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Numerical finite element method (FEM) calculation results reveal that the effective strain differs between the ductile Zr-based bulk metallic glass and brittle Hf-based bulk metallic glass during rolling at room temperature. The current results demonstrated that the deformation mechanism of ductile Zr-based bulk metallic glass can be explained by perceptibility of multiple shear bands formation, however, the deformation mechanism of brittle Hf-based bulk metallic glass is represented by uniformly distribution of effective strain through overall specimen rather than localized into shear band. The variation of stress and the distribution of effective strain change significantly near the surface region of a Hf-based bulk metallic glass plate with increasing number of rolling passes. Under elasto-plastic deformation by cold rolling, the brittle Hf-based BMG has a thickness strain (et) of -0.012, and the neutral effective strain (eeff) is 0.011 at the thickness direction, respectively. We present experimental confirmation that when the applied effective strain can be adjusted to below 1.1% during elasto-plastic deformation then even the brittle as-cast Hf-based BMG can be deformed up to 44% thickness reduction and 23% width expansion after multi-pass cold rolling without fracture by localization of shear stress.

The structure-property relationship, one of the central themes in materials science, is far from being well understood for metallic glasses (MGs) due to the great complexity of their amorphous structures. Based on the analysis of published experimental data for 165 MGs from more than 15 different alloy systems, the present study reveals a universal dependence of mechanical properties (Young’s moduli, shear moduli and yield strength) on simple structural parameters (the inter-atomic distance and/or valence electron density) originating from the interatomic potential and Fermi sphere-Brillouin zone interaction. This work establishes a structure-property relationship for metallic glasses and provides insights into the fundamentals of the mechanical properties of disordered systems.

In transition metal-based alloys, the nonlinearity of the current at large cathodic potentials reduces the credibility of the linear Tafel slopes for the evaluation of electrocatalytic hydrogen activity. High-precision nonlinear fitting at low current densities describing the kinetics of electrochemical reactions due to charge transfer can overcome this challenge. To show its effectiveness, we introduce a glassy alloy with a highly asymmetric energy barrier: amorphous NiP electrocoatings (with different C and O inclusions) via changing the applied DC and pulsed current and NaH2PO2 content. The highest hydrogen evolution reaction (HER) activity with the lowest cathodic transfer coefficient α = 0.130 with high J0 = -1.07 mA cm-2 and the largest surface areas without any porosity are observed for the pulsed current deposition. The calculated α has a direct relation with morphology, composition, chemical state and coating thickness defined by the electrodeposition conditions. Here, a general evaluation criterion with practicality in assessment and high accuracy for electrocatalytic reactions applicable to different metallic alloy systems is presented.

Bulk metallic glasses (BMGs) are materials with outstanding strength and elastic properties that make them tantalizing for engineering applications, yet our poor understanding of how their amorphous atomic arrangements control their broader mechanical properties (hardness, wear, fracture, etc.) impedes our ability to apply materials science principles in their design. In this work, we uncover the hierarchical structure that exists in BMGs across the nano- to microscale by using nanobeam electron diffraction experiments. Our findings reveal that local hardness of microscale domains decreases with increasing size and volume fraction of atomic clusters with higher local medium range order (MRO). Furthermore, we propose a model of ductile phase softening that will enable the future design of BMGs by tuning the MRO size and distribution in the nanostructure.

Increasing the efficiency of hydrogen storage and release using recent generation metallic glass nanofilms (MGNFs) offers green solutions for nanoscale energy applications. Contrary to flat nanofil...

The viscoelastic behavior of four different bulk metallic glass (BMG) systems, i.e., Cu46Zr46Al8, Cu44Zr44Al8Co4, Cu44Zr44Al8Hf4, and Cu44Zr44Al8Co2Hf2, is investigated concerning its deformation-mode dependence via dynamic mechanical analysis (DMA) in 3-point bending (TPB), tension, and torsion modes. At temperatures below the glass transition, the considered BMGs deform primarily elastic, and the mechanical response is independent of the testing frequency, whereas, in the glass transition region, the viscoelastic component dominates. Crystallization decreases the viscoelastic contribution, whereas plastic deformation leads to an increase in atomic mobility for all three deformation modes. Compared to tension and torsion, TPB is found to be more sensitive to dynamic mechanical stress. TPB generates a complex stress state in the matrix and can thus introduce substantial variations in loss modulus. Structural analyses carried out by transmission electron microscopy and X-ray diffraction confirmed the amorphous nature of the base composition and structural changes when heated to the intermediate peak temperature observed at 743 K for the TPB mode. Compared to Cu46Zr46Al8, 4 at. pct Co addition in the Cu46−x\2Zr46−x\2Al8Cox amorphous alloy leads to a glass showing relatively higher thermomechanical stability around its glass transition. This study provides evidence for the enhancement of the mechanical properties of CuZr-based BMGs at elevated temperatures by microalloying.

Nanostructured metallic glass films (NMGF) can exhibit surface and intrinsic effects that give rise to unique physical and chemical properties. Here, a facile synthesis and electrochemical, structural, and morphologic characterization of Pd-Au-Si based MGs of approximately 50 nm thickness supported on Si/SiO2 is reported. Impressively, the maximum total hydrogen charge stored in the Pd-Au-Si nanofilm is equal to that in polycrystalline Pd films with 1 μm thickness in 0.1 m H2 SO4 electrolyte. The same NMGF has a volumetric desorption charge that is more than eight times and 25 % higher than that of polycrystalline PdNF and Pd-Cu-Si NMGF with the same thickness supported on Si/SiO2 , respectively. A significant number of nanovoids originating from PdHx crystals, and an increase in the average interatomic spacing is detected in Pd-Au-Si NMGF by high-resolution TEM. Such a high amount of hydrogen sorption is linked to the unique density fluctuations without any chemical segregation exclusively observed for this NMGF.

In this study, high entropy metallic glasses (HEMGs) were developed through a combination of concepts for designing metallic glasses (main element + transition metal + metalloid element) and high-entropy alloys (more than five elements, each element having an atomic concentration between 5 at.% and 35 at.%). The developed metallic glass alloys are composed of Co-Fe-Ni main elements, transition metals (Cr, Mo) and metalloid elements (C, B). Moreover, the present work reports the thermal, mechanical and magnetic properties of (CoFeNi)50(CrMo)50-x(CB)x alloys with x = 20, 25, 30. The developed as-spun HEMGs exhibit typical paramagnetic properties even for a high amount of ferromagnetic elements (Co, Fe, and Ni) and have high elastic modulus (103–160 GPa) and hardness (14–27 GPa), thus possessing mechanical properties similar to well-known Co-based metallic glasses (Co-Cr-Mo-C-B system). In addition, some of the bulk samples prepared with a diameter of 2 mm form bulk metallic glasses with a high compressive strength around 3.5 GPa. The mechanisms determining the stability of the supercooled liquid, as well as the paramagnetic and mechanical properties for the developed non-equiatomic HEMGs, are discussed.

Using molecular dynamics simulations, we provide an atomistic description of the shear band multiplication mechanisms in a heterogeneous metallic glass consisting of two distinct amorphous regions with different amounts of free volume and degrees of short-range order. The structural differences and elastic fluctuations encountered by the developing shear band while crossing the interface between the two regions alter the autocatalytic activation of the shear transformation zones (STZs) and, subsequently, lead to shear band branching. The two-unit STZ-vortex mechanism sheds light on the correlation between the strain distribution during tensile deformation and the directional characteristics of the STZ activation process.

A new series of Ti40Zr10Cu36−xPd14Gax (x = 2, 4, 8 and 10) alloys with improved plasticity and glass-forming ability (GFA) are produced. The strategy is to create local structural heterogeneity in Ti-based bulk metallic glasses (BMGs) by alloying with a soft element which has high negative enthalpy of mixing with Ti-atoms. The GFA of Ti40Zr10Cu36−xPd14Gax rods increases with increasing Ga content up to 4 at.%, and for more Ga it decreases. Minor addition of Ga (x = 2) is beneficial to improve the strength and ductility of the TiZrCuPd-based BMG due to the formation of local structural heterogeneities which facilitate the formation of a highly organized pattern of multiple shear bands under quasi-static compression. Ti40Zr10Cu34Pd14Ga2 BMG shows the best combination of compressive global strain and maximum compressive strength of ∼4.5% and ∼2000 MPa, respectively. Comparing the Young's modulus of Ga-containing alloys in amorphous and crystalline state we find that the amorphous state shows significant elastic softening (40–48% smaller) which is useful to reduce stress shielding effect for biomedical applications. The alloys show a remarkable thermoplastic formability as the viscosity decreases sharply by an order of magnitude in the supercooled liquid region.

This study investigates the effect, that replacing Zr in a glass-forming Cu47.5Zr47.5Al5 alloy by Sc (0–2 at.%) has on the phase formation as well as on the thermal and mechanical properties. Even though it is not reflected in the thermal data, the glass-forming ability (GFA) is significantly reduced. This originates from the increased tendency to precipitate the shape-memory phase B2 CuZr(Sc), which, in turn, promotes the formation of bulk metallic glass (BMG) matrix composites. Sc appears to be very effective in stabilizing the B2 crystals because it forms the stable B2 CuSc phase with a similar lattice constant like B2 CuZr. By adjusting the casting parameters, the composite microstructure of Cu47.5Zr46.5Al5Sc1 can be controlled to a certain extent. The yield strength and the plasticity of the present composites depend on the crystalline volume fraction. In-situ high-energy X-ray diffraction reveals that deformation proceeds in three stages: (i) martensitic transformation of the B2 phase, (ii) yielding of the amorphous phase and continuing martensitic transformation, (iii) completion of the phase transformation and plastic deformation of all phases. Our work suggests that Sc is a promising candidate to adjust the microstructure and, thus, the mechanical properties of CuZr-based composites consisting of a glassy matrix and shape-memory crystals.

Most bulk metallic glass systems with high glass-forming ability are doped with precious metal(s) or toxic element(s), like palladium and beryllium, leading to relatively high cost or low safety in use. In this work, the glass-forming ability, crystallization kinetics and mechanical properties of the Zr55Co25Al20 and Zr52Co25Al23 bulk metallic glasses prepared by water-cooled copper mold suction casting are investigated in detail. The results show that Zr52Co25Al23 has a wider supercooled liquid region, a higher thermal stability and a higher glass-forming ability than the Zr55Co25Al20 amorphous alloy. At the same time, Zr52Co25Al23 has a higher activation energy of crystallization than Zr55Co25Al20 amorphous alloy, indicating that Zr52Co25Al23 has a higher resistance to crystallization than Zr55Co25Al20. Investigation of the mechanical properties reveals that Zr52Co25Al23 also has a higher fracture strength of up to 2308 MPa and a higher specific fracture strength of up to 3.78 × 105 N m kg−1 compared with 316L stainless steel, Ti-6Al-4V, as well as Mg-, Ti-, and other Zr-based BMGs.

To uncover the physical correlation between intrinsic structural heterogeneity and mechanical properties of metallic glasses, we performed high-pressure torsion (HPT) to tailor their microstructure with a rejuvenated state. Nanoindentation results show that the change of mechanical properties results from the convolution of the intrinsic thermal activation process and the structural inhomogeneity in the glassy phase. The spatial heterogeneity induced by severe plastic deformation can markedly operate to reduce the energy barrier for shear transformation zone (STZ) activation and increase the number of STZ events during tensile testing to accommodate the plastic strain, leading to a brittle-to-ductile transition. We highlight that by properly controlling the distribution of structural heterogeneities, one can purposely tune the mechanical performance of metallic glasses.

We investigate the influence of the ratio between carbon and boron on glass-forming ability, thermal stability and mechanical properties of the Co65Cr13Mo22 ternary eutectic alloy modified by carbon and boron addition. Besides, the correlation between the extension of the supercooled liquid region (SCLR) and three bonding parameters, i.e. electronegativity, atomic size, and valence electron is investigated. The results reveal that carbon is more effective than boron to form a glassy phase in this system, and fully glassy alloys or metallic glass composites can be obtained when the metalloid contents are in the range of 20–25 at.%. Fully glassy alloys are only obtained when the carbon-to-boron ratio is 2:1. The glassy alloys and the glass composites exhibit high strength and extraordinarily wide SCLRs of >120 K. The width of the SCLR of the Co-based BMGs can be articulated as a function of three bonding parameters, which is different to the relationship previously observed for Fe-, Mg- and Pd-based BMGs. Based on these findings we propose the optimum C/B ratio for glass phase formation and the new criterion for predicting the width of the SCLR of Co-based BMGs by the three bonding parameters.

To overcome the brittleness of metallic glasses (MGs), their structure, chemistry, or loading conditions are usually controlled. Here, the local stress state in MGs was modulated without affecting their structure. The elastically designed MG heterostructures provide enhanced ductility together with strain hardening during loading. The stress heterogeneity leads to shear band multiplication that consequently enhances the macroscopic ductility of MGs. In addition, the residual compressive stress significantly increases the strength of the glass and is responsible for the observed strain hardening.

The structure of matter at the nanoscale, in particular that of amorphous metallic alloys, is of vital importance for functionalization. With the availability of synchrotron radiation, it is now possible to visualize the internal features of metallic samples without physically destroying them. Methods based on computed tomography have recently been employed to explore the local features. Tomographic reconstruction, while it is relatively uncomplicated for crystalline materials, may generate undesired artifacts when applied to featureless amorphous or nanostructured metallic alloys. In this study we show that X-ray diffraction computed nanotomography can provide accurate details of the internal structure of a metallic glass. We demonstrate the power of the method by applying it to a hierarchically phase-separated amorphous sample with a small volume fraction of crystalline inclusions, focusing the X-ray beam to 500 nm and ensuring a sub-micrometer 2D resolution

The effects of severe plastic deformation (SPD) by means of high pressure torsion (HPT) on the soft magnetic properties of the iron-based metallic glass Fe$_{73.9}$Cu$_{1}$Nb$_{3}$Si$_{15.5}$B$_{6.6}$ are investigated. With this procedure, it has been possible for the first time to consolidate and deform this high-strength material with a minimum of cracks. X-ray diffraction, differential mechanical and differential scanning analyses, as well as optical and scanning electron microscopy were carried out for characterization of crystallization phenomena and crack formation. For magnetic properties, minor loop hysteresis measurements were done because of their high sensitivity for magnetic structures. Significant influences of HPT-processing on the coercivity as well as on magnetic anisotropy are found and discussed in detail.

Contrary to the electrochemical energy storage in Pd nanofilms challenged by diffusion limitations, extensive metal-hydrogen interactions in Pd-based metallic glasses result from their grain-free structure and presence of free volume. This contribution investigates the kinetics of hydrogen-metal interactions in gold-containing Pd-based metallic glass (MG) and crystalline Pd nanofilms for two different pore architectures and nonporous substrates. Fully amorphous MGs obtained by physical vapor deposition (PVD) co-sputtering are electrochemically hydrogenated by chronoamperometry. High-resolution (scanning) transmission electron microscopy and corresponding energy-dispersive X-ray analysis after hydrogenation corroborate the existence of several nanometer-sized crystals homogeneously dispersed throughout the matrix. These nanocrystals are induced by PdHx formation, which was confirmed by depth-resolved X-ray photoelectron spectroscopy, indicating an oxide-free inner layer of the nanofilm. With a larger pore diameter and spacing in the substrate (Pore40), the MG attains a frequency-independent impedance at low frequencies (∼500 Hz) with very high Bode magnitude stability accounting for enhanced ionic diffusion. On the contrary, on a substrate with a smaller pore diameter and spacing (Pore25), the MG shows a larger low-frequency (0.1 Hz) capacitance, linked to enhanced ionic transfer in the near-DC region. Hence, the nanoporosity of amorphous and crystalline metallic materials can be systematically adjusted depending on AC- and DC-type applications.

Compared to their conventional polycrystalline Pd counterparts, Pd79Au9Si12 (at%) – metallic glass (MG) nanofilm (NF) electrocatalysts offer better methanol oxidation reaction (MOR) in alkaline medium, CO poisoning tolerance and catalyst stability even at high scan rates or high methanol concentrations owing to their amorphous structure without grain boundaries. This study evaluates the influence of scan rate and methanol concentration by cyclic voltammetry, frequency-dependent electrochemical impedance spectroscopy and a related equivalent circuit model at different potentials in Pd–Au–Si amorphous NFs. Structural and compositional differences for the NFs are assessed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy dispersive X-ray (EDX) mapping and X-ray diffraction (XRD). The ratio of the forward to reverse peak current density ipf/ipb for the MG NFs is ∼2.2 times higher than for polycrystalline Pd NFs, evidencing better oxidation of methanol to carbon dioxide in the forward scan and less poisoning of the electrocatalysts by carbonaceous (e.g. CO, HCO) species. Moreover, the electrochemical circuit model obtained from EIS measurements reveals that the MOR occurring around −100 mV increases the capacitance without any significant change in oxidation resistance, whereas CO2 formation towards lower potentials results in a sharp increase in the capacitance of the Faradaic MOR at the catalyst interface and a slight decrease in the corresponding resistance. These results, together with the high ipf/ipb = 3.37 yielding the minimum amount of carbonaceous species deposited on the thin film during cyclic voltammetry and stability in the alkaline environment, can potentially make these amorphous thin films potential candidates for fuel-cell applications.

Unlike crystalline alloys, disordered amorphous alloys lack long-range order. Even today, the creep mechanisms for amorphous alloys are far from being fully understood. In this work, an Fe–Cr–Mo–B–C (Cr: 25–27 wt%, Mo: 16–18 wt%, B: 2.0–2.2 wt%, and C: 2.0–2.5 wt%) amorphous coating was fabricated on the surface of a 304 stainless steel via atmospheric plasma spraying with a NiAl bonding layer. In this study, the effects of peak load and loading rate on the creep deformation behavior of the Fe-based amorphous coatings were investigated. The results demonstrated that a macroscopic viscous flow behavior was obtained at low peak loads, which led to a larger creep strain rate sensitivity m. At high loading rates, the accumulation of free volume led to an increase in the shear deformation zone and a more uniform plastic rheology. It indicated that at higher loading rates, the amorphous coating had higher m values under steady-state creep.

We report a novel plastic deformation mechanism of bulk metallic glass composites (BMGCs) containing metastable $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ti}$ dendrites. Plastic deformation of the BMGCs beyond the ultimate tensile strength is mediated by cooperative shear events, which comprise a shear band in the glassy matrix and a continuous $\ensuremath{\omega}\text{\ensuremath{-}}\mathrm{Ti}$ band with a thickness of $\ensuremath{\sim}10\text{ }\text{ }\mathrm{nm}$ in the $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ti}$ dendrite. The cooperative shear leads to serrated shear avalanches. The formation of narrow $\ensuremath{\omega}\text{\ensuremath{-}}\mathrm{Ti}$ bands is caused by high local strain rates during the cooperative shear. The cooperative shear mechanism enriches the deformation mechanisms of BMGCs and also deepens the understanding of $\ensuremath{\omega}\text{\ensuremath{-}}\mathrm{Ti}$ formation.

Alloying elements play an important role in adjusting the magnetic and thermal properties of Fe-based amorphous alloys. In this work, the effect of Mo addition on the thermal stability, structural evolution, and magnetic properties of Fe76Si9B10P5 metallic glass was studied. The study revealed that the substitution of a small amount of Mo (1 at.%) for Si enhances the glass-forming ability (GFA) but reduces the thermal stability of the alloy, causing a reduction of the supercooled liquid region. Substitution of up to 3 at.% Mo for Si lowers the Curie temperature from 677 to 550 K and the saturation magnetization drops from 160 to 138 Am2/kg. The structural evolution was evaluated by annealing the glassy samples at different temperatures, revealing that the crystallization proceeds in multiple steps, beginning with the formation of different iron borides (FeB, Fe2B, FeB2 and Fe23B6) followed by transformation to a mixture of more stable phases., Metals, 10 (7), ISSN:2075-4701

In this work, the biomedical applicability and physical properties of magnesium-based metallic glass/polycaprolactone (PCL) composites are explored. The composites were fabricated via mechanical alloying and subsequent coextrusion. The coextrusion process was carried out at a temperature near to the supercooled liquid region of the metallic glass and the viscous region of the polymer. The structures, as well as thermal and mechanical properties of the obtained samples were characterized, and in vivo investigations were undertaken. The composite samples possess acceptable thermal and mechanical properties. Tensile tests indicate the ability of the composites to withstand more than 100% deformation. In vivo studies reveal that the composites are biologically compatible and could be promising for biomedical applications.

With the discovery of bulk metallic glasses (BMGs), there has been considerable interest in understanding their mechanical behavior. In spite of these previous observations on the relation between plastic deformation of metallic glasses and their diffusion behavior, a detailed understanding on the diffusion of BMGs is still unexplored. We evaluated the contribution of deformation-induced structural transformations (elastic, anelastic, viscoplastic or viscoelastic responsive and plastic strain) on the diffusion of Zr-based bulk metallic glasses in as-cast, elastostatically stressed and plastically deformed states. Experimental investigations of the diffusion process and the elemental distributions in the glassy alloy were performed following plastic deformation by multiple cold rolling and elastostatic cyclic compression, respectively. We compared the vacancy model and the transition state model to verify the diffusion mechanism in the deformed bulk metallic glass. The diffusion of tracer atoms, i.e., Fe, in the bulk metallic glass is affected by viscoelastic responsive strain governing the transition-state model. In contrast, the diffusion of constituent atoms, i.e., Ti, Zr, in the bulk metallic glass is dominantly affected by plastic strain governing the vacancy model. The results reveal that the diffusion behavior of bulk glassy alloys can be changed by variation of the constituent elements and applying different strain modes upon deformation.

Mg-Ca-Zn alloys are considered to be biodegradable materials and nontoxic to the living cell. Thus, they are promising candidates for use in orthopaedic applications. In the present work, two new Mg-based glassy alloys (Mg72Ca12Zn16 and Mg63Ca15Zn22) are synthesized by melt spinning. The prospect of these alloys for biomedical application is assessed by evaluating and comparing their mechanical properties, biodegradation and biocompatibility with the well established glassy alloy composition, Mg60Ca5Zn35, synthesized under identical conditions. With decreasing Zn content in the composition, the density of the melt spun alloys decreases. The new alloys show lower hardness and an about ~35 % lower Young's modulus than the Mg60Ca5Zn35 alloy. In vitro degradation behaviour in Hank's balanced salt solution (HBSS) solution yields a higher degradation rate for the new alloys than for the existing Mg60Ca5Zn35 alloy. Degradation products are characterized using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), confirming the presence of magnesium hydroxide (Mg(OH)2), hydroxyapatite (HAp), and ternary calcium magnesium zinc (Ca2Mg6Zn3) phases. In vitro biocompatibility studies with a mouse osteoblast cell line (MC3T3-E1) clearly demonstrates the non-cytotoxic nature of the alloys. The results altogether suggest possible biomedical applications of the new Mg-based glassy alloys.

NPG Asia Materials 12(1), 1-59 (2020). doi:10.1038/s41427-020-00241-4, The design of ductile heterogeneous metallic glasses (MGs) with enhanced deformability by purposely controlling theshear-band dynamics via modulation of the atomic-scale structures and local stress states remains a significantchallenge. Here, we correlate the changes in the local atomic structure when cooling to cryogenic temperature withthe observed improved shear stability. The enhanced atomic-level structural and elastic heterogeneities related to thenonaffine thermal contraction of the short-range order (SRO) and medium-range order (MRO) change thecharacteristics of the activation process of the shear transformation zones (STZs). The experimental observationscorroborated by Eshelby inclusion analysis and molecular dynamics simulations disclose the correlation between thestructuralfluctuations and the change in the stressfield around the STZ. The variations in the inclination axes of theSTZs alter their percolation mechanism, affect the shear-band dynamics and kinetics, and consequently delay shearfailure. These results expand the understanding of the correlation between the atomic-level structure and elementaryplastic events in monolithic MGs and thereby pave the way for the design of new ductile metallic alloys., Published by Macmillan Publishers Limited, part of Springer Nature, Tokyo

A new concept of soft ferromagnetic bulk metallic glass (BMG) with self-healing ability is proposed. The specific [Fe36Co36B19.2Si4.8Nb4]100−x(Ga)x (x = 0, 0.5, 1 and1.5) BMGs prepared by copper mold casting were investigated as a function of Ga content. The Ga-containing BMGs still hold soft magnetic properties and exhibit large plastic strain of 1.53% in compression. Local melting during shearing produces molten droplets of several µm size throughout the fracture surface. This concept of local melting during shearing can be utilized to produce BMGs with self-healing ability. The molten regions play a vital role in deflecting shear transformation zones, thereby enhancing the mechanical properties., Materials, 13 (6), ISSN:1996-1944

In the present study, the glass-forming ability (GFA) of (Cu50Zr43Al7)100-xYx (x = 0, 2, 4 and 6 at.%) bulk metallic glasses (BMGs) was investigated from thermodynamic and kinetic viewpoints. The amorphous structure of the alloys was confirmed using several techniques including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and differential scanning calorimetry (DSC). The thermal characteristics obtained through DSC and differential thermal analysis (DTA) were employed to establish a correlation between the GFA and thermodynamics/kinetics factors. The (Cu50Zr43Al7)98Y2 BMG has a promising GFA of 15 mm diameter, i.e. a 50% increase in GFA compared to the base alloy. Several GFA indicators ( Δ T x , Trg, γ and K) confirm the excellent glass-forming capability of 2 at.% Y-doped BMG. The role of the optimum yttrium content in improving the GFA was scrutinized considering atomic, topological, and thermodynamics concepts. The interpretations imply that Y enhances the GFA through two main ways including promoting a more densely packed atomic configuration and also through scavenging oxygen as an undesirable element in glass-forming systems.

We model the deformation behavior of metallic amorphous Cu 64 Zr 36 /crystalline B2 CuZr nanolaminate systems using molecular-dynamics computer simulations. Amorphous-crystalline nanolaminates with shape memory alloys may be a material class which is combining the advantageous properties of metallic glasses with large-strain homogeneous flow at low temperatures and high stresses. We find that the deformation of the glassy and crystalline phases is a coupled process: martensitic transformation leads to shear band formation while the stress at the shear band tip induces martensitic transformation in the shape memory crystal. Moreover, the martensitic transformation changes the shear band morphology, stabilizes the shear flow and avoids a runaway instability. Finally, the critical volume fraction of the B2 layer for which the composite laminate shows a brittle-to-ductile transition is identified. The value of the critical volume fraction can be further decreased when the structure of the metallic glass is rejuvenated. Therefore, tailoring the architecture of metallic glass laminates with shape memory phases may allow the development of materials that exhibit large tensile ductility.

The influence of pressure (0–764 MPa), on the densification and crystallization behavior of Al84Ni7Gd6Co3 glassy powders during sintering has been studied by hot pressing the amorphous alloy powders within the supercooled liquid region. The applied pressure significantly influenced the densification process: decreased the porosity level, changed the area of the inter-particle region and varied the volume fraction of α-Al in the inter-particle regions. The main densification mechanisms operating were particle (re)arrangement, plastic flow, viscous flow, and Al diffusion from the amorphous matrix to the inter-particle region at the surface. Furthermore, pressure affected the thermal stability of the amorphous powders during consolidation, influenced the amount of crystalline phases in the amorphous matrix, and the subsequent crystallization of the retained glassy phase. With increasing pressure, the primary crystallization of α-Al was hindered whereas crystallization of the intermetallic phases was promoted. This indicated that amorphous powders sintered at lower pressures allowed one to obtain higher volume fraction of α-Al and suppress the formation of brittle intermetallic phases. This study provides new and deep insights into pressure-assisted sintering of amorphous alloy powders and allows an innovative way of designing novel and advanced alloys with superior properties.

In this work, we investigated the effect of boron addition on glass-forming ability and mechanical properties of Co-Cr-Mo-C alloys. The starting alloy was (Co0.65Cr0.13Mo0.22)80C20 derived from the Co65Cr13Mo22 ternary eutectic composition. This alloy is almost fully crystalline and exhibits brittle mechanical properties. Replacing carbon with boron allows obtaining bulk metallic glasses (BMGs) and bulk metallic glass composites. The designed alloys show very high strength (∼4100 MPa), wide super-cooled liquid region (∼100 K) and large endothermic enthalpy of the supercooled liquid region (∼35 J/g), indicating high thermal stability. The electronegativity difference and supercooled liquid region of the present alloys are comparable to rare-earth elements containing BMGs. The physical properties of the present alloys corresponded well with the alloy design strategy based on the unified parameter using the glass transition temperature, fracture strength, and molar volume. Furthermore, we propose the optimum compositional condition for glass formation by controlling the carbon-to-boron ratio and the influence of carbon on phase formation in this alloy system is discussed.

B2 CuZr exhibits a stress-induced martensitic transformation from a B2 to a B19 structure during tensile deformation, and that is believed to be the reason for the pronounced ductility and work hardening of CuZr-based bulk metallic glass (BMG) composites. In order to gain a better insight into the structural transformation of CuZr precipitates, the phase stabilities as well as the anisotropic elastic and thermodynamic properties of both B2 cubic (CsCl-type) CuZr and B19 (β-AuCd-type) CuZr structures under hydrostatic pressures up to 30 GPa are investigated by first principles calculations. Moreover, the effects of the hybridization between the electronic orbitals of the constituent atoms on the variation of the elastic properties of the B2 CuZr structure are discussed. The results show that the Young's modulus (E), bulk modulus (B) and shear modulus (G) increase significantly with increasing pressure. Noticeably, for pressures up to 30 GPa, the B2 CuZr structure shows a stronger anisotropy along the (1 1 0) plane than for the (1 0 0) plane. Under high pressure, the stability of both B2 CuZr and B19 CuZr phases decrease while the Helmholtz free energy (F) and the formation enthalpy (H) of B2 CuZr increase monotonically. However, the different stability decreasing trajectories of both B2 and B19 CuZr phases result in a high propensity of martensitic transformation from the B2 to B19 structure. Our results may have implications for better understanding the phase stability of B2 and B19 CuZr structures under high pressure and can shed light on the structure-property relationships of BMG composites reinforced with shape-memory crystals.

Positron annihilation spectroscopy and differential scanning calorimetry were used to evaluate the changes of the atomic configurations in Zr-based metallic glasses (MGs) due to alloying and plastic deformation. The correlation between the atomic configurations of MGs and the amorphous-to-icosahedral phase transition due to heating was investigated. The results indicate that the free volume frozen in the as-cast Zr60Al15Ni25, Zr65Al7.5Ni10Cu17.5, and Zr65Al7.5Ni10Cu17.5Ag5 MGs substantially decreases in sequence. More excess free volume is introduced in Zr65Al7.5Ni10Cu17.5Ag5 MG due to cold rolling and milling. The annihilation of free volume due to alloying considerably stabilizes the icosahedral structure of MGs, which enhances the nucleation and growth of quasicrystals upon heating. However, the nucleation and growth of quasicrystals are considerably suppressed in Zr65Al7.5Ni10Cu17.5Ag5 MG due to cold rolling and milling, during which the more introduced excess free volume results in substantial destruction of short-range order with 5-fold symmetry. The present work further provides direct evidence for the prevalence of icosahedral short-range order in MGs.

In the present work, the influence of a minor Hf addition on the atomic structure, crystallization behavior, thermal stability and magnetic properties of a Co-based metallic glass was studied. Thermal analysis indicates that the thermal stability of new glassy ribbons microalloyed with 2.5 at.% Hf is notably enhanced through increasing the incubation time prior to devitrification and enlarging the width of the supercooled liquid region from 72 K to 96 K. Magnetic studies reveal that the new glass exhibits an excellent soft magnetic behavior, i.e., a very low coercivity of 0.26 A/m in the relaxed state, and a comparable saturation magnetization as the Hf-free ribbon. Structural relaxation of the Hf-containing alloy upon isothermal annealing below the glass transition temperature, Tg, was investigated by differential scanning calorimetry (DSC) and high-energy synchrotron X-ray diffraction (XRD). The evolution of the recovered enthalpy with annealing time can be expressed by the Kohlrausch-Williams-Watts (KWW) exponential function with a Kohlrausch exponent of 0.88, indicating a broad spectrum of relaxation times. Analysis of the reduced pair correlation functions, G(r), reveals volume shrinkage upon annealing caused by elimination of liquid-like sites including free-volume and anti-free-volume according to the shift in the positions of the G(r) maxima in the medium-range scale. The influence of structural relaxation on the variation of the Curie temperature and the coercivity of the new Hf-microalloyed glassy ribbon is discussed. A faster evolution of the Curie temperature with annealing time compared to the coercivity indicates a preferential dependence of the former on the chemical short-range order (SRO).

Ti-based metallic glass matrix composites (MGMCs) with a composition of Ti50Zr20V10Cu5Be15 (atomic percent, at. %) exhibit excellent tensile ductility and distinct work-hardening capability. A dislocation pile-up model (DPM) has been established to elaborate the dislocation motion near the yield point and to theoretically derive a linear Hall-Petch-like relationship between the yield strength, σ, and the inverse square root of the diameter of dendrite arms, d−1/2. The materials constant, k, in the present Hall-Petch-like relationship can be calculated on the basis of the pile-up model, and is very close to the experimental value. The hardness variation in the dendrites and the strengthening effect from unloading-reloading tests prove the reasonability of the DPM during tension. The Hall-Petch-like relationship is verified for a variety of MGMCs, whose plastic deformation is only dominated by dislocation motion. Mean-field theory (MFT) has been first utilized to build a relationship between the critical diameter of dendrites, dc, and the composition in MGMCs with the similar atomic percentages of low solubility elements. By tuning the composition, one can universally scale the Hall-Petch-like relationship, and accurately predict the yield strength of such in-situ MGMCs.

As an efficient potential hydrogen storage and conversion system, hydrogen electrosorption and evolution mechanisms in Pd-based metallic glass thin films (MGTFs) are investigated. In this study, th...

The influence of cryothermal treatment on the mechanical properties of metallic glasses with different compositions was investigated in the present work. It was found that cryothermal cycling can induce rejuvenation as well as relaxation of the metallic glasses. The local apparent Young’s modulus and its spatial distribution width on the surface of the metallic glass increase after cryothermal cycling, while in the bulk the effect depends on the glass composition. It appeared that this increase is temporary and disappears after a period of room temperature aging. This effect is connected with a large distribution of relaxation times in the metallic glasses due to their heterogeneous structure and the formation of complex native oxides on the outer surfaces of the glasses. Our findings reveal that a cryothermal cycling treatment can improve or degrade the plasticity of a metallic glass, and the atomic bond structure appears to be very important for the outcome of the treatment. Cryothermal cycling affects the structure and mechanical properties of metallic glasses in ways that depend on their composition. These metallic alloys, with their disordered atomic structures, have promising characteristics but before widespread applications are possible their behavior needs to be better understood. An international team of researchers led by Sergey Ketov from the Erich Schmid Institute of Materials Science, Leoben, Austria, have now subjected three such glasses with the general formula Zr60Cu20M10Al10 (where M is iron, cobalt or nickel) to temperature cycles between 77 and 320 K. The treatment was found to temporarily increase their surface stiffness, to a greater extent for the iron-based sample. It also affected their bulk plasticity: an increase was noticed for the iron-based glass, “rejuvenating” the material, whereas the cobalt-based glass was degraded and the nickel-based glass remained largely unaffected. Cryothermal cycling can induce rejuvenation as well as relaxation of metallic glasses. The surface apparent Young’s modulus and its spatial distribution width increase after the treatment, while in bulk effect depends on the glass composition. This increase is temporary and disappears after some time of room temperature aging. Effect is connected with a large distribution of relaxation times in metallic glasses due to their heterogeneous structure and the formation of complex native oxide on the glass surface. Cryothermal cycling can improve or degrade the plasticity of metallic glasses and the atomic bond structure determines the outcome of the treatment.

The multiplication and interaction of self-organised shear bands often transform to a stick-slip behaviour of a major shear band along the primary shear plane, and ultimately the major shear band b...

Transformation-induced plasticity in CuZr-based bulk metallic glass (BMG) composites opens up a new perspective for the design of ductile BMG composites. In this work, rapid and partial crystallization of BMGs is proposed to tailor microstructures of ductile BMG composites. During the early crystallization stage, the precipitation of B2 CuZr crystals in the Cu42Zr42Al10Co6 BMGs is growth-controlled at low or high heating rates after having passed through a nucleation-dominated temperature region. Abundant nucleation is beneficial for the uniform precipitation of B2 crystals in the glassy matrix. Compared with annealing processes at slow heating, the degree of the free volume annihilation could be reduced to some content by employing a high heating rate. The microstructures of CuZr-based BMG composites can be effectively tuned by changing rapid annealing parameters, which enhances the ductility of the resulting BMG composites. This approach proves to be efficient for the development of ductile metastable alloys and composites. Keywords: Metallic glasses, Crystallization, Annealing, Composites, Deformation

Vitreous alloys exhibit changes in the specific heat capacity (Δcp) as well as configuration at the characteristic glass transition temperature (Tg). A wide variety of bulk metallic glasses (BMGs) such as, Zr47.5Cu47.5Al5, Zr55Cu30Ni5Al10, Zr52.5Cu17.9Ni14.6Al10Ti5 and Zr41.2Ti13.8Cu12.5Ni10Be22.5 were rolled at room temperature, and the variation of Tg, Δcp at Tg, configurational entropy (ΔSconf.), pressure coefficient of Tg (dTg/dP) and bending strength were measured to understand the configurational change in BMGs upon cold rolling. The normalized bending moment during yielding of a cold rolled Zr55Cu30Ni5Al10 has increased by 43.4% than the as-cast specimen. The ΔSconf. of the Zr55Cu30Ni5Al10 plate increased upon rolling from 0.537±0.041 J/mol∙K for as-cast to 0.654±0.032 J/mol∙K for 70% cold rolled sample. A relaxed glassy state with low Tg exhibits high dTg/dP value and vice versa.

Bulk metallic glasses (BMGs) have attracted wide interest, but their successful application is hindered by their low ductility at room temperature. Therefore, the use of composites of a BMG matrix with crystalline secondary phases has been proposed to overcome this drawback. In the present work we demonstrate the fabrication of a tailored BMG nanocomposite containing a high density of monodisperse nanocrystals with a size of around 20 nm using a combination of mechanical and thermal treatment of Cu36Zr48Al8Ag8 well below the crystallization temperature. Direct observations of the interaction of the nanocrystals with a shear band during in situ deformation in a transmission electron microscope demonstrate that the achieved nanocomposite has the potential to inhibit catastrophic fracture in tension. This demonstrates that a sufficient number of nanoscale structural heterogeneities can be a route towards BMG composites with superior mechanical properties.

Short term sub-glass transition annealing controls the structural reordering and nanocrystallization of CuZr − based bulk metallic glasses (BMGs). Using this method, 5 times higher resistance against strain softening as confirmed by three-point bending tests is attained. Homogenously dispersed nanoparticles in sizes of 20–50 nm accounts for the remarkable shifts and peak formations on reciprocal space together with the diffraction spots observed on 2D diffraction pattern. Real-space pair distribution function analysis reveals noticeable changes in peak shapes and positions correlated with the changes in short- to medium-range ordering. The differences in the partial nearest neighbor numbers upon annealing mark the dominant Zr − Zr pair to diminish whereas the Cu − Cu pair becomes prominent, hinting that Cu diffusion is mainly responsible for structural reordering and formation of new phases. The pursued study using hard X-ray synchrotron radiation reveals important aspects of structural changes preceding nanocrystallization, which in turn enhances the mechanical performance in BMGs.

Melt-spun amorphous Fe80Nb10B10 ribbons were subjected to ball milling for different periods. The results show that ball milling not only induces the formation of nano-crystalline α-Fe during milling, but also drastically changes the crystallization behavior of the residual amorphous phase. In contrast to the as-spun ribbon, which crystallizes during heating through the formation of the metastable Nb5Fe33B10 phase and subsequent decomposition into stable α-Fe and a small fraction of Fe2B and NbFeB, ball milling progressively suppresses the formation of Nb5Fe33B10 as primary crystallization product and promotes the formation of α-Fe at low temperatures. The ball-milled nano-crystalline/amorphous powders have a saturation magnetization 2.5–3 times higher than the as-quenched amorphous ribbon along with Curie temperature increase by about 70 K with respect to the parent as-spun material. The structure and related magnetic properties of the milled material do not significantly change in the bulk samples synthesized by powder consolidation at temperatures up to ∼900 K, which indicates that the stable nature of α-Fe makes this nano-crystalline/amorphous composite material potentially suitable for a wide range of high temperature applications.

Here we provide a systematic study of tensile deformation mechanisms in PdSi metallic glasses employing polymer-supported sputter deposited films. Due to the polymer substrate, global strain localization within one critical shear band is inhibited and thus thousands of different shear bands and cracks can be examined within a single tensile sample. Two distinctly different types of shear bands, allowed by the sample geometry, are observed and analyzed. It is shown that crack formation occurs through activation of multiple out-of-plane shear bands leading to local plasticity and neck-like shape of crack edges. The in-plane shear bands, which appear at higher applied strains, typically do not lead to crack formation and can accommodate both applied tensile strain and transverse compressive strain. The mechanism of effective strain hardening through de-activation of in-plane shear bands is demonstrated by means of quasi-in-situ studies. Shear banding is shown to be the fundamental deformation mechanism for different film thicknesses ranging from 25 to 1000 nm. Polymer-supported films represent a unique model material system which enables easy characterization of the tensile behavior of metallic glasses and provides an opportunity to gain new insights into the fundamental mechanisms of shear banding.

This study focuses on the production and characterization of ferromagnetic Fe80P13C7 alloys with cobalt and/or nickel additions. Starting from the base composition of Fe80P13C7, an alloy series of Fe(80−x-y)NixCoyP13C7 (x,y = 0, 5, 10, 15, 20 at%; x + y = 20 or 0) has been synthesized using the melt spinning technique. For a fixed amount of metalloids (P13C7), three novel alloys, namely, Fe60Ni15Co5P13C7, Fe60Ni10Co10P13C7 and Fe60Co15Ni5P13C7 have been synthesized and characterized for the first time. The Fe80P13C7, Fe60Ni20P13C7 and Fe60Co15Ni5P13C7 alloys were also subjected to short-term fluxing treatment. Nine types of samples from six different alloy compositions, including three fluxed alloys, were comparatively investigated for their glass-forming abilities, compositional deviations, as well as thermal and magnetic properties.

A novel method for measurement of atomic interdiffusivity is applied to amorphous Cu-Zr close to its glass-transition temperature Tg. Sputter-deposited multilayers are examined in cross-section by transmission electron microscopy and energy-dispersive X-ray spectroscopy. Mapping the evolution of composition profiles gives the interdiffusivity, which is orders of magnitude higher than if coupled to the viscosity expected near Tg. Kirkendall drift of interlayer interfaces in both amorphous and supercooled liquid states (i.e. below and above Tg), and associated voiding in the liquid, show that the diffusivity of copper greatly exceeds that of zirconium. Amorphous Cu-Zr is known to show maxima in atomic packing density at sharply defined compositions. The comparison of the two compositions in the present work provides the first direct evidence that denser packing is associated with lower atomic interdiffusivity. The lower interdiffusivity is governed by a lower diffusivity of copper, and reflects a lessened degree of decoupling of the copper (fast) and zirconium (slow) diffusivities in an efficiently packed glass. The new insights help to understand issues ranging from glass-forming ability to the controlled generation of nanovoided structures.

In this paper, the tensile creep behavior of Cu64Zr36 metallic glass (MG) is investigated by molecular dynamics simulations. The effects of stress and temperature on the evolution of activated states during nanoscale creep tests are discussed, the creep failure time is predicted by linear regression analysis and three creep mechanisms are found. The creep mechanisms at the low stress and high temperature regime is dominated by thermally activated atomic diffusional creep; the low temperature and low stress regime is dominated by anelastic deformation and homogeneous activation of elastic events, while the low temperature and high stress regime is governed by local inhomogeneous shear deformation through shear transformation zones (STZs) percolation and embryonic shear band formation. These mechanisms help to further understand the creep behavior and derive an atomistic description of the relationship between the creep behavior and the three mechanisms of mechanical relaxations in MGs: atomic diffusion via cooperative atomic motion or α relaxation, stress-induced STZs activation such as slow β relaxation and anelastic deformation (fast β’ relaxation), respectively.

The temperature-dependent atomic structural evolution of a Zr 64.13 Cu 15.75 Ni 10.12 Al 10 metallic glass is studied by in-situ high-energy X-ray diffraction in the temperature range of 79–553 K. The interatomic distance in the first nearest neighbor shell increases with decreasing temperature, while the interatomic distance in other atomic shells decreases. This decrease in the interatomic distances causes volume shrinkage and an increase in the strength of the metallic glass. The volume shrinkage can be quantitatively described by the Varshni function. The structural information is helpful for deeply understanding of the origin of the rejuvenation of metallic glasses at the cryogenic temperatures.

Bulk metallic glass composites (BMGCs) containing an in-situ formed metastable β phase (β-type BMGCs) being capable of transforming to martensites, are a promising class of alloys. Nonetheless, the influence of the structural metastability of β phases on the deformation mechanisms of BMGCs remains largely unclear. This, however, is crucial for understanding and optimizing plastic deformability. In this study, three Ti-based β-type BMGCs with nominal compositions of (Ti0.505Zr0.353Fe0.028Be0.114)100−xCux (x = 1, 5, 8) have been developed, in each of which the β-Ti phase has a different metastability. For x = 8, the β phase precipitates exclusively and does not transform martensitically during deformation. At the lowest Cu content (x = 1), however, the β phase contains a high density of fine ω phase upon quenching, which hinders the deformation-induced martensitic transformation. β phases in both BMGCs deform plastically by dislocation glide. Only when x = 5 does the β phase undergo the martensitic transformation to α″-Ti due to an appropriate degree of metastability. The reversible phase transformation between β and α″ followed by twinning in α″ on continuing deformation leads to a “double yielding” behavior and results in strong work-hardening capability as well as superelasticity during compression (cyclic loading) tests.