Here, vibrational loading is introduced through nanoscale dynamic mechanical analysis technique to probe vibration-accelerated atomic level flow that plays a crucial role in the mechanical behavior of metallic glasses. The intriguing finding is that high vibrational frequency induces deep indentation depth, prominent pop-in events on load–depth curves and low storage modulus, exhibiting a vibration-facilitated activation of flow units in Pd 40 Cu 30 Ni 10 P 20 metallic glass. Theoretical analysis revealed that vibration-moderated activation time-scale accelerate the activation of flow units and responsible for the above scenario. [ABSTRACT FROM AUTHOR]

Highlights: [•] The oxidation kinetics follows a multiple-stage parabolic rate law. [•] The transition of oxidation kinetics is found to be caused by crystallization. [•] We investigate the effect of thermal parameters on surface segregation. [•] We explain the ionic diffusion mechanism of oxidation. [Copyright &y& Elsevier]

Tensile rheological behavior of the Zr 35 Ti 30 Be 27·5 Cu 7.5 metallic glass under conditions of high strain rates (strain rates ranging from 0.05 s−1 to 0.5 s−1) within the supercooled liquid region was studied. The stress-strain curve reveals that this metallic glass demonstrates non-Newtonian flow characteristics under these circumstances. The peak stress associated with stress overshoot increases as temperature decreases and strain rate increases. However, the steady-state flow stress displays an anomalous decrease due to rapid necking at high strain rates. Subsequently, the thermal processing map of the Zr 35 Ti 30 Be 27·5 Cu 7.5 metallic glass was established and analyzed. It reveals that the alloy has relatively excellent thermoplastic forming ability at temperature between 630 K and 655 K and strain rates between 0.05 s−1-0.2 s−1. We found that under strain rates ε ˙ ≥ 10 − 2 s − 1 , the strain rate is the primary factor influencing normalized viscosity. Therefore, we developed a simplified Maxwell-Pulse constitutive model applicable to metallic glass in the supercooled liquid region under high strain rate conditions, by simplifying the constitutive equation for steady-state flow stress using a simplified formula for normalized viscosity calculation. The predictions of this simplified modified model align well with experimental values. By comparing the simplified and unsimplified Maxwell-Pulse constitutive models, the overall fitting accuracy of the simplified model was improved by 3–5%, with the highest improvement of 67% in the steady-state flow stress stage. These results indicate that the modified model accurately reflects the stress-strain relationship for the Zr 35 Ti 30 Be 27·5 Cu 7.5 metallic glass under high temperature and high strain rate tensile conditions. • The tensile rheological behavior of Zr 35 Ti 30 Be 27·5 Cu 7.5 metallic glass under high temperature and high strain rate conditions was studied. • Zr 35 Ti 30 Be 27·5 Cu 7.5 metallic glass exhibits non-Newtonian flow behavior (temperature: 618–663 K, strain rate: 0.05–0.5 s−1). • A thermal processing map for Zr 35 Ti 30 Be 27·5 Cu 7.5 metallic glass has been established to determine the relative optimal range of thermal processing parameters. • The proposed modified Maxwell-Pulse constitutive model accurately characterizes the rheological behavior. [ABSTRACT FROM AUTHOR]

• Electric currents endow Zr-based metallic glasses with an exceptional combination of ultra-high strength, considerable plasticity, and enhanced strain-hardening ability. • Electric currents construct a unique structural fluctuation featuring soft zones surrounded by hard zones. • Plausible mechanisms for such electric current-regulated plasticity in metallic glasses are proposed. Structural applications of metallic glasses are limited by their room-temperature brittleness and strain-softening, associated with the extreme localization of plastic flow in shear bands. Herein, we alleviate this dilemma by the structural fluctuation induced by electric currents, achieving simultaneous improvement of strengths, compression plasticity, and strain hardening capacity. The electric current enhances the structural fluctuations at the atomic scale, introducing more soft zones while densifying their surroundings, which differs from previously reported rejuvenation strategies. This unique structural pattern featuring soft zones surrounded by hard zones leads to extensive sprouts and interactions of shear bands, capturing substantial atoms to participate in the deformation. The plausible mechanism behind the electric current-influenced dynamic evolution of amorphous clusters is proposed. Electric current introduces interatomic electrostatic forces between shell atoms of amorphous clusters through charge transfer. This force decreases the coordination of Zr- and Cu-centered clusters and increases the Al-centered icosahedra in the glassy matrix. Differential evolution of clusters under electric currents enhances atomic structural fluctuations and originates from the cohesion mechanism determined by electron distribution. These findings are suggestive of vanishing room-temperature brittleness and shed atomic-scale light on modulations of the structure and properties of metallic glasses by electric currents. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract The size-dependent microstructural evolution behavior has a significant impact on the thermoplastic flow behavior of metallic glass in the supercooled liquid region. In this work, double-cup extrusion tests, corresponding finite element method simulations and nanoindentation measurements of Zr 55 Cu 30 Al 10 Ni 5 metallic glass samples with different sizes, are employed to study the relationship between the size-dependent free volume variations and thermoplastic flowability. Experimental results and theoretical analyses demonstrate that the free volume content increases with increasing sample size. The change tendencies of free volume content and micro-hardness are in a negative correlation. The variation regularity of extruded cup height ratio verifies that the thermoplastic flowability of Zr-based metallic glass is enhanced with rising sample size. Furthermore, the size-dependent free volume variations and distributions induced by internal microstructural evolution behaviors result in the evident variations of micromechanical properties. The results of this study will contribute to a more in-depth understanding of the size-dependent thermoplastic deformation behavior of Zr-based metallic glass. Highlights • Thermoplastic flow ability of Zr-based metallic glass rises with rising sample size. • The ratio of upper and lower cup heights decreases with sample size rising. • The average free volume distribution increases with sample size increasing. • The free volume and micro-hardness change regularities are in a negative correlation. [ABSTRACT FROM AUTHOR]

Highlights • Preexisting scales fracture in a brittle and intergranular way in the early stage of deformation. • t → m transformation of t-ZrO 2 takes place in the front of the crack tip due to the concentration strain field. • Multiple t-ZrO 2 grain regions form around fractures due to the combined action of oxidation, deformation and fracture. Abstract The microstructure evolution behavior of a Zr-based bulk metallic glass during thermal oxidation under plastic strain has been investigated. In the early stage of deformation, preexisting scale fractures in a brittle and intergranular way. Due to the concentrated strain field formed in front of crack tip, t→m martensitic transformation of t-ZrO 2 takes place. With tensile strain increasing, multiple oxide regions consisting of t-ZrO 2 dendrites form around fractures due to the combined action of oxidation, deformation and fracture. The formation and fracture of scales under large strains seriously deteriorate the surface finish and dimension accuracy of thermoplastic forming parts. [ABSTRACT FROM AUTHOR]

Highlights • Thermal oxidation of Ti 16.7 Zr 16.7 Hf 16.7 Cu 16.7 Ni 16.7 Be 16.7 HE-BMG is not a diffusion-controlled process. • The mass gain decreases with the prolonging of oxidation duration. • The mass gain reduction is attributed to the decomposition of titanium oxides. Abstract In this study, the oxidation behavior of a supercooled Ti 16.7 Zr 16.7 Hf 16.7 Cu 16.7 Ni 16.7 Be 16.7 HE-BMG was investigated. The mass gain of specimens during oxidation is very limited, and the maximum value is lower than 0.08 × 10−2 g/cm2 after two hours of insulation. With the insulation duration prolonging, the mass gain decreases, indicating that the oxides formed are not stable and could decompose after long-term heating. Meanwhile, the oxidation kinetics doesn't follow the parabolic rate law, indicating that it is not a diffusion-controlled process. Even for specimens oxidized at 713 K for 120 min, they are still fully amorphous. No surface precipitates, as well as continuous oxide scale structures, are formed even after oxidation for 120 min at 753 K. Therefore, it is indicated that this HE-BMG has strong thermal stability and oxidation resistance, which is much promising for thermoplastic forming. [ABSTRACT FROM AUTHOR]

The poor thermoplastic formability greatly restrains the wide application of Ti-based bulk metallic glasses. In this study, the thermoplastic formability of Ti-Zr-Be BMG has been dramatically improved by alloying method. The representative Ti 41 Zr 25 Be 28 Fe 6 alloy exhibits a wide supercooled liquid region of ∼123 K and low viscosity of ∼10 6  Pa s in its supercooled liquid region, resulting in the best thermoplastic formability among all the developed Ti-based BMGs. Nanorods arrays can be successfully fabricated even in air with the help of a wetting layer. Moreover, the good glass-forming ability and excellent mechanical properties also make it applicable as engineering materials. [ABSTRACT FROM AUTHOR]

In this study, a series of in-situ Ta-rich particle reinforced Zr-based bulk metallic glass composites were successfully fabricated by arc-melting copper-mold spray casting. The effects of Ta content on the room temperature plasticity, compressive strength and thermoplastic formability were studied. (Zr 55 Cu 30 Al 10 Ni 5) 94 Ta 6 showed good comprehensive performance, and it was selected to systematically study the deformation behavior in the supercooled liquid region. Different from the strain softening after stress overshoot in bulk metallic glass, the composites showed work hardening in the late stage. Some classical constitutive models cannot accurately describe these phenomena. The back-propagation artificial neural network optimized by particle swarm optimization and genetic algorithm was used to establish the constitutive model. The particle-swarm-optimization back-propagation network with the optimal topology showed high accuracy and good generalization ability. The results predicted with this model were consistent with the experimental data, providing a powerful approach for describing the hot-deformation behavior of these Zr-based bulk metallic glass composites in the supercooled liquid region. [Display omitted] • Ta-rich particle reinforced bulk metallic glass composites with good room-temperature plasticity were developed. • The effect of temperature, strain rate and Ta-rich particle on thermoplastic formability was investigated. • The constitutive model is established by using evolutionary artificial neural network. • The constitutive model has high accuracy and good generalization ability. [ABSTRACT FROM AUTHOR]

The specimen size has a significant influence on the flow behavior of Zr 55 Cu 30 Al 10 Ni 5 bulk metallic glass throughout thermoplastic compression processes in the supercooled liquid region. Due to enhanced structural relaxation, the recovery enthalpy increases with specimen size decreasing. Based on combined differential scanning calorimetry experiments and finite element modeling simulations, the different free volume annihilations caused by the specific surface area discrepancy are considered as the main cause of size-dependent free volume variation during the pre-heating process. Thermodynamics and structural relaxation theory are introduced to provide a reasonable explanation of the size-dependent flow behavior at elevated temperatures. A novel prediction model involving size-induced free volume variations has been established by considering the influences of compression deformation variables. The quantitative theoretical prediction agrees well with experimental data, which indicates that the developed prediction model can accurately describe the size-dependent free volume annihilation and generation during the thermoplastic forming process. Furthermore, the reasonable theoretical analysis and size-dependent free volume prediction model provide a profound understanding of the size-dependent flow behavior and its relation to internal structural evolution behaviors of metallic glasses. [ABSTRACT FROM AUTHOR]

The room temperature creep behavior of newly developed Ti 20 Zr 20 Hf 20 Be 20 Cu 20 and Ti 20 Zr 20 Hf 20 Be 20 Cu 20 Ni 10 high entropy bulk metallic glasses (HE-BMGs) was investigated using nanoindentation technique under different loading rates. A generalized Kelvin model was applied to describe the experimental creep curves. The strain rate sensitivity parameter, creep compliance and retardation spectra were derived to study the creep mechanism. The experimental results show that the studied HE-BMGs possess good creep resistance. Moreover, it has been found that the addition of Ni effectively enhances the creep resistance of Ti 20 Zr 20 Hf 20 Be 20 Cu 20 HE-BMG and the related mechanism has also been discussed. [ABSTRACT FROM AUTHOR]

During the oxidation of Zr 55 Cu 30 Al 10 Ni 5 metallic glass in its supercooled liquid region, multilayered scale structures were found. An amorphous oxide layer forms first, beneath which, a layer of t-ZrO 2 columnar dendrites nucleate and grow into the supercooled amorphous substrate. After the substrate fully crystallizes, a layer of flaky t-ZrO 2 grains forms beneath the columnar dendrite layer. The continuous outward diffusion of Cu 2+ cations from the grain boundaries of columnar grains results in interconnected fissures inside this layer, which provide pathways for gaseous O 2 transportation. Thus, another distinguishable layer is generated between the columnar grain layer and the flaky grain layer. [ABSTRACT FROM AUTHOR]

The effect of surface morphology on the oxidation behavior of a Zr 55 Cu 30 Al 10 Ni 5 bulk metallic glass was investigated. The results revealed that specimens with rougher surfaces oxidized faster than specimens with smoother surfaces, resulting in larger and denser segregated particles, as well as thicker scales. The effect of surface morphology on the oxidation behavior was explained from the aspects of thermodynamics and diffusion kinetics. It was found that it is the scratch sharpness, rather than the roughness, should account for the acceleration effect on BMG, which is different from that of crystalline materials. [ABSTRACT FROM AUTHOR]

• Wp/BMGcs were successfully prepared by spark plasma sintering. • The reliability of mechanical properties were described by Weibull distribution. • The volume fraction of Wp plays an important role on the densification. • The tungsten particle tend to agglomerate with increasing Wp volume fraction. Tungsten-particle (Wp) reinforced Zr-based bulk metallic glass composites (Wp/BMGCs) were prepared by spark plasma sintering (SPS). The effects of the volume fraction of Wp on the densification and mechanical properties of the composites were studied. Weibull statistics were used to analyze the variability of compressive strength of Wp/BMGCs. The Weibull modulus (m) has a strong correlation with the volume fraction of Wp. The lower the volume fraction of Wp, the more uniform the compressive strength of the Wp/BMGCs, and the more reliable the materials should be in practical application. [ABSTRACT FROM AUTHOR]

[Display omitted] • A strategy for designing ductile metallic glass composites by forming nanocrystals in the high-entropy alloy particles during deformation was proposed. • The ex situ high-entropy alloy particle toughened Zr-based metallic glass composites were prepared by spark plasma sintering. • By adding CoCrFeNiAl high-entropy alloy particles, the Zr-based metallic glass composites exhibit enhanced strength-ductility synergy. • The toughening mechanisms were discussed in accordance with experiments and modeling. In this work, we prepared equiatomic AlCoCrFeNi high-entropy alloy (HEA)-particle-toughened, Zr-based metallic glass composites by spark plasma sintering. By adding HEA particles as the second phase, the strength and plasticity of the Zr-based metallic glass composites improved concomitantly. After fracture, high-density dislocations and nanocrystals were formed in the HEA particles due to local severe plastic deformation, which consumed massive strain energy to enable the resistance to crack formation. Substantial lattice distortion imparted a remarkable work-hardening capacity to the HEAs and enhanced crack-tip dislocation trapping, and thus led to an extreme refinement of the grain size. Finite-element analyses indicated that the strain hardening behavior of HEA particles reduced the magnitude of strain localization, promoted generation of multiple shear bands, and stabilized shear band propagation. We attribute the enhanced strength-ductility synergy in the current composites to high-density dislocations and nanocrystal formation in the HEA particles, and stable propagation of multiple shear bands. [ABSTRACT FROM AUTHOR]

Tungsten-particle (Wp)-reinforced bulk-metallic-glass composites (Wp/BMGCs) with a Wp volume fraction of 0%–50% were prepared by spark plasma sintering (SPS). The sintering process parameters were optimized through an orthogonal experiment. With increasing volume fraction of Wp, the compressive yield strength of the Wp/BMGCs decreased, while their ultimate strength and plastic strain increased. Nanoindentation experiments revealed that the elastic modulus and hardness of the composites increased with increasing volume fraction of Wp. The elastic modulus mismatch between the tungsten particles and metallic glass matrix resulted in a stress concentration at the interface of the two phases, promoting the initiation and propagation of shear bands in the metallic glass matrix. Fracture morphology analysis of the composites showed that the failure mode of the composites was shear fracture mode, which is beneficial to the self-sharpening of the composites. With increasing volume fraction of Wp, the plastic deformation of Wp became more obvious, and the deformation was most obvious near the shear fracture surface. The degree of plastic deformation of Wp in the region far away from the shear fracture surface gradually decreased. In addition, the reliability of the mechanical properties of Wp/BMGCs sintered in the same preparation parameters was verified by three-parameter Weibull statistical analysis. These results help deepen our understanding of the preparation and mechanical properties of Wp/BMGCs and further promote their application. [ABSTRACT FROM AUTHOR]

• The oxidation kinetics follows the inverse logarithm law in the early stage. • About 1 at. % of oxygen is beneficial to the stabilization of MG short-range ordering. • Oxidative stress leads to the increase of oxide-glass interface roughness. • The ductility of the oxide-bonded MG is increased while the strength is reduced. A layer of oxygen is introduced into a Cu 64 Zr 36 MG to achieve interfacial bonding. The oxidation kinetics, diffusion behavior, local structure, and mechanical performance of the oxide-bonded MG were investigated via molecular dynamics simulations. The growth of the oxide layer follows the inverse-logarithmic law, indicating that this process is controlled by the ionic drift through the oxide in response to electric fields. The introduction of oxygen atoms seriously deteriorates the short-range ordering of MG by suppressing the formation of icosahedral clusters. Uniaxial tensile tests indicate that the oxide-bonded MG has favorable ductility, while its ultimate tensile strength is reduced. [ABSTRACT FROM AUTHOR]

In this work, a two-step spark plasma sintering process for amorphous alloys was proposed. Densification mechanisms and the effects of the pretreatment process on the microstructure and mechanical properties were studied. The results show that the two-step spark plasma sintering process could significantly improve the density and fracture strength of sintered samples and even reduce the final sintering temperature. The lower-temperature pretreatment process resulted in higher density and more uniform density distribution of the powder billets, which reduced the temperature gap between inside of the powder particles and the contacts, as well as reducing the electrical resistivity of the powder billets. Starting from highly homogeneously packed powder billets, more current flowed to the powders, promoting the discharge effect and internal Joule heat generation. The pre-treated high-quality powder billets enhanced the sintering process, which enabled the realization of a high strength and reduced sintering temperature. • A two-step spark plasma sintering strategy for amorphous alloys was proposed. • The two-step sintering strategy improves the sintering quality and reduces the final sintering temperature. • Local overheating during the two-step sintering was observed experimentally and discussed in detail. • The related densification mechanisms have been explored. [ABSTRACT FROM AUTHOR]

The effects of deep cryogenic cycling treatment (DCT) on the rejuvenation and room-temperature compressive properties of a TiZrHfBeCu high-entropy bulk metallic glass (HE-BMG) were studied. After DCT, the fully amorphous structure of TiZrHfBeCu HE-BMG remained, but the relaxation enthalpy showed obvious rejuvenation behavior. Upon increasing the number of cycles, the room-temperature plasticity of TiZrHfBeCu HE-BMG gradually increased from 0.62% to 5.38%, with no obvious change in the yield strength. The hardness and density of the high-entropy amorphous alloy decreased with the number of DCT cycles. The DCT-induced rejuvenation and plasticization mechanisms were explained based on the shear transformation zone (STZ) model. The effects of high mixing entropy and fragility on the rejuvenation behavior were also considered. This work confirmed that DCT is an effective method for the rejuvenation and plasticization of TiZrHfBeCu HE-BMG. • TiZrHfBeCu high entropy metallic glass can be rejuvenated by deep cryogenic cycling treatment. • The room temperature plasticity can be improved from 0.62% to 5.38%. • The plasticization results from the induced free volume content. • The small fragility of TiZrHfBeCu alloy favors the rejuvenation and plasticization. [ABSTRACT FROM AUTHOR]

• The addition of Ni deteriorates the thermal stability but improves the glass-forming ability of TiZrHfBeCu HE-BMG. • The addition of Ni increases the activation energies of crystallization. • The addition of Ni changes the trend of local Avrami exponent during the crystallization process. • The addition of Ni suppresses the precipitation of CuHf 2 and introduces more crystalline phases. The crystallization kinetics of TiZrHfBe(Cu, Ni) high entropy bulk metallic glasses have been studied using differential scanning calorimetry. Each alloy possesses a complex multi-step crystallization process due to the high entropy effect. It was found that substitution of Cu by Ni significantly improves the glass-forming ability but deteriorates the thermal stability of the TiZrHfBeCu alloy. The activation energies of glass transition and crystallization together with the fragility parameter for the Ni-free and Ni-containing alloys have been calculated. According to the calculated values of local Avrami exponent, the first crystallization event for the studied TiZrHfBe(Cu, Ni) alloys is dominated by a direct growth of crystal nuclei. Substitution of Cu by Ni favors the precipitation of Ni-containing intermetallic compounds and inhibits the formation of CuHf 2 phase, resulting in the improvement of glass-forming ability. [ABSTRACT FROM AUTHOR]

Nanoscratch tests have been conducted to study the tribological behavior of TiZrHfBeCu(Ni) high entropy bulk metallic glasses (HE-BMGs). A classic Zr-based BMG, vit-1 alloy, has been selected as a reference. For all the four adopted alloys, the effect of scratch velocity on the nano-scratch behavior is not obvious. It is found that TiZrHfBeCu(Ni) HE-BMGs possess better nanoscratch resistance than that of vit-1 alloy. Moreover, with the Ni content increasing, the glass transition temperature and hardness increase while the free volume content decreases, resulting in a better nanoscratch resistance. The addition of Ni also changes accumulation mode of the removed chips during the scratching process. Based on the variation of coefficient of friction (COF) during the scratching process, all the four alloys exhibit similar scratching mechanisms, which have been explained in detail. • Scratching velocity has no obvious influence on the nanoscratching behavior of TiZrHfBeCu(Ni) HE-BMGs. • TiZrHfBeCu(Ni) HE-BMGs exhibit better scratching resistance than that of vit-1 alloys. • The addition of Ni effectively enhances the scratching resistance of TiZrHfBeCu HE-BMG. • The addition of Ni doesn't change the scratching mechanism of TiZrHfBeCu HE-BMG. [ABSTRACT FROM AUTHOR]