High-entropy Metallic Glasses Research Articles

Structure, mechanical properties and nanocrystallization of (FeCoCrNi)-(B,Si) high-entropy metallic glasses

New high-entropy metallic glasses were produced by adding B and Si to the Fe25Co25Cr25Ni25 high entropy alloy. Extremely hard materials were obtained by inducing structural relaxation and nanocrystallization in the original amorphous metals. In this work we study the effects of the composition and annealing treatments on the structure and the mechanical properties. The changes in structure during heating treatments were determined by in situ synchrotron X-ray diffraction and the mechanical behavior was studied by nanoindentation. The mechanical properties of the as-quenched glasses were tuned through the annealing treatments, obtaining amorphous/nanocrystalline composites appropriate for highly demanding, wear resistance applications. The relative amount of B and Si is shown to determine the main nanocrystalline phase, FCC or BCC, which has a key influence on the mechanical properties.

Enhancing the magnetocaloric response of high-entropy metallic-glass by microstructural control

Non-equiatomic high-entropy alloys (HEAs), the second-generation multi-phase HEAs, have been recently reported with outstanding properties that surpass the typical limits of conventional alloys and/or the first-generation equiatomic single-phase HEAs. For magnetocaloric HEAs, non-equiatomic (Gd36Tb20Co20Al24)100−xFex microwires, with Curie temperatures up to 108 K, overcome the typical low temperature limit of rare-earth-containing HEAs (which typically concentrate lower than around 60 K). For alloys with x = 2 and 3, they possess some nanocrystals, though very minor, which offers a widening in the Curie temperature distribution. In this work, we further optimize the magnetocaloric responses of x = 3 microwires by microstructural control using the current annealing technique. With this processing method, the precipitation of nanocrystals within the amorphous matrix leads to a phase compositional difference in the microwires. The multi-phase character leads to challenges in rescaling the magnetocaloric curves, which is overcome by using two reference temperatures during the scaling procedure. The phase composition difference increases with increasing current density, whereby within a certain range, the working temperature span broadens and simultaneously offers relative cooling power values that are at least 2-fold larger than many reported conventional magnetocaloric alloys, both single amorphous phase or multi-phase character (amorphous and nanocrystalline). Among the amorphous rare-earth-containing HEAs, our work increases the working temperature beyond the typical <60 K limit while maintaining a comparable magnetocaloric effect. This demonstrates that microstructural control is a feasible way, in addition to appropriate compositional design selection, to optimize the magnetocaloric effect of HEAs.

Ultrastrong behaviors of a non-equiatomic Ti-Zr-Ni-Cu-Be high entropy metallic glass with ultrahigh glass forming ability

We studied the thermodynamics and kinetics of a non-equiatomic Ti32.8Zr30.2Ni5.3Cu9Be22.7 high entropy metallic glass (HEMG, ΔSconfig.∼1.44R) with a critical size for glass formation over 50 mm. The characteristic transition temperature, enthalpy of transition and specific heat capacity along with the relaxation time at different heating rate were determined using differential scanning calorimeter (DSC) measurements. It reveals that the present HEMG supercooled liquid shows the lowest thermodynamic driving force against the nucleation and growth of crystalline phases with comparing to other typical MG formers. Meanwhile, it exhibits the ultrastrong behavior near glass transition in terms of the Angell's fragility concept. The alloy has a fragility index of D* = 45.2, higher than all the typical MG formers reported so far. Both thermodynamic and kinetic factors are responsible for the ultrahigh glass-forming ability of the HEMG alloys.

Role of Co content on the microstructure and anti-corrosion performance of high-hardness AlNiYCox high entropy metallic glasses

In the present paper, the role of Co content on phase structure, thermal stability and anti-corrosion performance of melt-spun (Al1/3Ni1/3Y1/3)100-xCox (x= 5, 8, 12, 15 and 20 at.%) high entropy metallic glasses has been investigated. The microstructure and average hardness of AlNiYCox alloy ribbons were characterized and detected using a scanning electron microscope (SEM) and Vickers-type hardness tester, respectively. The corrosion behavior in 3.5 wt% NaCl solution was tested through the conventional potentiodynamic polarization curves (Tafel curves) and electrochemical impedance spectroscopy (EIS). Results suggested that all kinds of high entropy metallic glass ribbons could remain amorphous nature and the crystallization onset temperature are all above 750 K. The average hardness value of each ribbon sample is more than 530 HV0.1 as well. With the increase of Co content, the corrosion resistance begins to enhance and then lowers gradually. When the Co content increases from 5 at.% to 20 at.%, it can result in a decrease of corrosion current density from 9.546 µA/cm2 of the (Al1/3Ni1/3Y1/3)95Co5 to 0.508 µA/cm2 of the (Al1/3Ni1/3Y1/3)92Co8 and 0.741 µA/cm2 of the (Al1/3Ni1/3Y1/3)80Co20 alloy ribbons. Therefore, it is assumed that adding appropriate content of Co element into (Al1/3Ni1/3Y1/3)100-xCox alloys can enhance ability to resist corrosion in 3.5 wt% NaCl solution. The ribbons with 8 at.% Co exhibited the excellent anti-corrosion performance in this current study.

Dislocation slip induced tensile plasticity and improved work-hardening capability of high-entropy metallic glass composite

The high-entropy metallic glasses (HEMGs), combining the characteristics of high-entropy alloys (HEAs) and metallic glasses (MGs), have recently appeared and become the research focus, due to their good glass-forming ability and high strength. However, the application of HEMGs as structural material is restricted owing to the limited tensile plasticity and absence of dislocation-dominated deformation mechanism. To resolve this pressing issue, a stable β-phase HEMG composite (β-type HEMGC) Ti20Zr20Hf20Nb16Co5Be19 composed of the bcc refractory HEA (RHEA) dendrites and the HEMG matrix was fabricated. Ti20Zr20Hf20Nb16Co5Be19 not only exhibits good tensile ductility, but also has the improved work-hardening capability under room temperature tension. Essentially, the dislocation-dominated deformation mechanism plays a significant role in achieving the excellent combination of good tensile plasticity and improved work-hardening capability of the stable β-type HEMGC. The tensile ductility of Ti20Zr20Hf20Nb16Co5Be19 is attributed to the dislocation-slip mechanism induced by high-stable HEA dendrites, and the dislocation pile-up phenomenon has a great influence on the improved work-hardening capability of composite. The present results provide an indepth understanding of the deformation behavior of stable β-type HEMGCs, and a reference for how to improve the strength-ductility combination of HEMG-matrix composites.

Nanocrystalline single-phase high-entropy alloy synthesized by using intermetallic compound type (TiZrHf)-(NiCuCo) high-entropy metallic glass precursor

‘AB’ intermetallic compound (IC) type (TiZrHf)-(NiCuCo) high-entropy metallic glass precursor (HE-MGP) has been designed under considering physical and thermodynamic relations between constitutive elements, such as atomic size difference, electronegativity difference, mixing enthalpy and entropy, and phase stability, for development of nanocrystalline single-phase high-entropy alloy. The IC type HE-MGP exhibits high thermal stability and polymorphic crystallization behavior. From the systematic investigations on crystallization kinetics and microstructural evolution, it was found that the HE-MGP crystallized into nanocrystalline single-phase high-entropy alloy with B2 structure by polymorphic crystallization, and nucleation and growth rates for crystallization processing strongly depend on the annealing temperature. Our approach and discovery provide an effective way to develop the nanocrystalline single-phase high-entropy alloys by designing IC type HE-MGP.

Laser-generated high entropy metallic glass nanoparticles as bifunctional electrocatalysts

High entropy metallic glass nanoparticles (HEMG NPs) are very promising materials for energy conversion due to the wide tuning possibilities of electrochemical potentials offered by their multimetallic character combined with an amorphous structure. Up until now, the generation of these HEMG NPs involved tedious synthesis procedures where the generated particles were only available on highly specialized supports, which limited their widespread use. Hence, more flexible synthetic approaches to obtain colloidal HEMG NPs for applications in energy conversion and storage are highly desirable. We utilized pulsed laser ablation of bulk high entropy alloy targets in acetonitrile to generate colloidal carbon-coated CrCoFeNiMn and CrCoFeNiMnMo HEMG NPs. An in-depth analysis of the structure and elemental distribution of the obtained nanoparticles down to single-particle levels using advanced transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) methods revealed amorphous quinary and senary alloy phases with slight manganese oxide/hydroxide surface segregation, which were stabilized within graphitic shells. Studies on the catalytic activity of the corresponding carbon-HEMG NPs during oxygen evolution and oxygen reduction reactions revealed an elevated activity upon the incorporation of moderate amounts of Mo into the amorphous alloy, probably due to the defect generation by atomic size mismatch. Furthermore, we demonstrate the superiority of these carbon-HEMG NPs over their crystalline analogies and highlight the suitability of these amorphous multi-elemental NPs in electrocatalytic energy conversion.

Developing a multilateral-based neural network model for engineering of high entropy amorphous alloys

Due to a great number of composition-processing factors, it is very difficult to design high entropy amorphous alloys without performing manifold trial-and-error experimentations. To solve this problem, in this study we developed a machine learning-based approach, namely multilateral-based neural network, which is able to predict new high entropy amorphous compositions through estimating the highest glass forming ability and the critical casting thickness. In this approach, the entropy parameters were individually correlated to each input, which leads to the improvement of predictive model in evaluating the high entropy glassy alloys. As a case study, Ti20Zr20Hf20Be20Co20 high entropy metallic glass (MG) was considered and the effects of added elements such as Y, Ni, Cr and V and Cu on the glass formation and critical casting thickness were investigated. According to the results, it is determined that the Y addition acts as a microalloying process in the base composition, while other elements improve the configurational entropy and the total negative heat of mixing, which lead to the engineering of equi-atomic high entropy MGs.

Identifying the high entropy characteristic in La-based metallic glasses

High temperature deformation was probed in a La-based high entropy metallic glass with an evident slow β relaxation. The correlation between high configurational entropy and high temperature deformation mechanism was analyzed. On the one hand, by increasing the strain rate, the degree of deviation from Newtonian behavior in high entropy metallic glasses is lower than that of conventional metallic glasses, which is ascribed to the high configurational entropy. On the other hand, high configurational entropy leads to a decrease in activation volume in high temperature deformation of metallic glasses. Identifying the influence of high configurational entropy on the deformation mechanism paves the way for further understanding of the mechanical behavior of metallic glasses.

Intrinsic correlation between β relaxation and electronic heterogeneity in high entropy metallic glasses

In this work, we have successfully produced a series of equal atomic high- and medium-entropy metallic glasses (HE- and ME-MGs) with pronounced β relaxation. However, compared with the conventional MGs, it is difficult to understand the origin of the strong β relaxation behaviors in these HE-MGs based on the existing empirical knowledge. Here we studied the origin of β relaxation from the viewpoint of electronic heterogeneity by using X-ray photoelectron spectroscopy. It is found that the behaviors of β relaxation are closely associated with the valence states of the constituent elements, .i.e., the larger number of multivalent constituent elements, the more obvious of β relaxation. Moreover, it is found that the high percentage of high energy valence state in the same multivalent system can also promote β relaxation of the amorphous alloy. Our results provide new insight into understanding the origin of β relaxation and the correlation between dynamics and electronic structure in MGs.

Decoupling between calorimetric and dynamical glass transitions in high-entropy metallic glasses

Glass transition is one of the unresolved critical issues in solid-state physics and materials science, during which a viscous liquid is frozen into a solid or structurally arrested state. On account of the uniform arrested mechanism, the calorimetric glass transition temperature (Tg) always follows the same trend as the dynamical glass transition (or α-relaxation) temperature (Tα) determined by dynamic mechanical analysis (DMA). Here, we explored the correlations between the calorimetric and dynamical glass transitions of three prototypical high-entropy metallic glasses (HEMGs) systems. We found that the HEMGs present a depressed dynamical glass transition phenomenon, i.e., HEMGs with moderate calorimetric Tg represent the highest Tα and the maximum activation energy of α-relaxation. These decoupled glass transitions from thermal and mechanical measurements reveal the effect of high configurational entropy on the structure and dynamics of supercooled liquids and metallic glasses, which are associated with sluggish diffusion and decreased dynamic and spatial heterogeneities from high mixing entropy. The results have important implications in understanding the entropy effect on the structure and properties of metallic glasses for designing new materials with plenteous physical and mechanical performances.

Stress relaxation in high-entropy Pd20Pt20Cu20Ni20P20 metallic glass: Experiments, modeling and theory

The viscoelastic properties of Pd20Pt20Cu20Ni20P20 high-entropy metallic glass were probed by dynamic mechanical spectroscopy and stress relaxation. The experimental evolution of stress can be characterized by the empirical Kohlrausch-Williams-Watts function during the tensile stress relaxation measurement. We develop a linear relaxation model able to describe the whole process of stress relaxation in a wide range of temperatures. The linear relaxation model takes into account the microstructural heterogeneity of the glass, and thus its dynamical heterogeneity, so that the thermal effect and stress-driven process are physically decoupled. The activation energy spectra at various temperatures reveal the changes of the deformation units during stress relaxation, which is the result of the interplay between stress and temperature. This study decomposes the widespread hierarchical dynamics due to structural heterogeneity which accommodates the viscoelastic deformation of the high-entropy metallic glasses.

A Self‐Supported High‐Entropy Metallic Glass with a Nanosponge Architecture for Efficient Hydrogen Evolution under Alkaline and Acidic Conditions

AbstractDeveloping highly efficient and durable electrocatalysts for hydrogen evolution reaction (HER) under both alkaline and acidic media is crucial for the future development of a hydrogen economy. However, state‐of‐the‐art high‐performance electrocatalysts recently developed are based on carbon carriers mediated by binding noble elements and their complicated processing methods are a major impediment to commercialization. Here, inspired by the high‐entropy alloy concept with its inherent multinary nature and using a glassy alloy design with its chemical homogeneity and tunability, we present a scalable strategy to alloy five equiatomic elements, PdPtCuNiP, into a high‐entropy metallic glass (HEMG) for HER in both alkaline and acidic conditions. Surface dealloying of the HEMG creates a nanosponge‐like architecture with nanopores and embedded nanocrystals that provides abundant active sites to achieve outstanding HER activity. The obtained overpotentials at a current density of 10 mA cm−2 are 32 and 62 mV in 1.0 m KOH and 0.5 m H2SO4 solutions, respectively, outperforming most currently available electrocatalysts. Density functional theory reveals that a lattice distortion and the chemical complexity of the nanocrystals lead to a strong synergistic effect on the electronic structure that further stabilizes hydrogen proton adsorption/desorption. This HEMG strategy establishes a new paradigm for designing compositionally complex alloys for electrochemical reactions.

Design of Fe-containing GdTbCoAl high-entropy-metallic-glass composite microwires with tunable Curie temperatures and enhanced cooling efficiency

Through designing the composition and processing approach, the non-equiatomic (Gd36Tb20Co20Al24)100-xFex (x = 0, 1, 2 and 3 at.%) high-entropy-metallic-glass (HE-MG) alloy microwires were successfully fabricated by melt-extraction technique. The microstructure and magnetocaloric properties of the microwires were systematically investigated. The microwires possess tunable Curie temperatures, i.e. 81–108 K, above the typical rare-earth (RE) containing HE-MG reports. The high Curie temperatures are attributed to the designed composition. Magnetocaloric response peak values of Fe-containing GdTbCoAl alloy microwires range 7.6–8.9 J kg−1 K−1 (5 T), which are comparable to those of many outstanding RE-containing magnetocaloric HE-MGs. The characteristics of the melt-extraction method, combining with compositional effects, favor the formation of amorphous and nanocrystalline phases. The increase in the cooling efficiency for microwires with higher Fe content can be attributed to the broadening of the Curie temperature distribution induced by the composition difference between nanocrystalline phase and amorphous matrix. The designed composition and the melt-extraction processing approach for Fe-containing GdTbCoAl alloys can tune their Curie temperatures towards a temperature range of natural gas liquefaction and improve their magnetocaloric properties. This demonstrates that Fe-containing GdTbCoAl HE-MG composite microwires have great potential as high-performance magnetic refrigerants.

Research progress in corrosion resistance of high-entropy metallic glasses

Research progress in corrosion resistance of high-entropy metallic glasses

Inelastic deformation of metallic glasses under dynamic cyclic loading

Dynamic mechanical analysis was performed in two metallic glasses that show markedly distinct β relaxation features. The confined flow defects of free-volume zones and the inelastic deformation behavior result into a significant mechanical hysteresis loop in dynamic cyclic loadings, which is described with a viscoelastic model. Annealing below the glass transition temperature Tg decreases the concentration of free-volume zones. The understanding of the inelastic deformation, which is related to free-volume zones, is critical and may provide a new avenue into the mechanical properties’ improvement of high entropy metallic glasses.

Structure, thermal and electrical properties of Fe-B-Co-Nb-Ni-Si high-entropy metallic glass

New nanostructured Fe25B17.5Co21.35Nb3.65Ni25Si7.5 high-entropy metallic glass is synthesized by dint of splat-quenching technique. The estimated cooling rate is ~106 K/s. The coherently scattering domain size (crystallite size) of films of Fe 25B17.5Co21.35Nb3.65Ni25Si7.5 high-entropy metallic glass estimated by the Sherer formula is ~ 3 nm. The high-entropy metallic glass shows a high temperature of glass transition Tg ~ 800 K and the initial temperature of crystallization Tx ~ 863 K. After annealing to ~ 1010 K (above the crystallization temperature) and subsequent slow cooling, the amorphous films crystallize with the formation of several crystalline phases. The temperature dependencies of electrical resistivity of the Fe25B17.5Co21.35Nb3.65Ni25Si7.5 films indicate the occurrence of phase transformations. The phase transformation temperature (temperature of crystallization) is ~ 869 K. This value agrees well with the value obtained from the differential thermal analysis.

Effect of Yttrium addition on magnetocaloric properties of Gd-Co-Al-Ho high entropy metallic glasses

The Gd25Co25Al25Ho25-xYx (x = 1, 5, 10, and 15) high entropy metallic glasses are fabricated by copper-mold quenching technique. Substituting moderately Ho with Y effectively improves the glass-forming ability, magnetic entropy change (|ΔSM|), and magnetic refrigeration capacity (RC) of Gd25Co25Al25Ho25-xYx. Besides, the Curie temperature of this quaternary alloy can be tuned from 51 to 41 K with Y addition. Among these alloys, Gd25Co25Al25Ho20Y5 exhibits the largest peak value of |ΔSM| up to 8.79 J kg−1K−1 and the corresponding RC up to 547 J kg−1 under a magnetic field of 5 T. It is worthy to note that a reverse changing trend can be observed between the maximum |ΔSM| and the temperature range of magnetic transition, which may be ascribed to the spatial inhomogeneity induced by yttrium microalloying. Our studies show that yttrium is an effective element to manipulate magnetocaloric properties of high entropy metallic glasses as potential refrigerants.

New (FeCoCrNi)-(B,Si) high-entropy metallic glasses, study of the crystallization processes by X-ray diffraction and Mössbauer spectroscopy.

The role of B and Si in the formation of (FeCoCrNi)100-x-yBxSiy high-entropy metallic glasses is studied. It is found that a content of B between 10 and 20 at% and of Si between 5 and 15 at% is able to produce a completely amorphous structure. The microstructural evolution of two of this high-entropy metallic glass compositions, (FeCoCrNi)80B20 and (FeCoCrNi)80B10Si10, have been studied by X-ray diffraction and Transmission Mössbauer Spectroscopy. In both compositions, the first crystallization process corresponds to the formation of metastable, M3B, and stable, M2(B,Si), borides where M stands for metallic atoms. In the Si containing sample a BCC phase also appears. At the second crystallization stage the metastable and the BCC phases disappear and stable M2B or M2(B,Si) phases begin to grow simultaneously with an FCC structure that presents a distribution of possible environs. The fully crystallized structure consists of boride and silicide phases and a paramagnetic FCC phase. The presence of Si promotes the crystallization of a BCC phase and the refinement of the microstructure leading to smaller and more uniform grains.

Unique energy-storage behavior related to structural heterogeneity in high-entropy metallic glass

Structural heterogeneity dominates energy-storage behavior of metallic glasses. Ti20Zr20Hf20Be20Cu7.5Ni12.5 high-entropy bulk metallic glass (HE-BMG) shows extraordinary energy-storage behavior under cryothermal cycling: the relaxation enthalpy monotonically increased with cryothermal cycling cycles, even after 240 cycles of treatment. By comparison, the relaxation enthalpy of Ti41Zr25Be22Ni12 BMGs reached a maximum only at 30 cycles and then dramatically decreased as prototypical BMGs reported. The as-cast HE-BMG demonstrates an extremely low fraction of loose-packing regions (~7.8%, fractions for Ti- and Zr-based BMGs are above 15.6%), indicating degraded structural heterogeneity that would induce smaller local internal stress during cryothermal cycling and eventually lead to the sluggish energy-storage behavior. In addition, the increase in the fraction of loose-packing regions accompanied by the energy storage also lead to significant improvement in ductility of the HE-BMG, the plastic strain increased from 0.6% (the as-cast sample) to 5.2% (the sample treated 240 cycles).