Al Bulk Metallic Glasses Research Articles

Composition design and crystallization behavior of Zr–Cu–Ni–Al bulk metallic glasses

The Zr–Cu–Ni–Al system was explored in this study, and seven new compositions of Zr-based bulk metallic glasses (BMGs) were designed by combining different proportions of binary eutectic phases Zr38Cu62, Zr76Ni24 and Zr51Al49, and rod-like shape samples were prepared with different diameters using the copper mold casting method. Modifying the proportion coefficient of these eutectic units, the width of the supercooled liquid region was increased from 79 to 95 K, and the thermal stability of the prepared metallic glass was improved. Differential scanning calorimetry was used to investigate the non-isothermal and isothermal crystallization behavior of the prepared samples. Noticeably, there was a small low-temperature exothermic peak before the glass transition temperature Tg in the non-isothermal crystallization process, which was related to the structural relaxation. The crystallization transition activation energy fitted by the Kissinger equation had the same variation trend as that obtained by the Arrhenius equation. Among the four metallic glasses studied, the glassy phase of Zr55·5Cu23·25Ni9Al12.25 was more stable, showing higher glass transition activation energy and longer incubation time. In addition, the isothermal crystallization processes of four metallic glasses were studied according to the Johnson-Mehl-Avrami (JMA) model. The calculated average Avrami exponents were >2.5, indicating that the nucleation rate increased during the isothermal crystallization process.

Improving fatigue property of Zr–Ni–Al metallic glass by tailoring chemical composition with high structure stability

Improving the tolerance to fatigue damage for bulk metallic glass (BMG) is of vital importance for its industrial applications during long-term mechanical service. In this work, the composition dependence of structure stability against thermal and mechanical stimuli are studied with Zr–Ni–Al BMGs from the perspective of atomic topological packing and chemical affinity. Large atomic size disparity and strong interactions between component elements facilitate the stability of glass structure, which impart BMGs with relatively higher glass transition temperature and yield strength. Fatigue failure of BMG initiates from the accumulation of inelastic shear transformation events on the sample surface under cyclic loading. The fatigue endurance limit of Zr–Ni–Al BMG is effectively improved by tailoring the chemical composition with increased structure stability, which is characterized by high mismatch entropy and mixing enthalpy. Aluminum plays a role in enhancing the fatigue tolerance of Zr–Ni–Al BMGs, which presumably arises from the covalent-like bond character of aluminum that promotes the formation of rigid atomic packing motifs with great resistance to shear rearrangement, and consequently mitigates the fatigue crack initiation process.

Tailoring the glass forming ability, mechanical properties and corrosion resistance of Cu–Zr–Al bulk metallic glasses by yttrium addition

The effect of yttrium (Y) on the glass-forming ability (GFA), mechanical properties and corrosion of (Cu50Zr43Al7)100-xYx (x = 0, 2 and 4, in at. %) alloys was investigated. The GFA of Cu50Zr43Al7 alloy was improved by the addition of Y, with a GFA of 15 mm in diameter being obtained for (Cu50Zr43Al7)98Y2. In uniaxial compression, the bulk metallic glasses (BMGs) exhibited a limited plastic strain of less than 1%, however compressive fracture strength and Young's modulus of 1787–2006 MPa and 101–122 GPa were achieved, respectively. Fracture surfaces and shear bands were studied via scanning electron microscopy. The corrosion behavior of BMGs was analyzed by potentiodynamic polarization experiments and electrochemical impedance spectroscopy (EIS) in acidic chloride (1 M HCl) and neutral chloride (0.6 M NaCl) environments. As a result of the Y addition, the corrosion resistance of the base BMG was improved.

Primary and secondary phase separation in Cu–Zr–Al bulk metallic glass by control of quenching conditions

Here, we report primary and secondary nanoscale phase separation in Cu46Zr46Al8 (at.%) bulk metallic glass (BMG) achieved by altering the casting temperature. The BMG rods were cast by using the melting currents of 160 A and 220 A which provide, respectively, higher and lower cooling rates. The transmission electron microscopy and energy-dispersive X-ray analysis revealed that the higher cooling rate conditions brings a primary phase separation of nanoscale Cu-rich amorphous regions which are evenly dispersed in the Cu-depleted amorphous matrix phase. However, lower cooling rates results in a bimodal microstructure comprising coarsened primary-separated nanograins and very fine secondary-separated nanograins throughout the amorphous matrix containing nanocrystals of smaller than 5 nm in size. This unique microstructure developed under lower cooling rates gives lower relaxation enthalpy, higher short-range ordering, and enhanced strain-hardening. Our study emphasizes the significant role of casting temperature on controlling the morphology of nanoscale phase separation in BMGs.

Comprehensive thermal analysis of a high stability Cu–Zr–Al bulk metallic glass subjected to high-pressure torsion

Bulk metallic glass of Cu38Zr54Al8 nominal composition was synthesized by copper mold casting into 6 mm diameter rods. Disks of the as-cast glass were subjected to severe plastic deformation by high-pressure torsion for different number of revolutions. The microstructure and the thermal behavior of the as-cast, isothermally annealed and deformed glass have been investigated by X-ray diffraction and differential scanning calorimetry, respectively. Continuous heating experiments revealed a two-stage devitrification event with excellent glass forming parameters, such as glass transition (Tg = 671 K), supercooled liquid region (∆Tx = 80 K), reduced glass transition (Tr = 0.57) and gamma parameter (γ = 0.41). Power law crystal growth during diffusion-controlled homogeneous nucleation was observed for isothermal annealings. Glassy state was preserved almost in the entire sample volume of the as-cast alloy during the high-pressure torsion process, corresponding to the extreme stability of the Cu38Zr54Al8 alloy against deformation-induced devitrification. This is in accordance with the transition of the reversible specific heat from the glassy to supercooled liquid state measured by modulated calorimetry. It was also concluded that glassy structure is more ordered in the severely deformed state.

The improvement of the plasticity of a Zr – Ni – Al bulk metallic glass by static quenching

A method of static quenching has been put forward to prepare the Zr-based bulk metallic glass (BMG) in this work. Compared with the traditional method of copper mold suction casting, the compressive plasticity of Zr60Ni25Al15 BMG can be improved from nearly 0 to 6.5% through the method of static quenching. An underlying reason of the improved plasticity has been clarified as that the cooling rate of static quenching is larger than that of copper mold suction casting, and there are more soft spots in the static quenching sample than suction casting. The corresponding mechanism has also been discussed.

Optimizing mechanical and electrical properties of Cu-coated Cu–Zr–Al bulk metallic glass composites by adjusting glassy powder size

In the rapidly developing modern industrial fields, the materials used are often required to have simultaneously high strength and high electrical conductivity. However, these two properties often conflict and cannot be met at the same time. In the present study, we developed the Cu/Cu–Zr–Al bulk metallic glass (BMG) composites which exhibited simultaneously high strength and high electrical conductivity, as well as large plasticity. The BMG composites were fabricated by using spark plasma sintering crystalline Cu-coated Cu–Zr–Al metallic glass composite powders. The effect of particle size of the Cu–Zr–Al metallic glass powders on mechanical properties and electrical properties of the BMG composites was investigated systematically. Microstructure of contact interfaces between crystalline Cu and Cu–Zr–Al metallic glass was also characterized and discussed. This study is expected to provide an effective technical approach for the development of new high strength and high conductivity materials.

Annealing-induced phase transitions in a Zr–Ti–Nb–Cu–Ni–Al bulk metallic glass matrix composite containing quasicrystalline precipitates

Abstract The phase formation of a copper-mold-cast Zr60Ti2Nb6Cu14 Ni9Al9 alloy has been investigated upon cooling from the melt as well as upon annealing of as-cast specimens. The different states of the samples are characterized by X-ray diffraction, optical and electron microscopy, and differential scanning calorimetry. The cooling rate as realized upon copper-mold casting leads to micrometer-sized quasicrystals, which are embedded in a glassy phase. The thermal stability ΔTx of the supercooled liquid state of the glassy phase that forms near to the wall of the copper mold, differs from that of the glassy matrix in the center of the rod due to different compositions of the glassy phase. This is a consequence of the change in local cooling conditions, which affects the phase formation upon solidification as well as the subsequent transformation behavior of the alloy upon constant-rate heating.

On the devitrification of Cu–Zr–Al alloys: Solving the apparent contradiction between polymorphic liquid-liquid transition and phase separation

In this work, the crystallization pathways and kinetics of the Cu47.5Zr45.1Al7.4 bulk metallic glass (BMG) are studied systematically using a broad collection of laboratory and synchrotron-based techniques, such as differential scanning calorimetry, transmission electron microscopy, in situ high-energy synchrotron X-ray diffraction (HEXRD), simultaneous small- and wide-angle X-ray scattering, and atom probe tomography. The crystallization sequence during heating at conventional slow rates and isothermal annealing near the glass transition temperature was determined to consist in preferential formation of (Cu, Al)10Zr7 followed by the formation of CuZr2 and AlCu2Zr induced by long-range atomic diffusion. In situ HEXRD shows that the formation of the latter two phases is accompanied by the partial dissolution of the primary Cu-rich phase. Microstructural characterizations of annealed samples suggest that a new liquid phase with enhanced local order and obscure compositional changes has formed from the homogeneous supercooled liquid before crystallization. A concept analogous to the disorder-to-order transition in crystalline materials is applied here to describe the observed transition in liquid, resolving the longstanding apparent contradiction between observing a nanoscale phase separation versus a liquid–liquid phase transition during devitrification. Furthermore, we suggest that the formation of an intermediate liquid phase prior to crystallization is the structural origin of the rapid devitrification kinetics during the primary crystallization. On the basis of our combined experimental results, a scheme is proposed to generalize the complex multistep devitrification mechanisms of the Cu47.5Zr45.1Al7.4 BMG during isothermal annealing or slow heating. This work may provide new insights into the crystallization behavior and thermal stability of Cu–Zr–Al BMGs in terms of thermodynamics and kinetics and might lead to deeper understanding of the underlying mechanisms of the often observed rapid crystallization processes of deeply undercooled metallic liquids.

Improvement the Plasticity of a Zr - Ni - Al Bulk Metallic Glass by Static Quenching

Improvement the Plasticity of a Zr - Ni - Al Bulk Metallic Glass by Static Quenching

Time-dependent deformation mechanism of metallic glass in different structural states at different temperatures

There is limited understanding of time-dependent plastic deformation behavior of amorphous alloys as a function of their structural state. Here, the creep behavior of Zr52.5Ti5Cu17.9Ni14.6Al10 bulk metallic glass was investigated in its as-cast and relaxed states using nanoindentation technique with applied load in the range of 500–1500 mN and temperature in the range of room temperature to 573 K. The creep displacement increased with increasing load and temperature since the creep process is thermally activated and diffusion rate is enhanced at elevated temperature and higher load. The creep strain rate sensitivity, which is a measure of the creep mechanism, increased with increase in temperature and decrease in applied load. This was attributed to the transition from localized to more homogeneous creep. Reduction in free volume for the relaxed alloy resulted in lower creep displacement and larger strain rate sensitivity compared to its as-cast counterpart. The results suggest that diffusion-based deformation dominate at higher temperature in contrast to shear transformation mediated plasticity at room temperature. The volume of shear transformation zone for the metallic glass was calculated using cooperative shearing model and correlated with the structural state for fundamental insights into the deformation process as a function of temperature and load.

Effect of Gd addition on non-isothermal and isothermal crystallisation of Cu–Zr–Al bulk metallic glass

Abstract The crystallisation kinetics of (Cu46Zr46Al8)100–xGdx (x = 0, 2 and 4 at.%) bulk metallic glasses in non-isothermal and isothermal conditions were studied by differential scanning calorimetry and X-ray diffraction. X-ray diffraction analysis shows that the crystallisation product Cu10Zr7 changes to Cu10Zr7 and Cu2Gd in the presence of Gd in non-isothermal and isothermal conditions. Crystallisation activation energy was calculated using the Kissinger and Ozawa methods in non-isothermal conditions and using the Arrhenius model in isothermal conditions. The results show that Gd addition triggers an increase in the energy barrier during crystallisation. The Johnson–Mehl–Avrami model was employed to analyse the crystallisation kinetics in the isothermal condition. The Avrami exponent, n, for Cu46Zr46Al8 is more than 2.5, which demonstrates that the crystallisation is mainly governed by diffusion-controlled three-dimensional growth with increasing nucleation rate. Comparably, n for (Cu46Zr46Al8)98Gd2 and (Cu46Zr46Al8)96Gd4 is 1.5 < n < 2.5, which suggests that the crystallisation is mainly determined by diffusion-controlled three-dimensional growth with decreasing nucleation rate.

Formation, microstructure and mechanical properties of ductile Zr-rich Zr–Cu–Al bulk metallic glass composites

We examined the microstructure, phase stability, mechanical properties and deformation behaviors of cast (Zr0.58Cu0.42)100-xAlx (x = 0, 3, 5, 7, 10) bulk metallic glass composites (BMGCs). With increasing Al content, the glass-forming ability of the new Zr-rich Zr–Cu–Al alloys gradually increases, enabling the fabrication of BMGCs for the alloys containing more than 3 at.% Al. The as-cast structure changes from Cu10Zr7 + CuZr2 for the Al-free base alloy to glass + crystal for the Al-added alloys. The new Zr-rich Zr–Cu–Al BMGCs exhibit a large fracture strain of ∼3.4–7.8% and a high fracture strength of ∼1731–1984 MPa under compression. The compressive fracture strain of Zr-rich Zr–Cu–Al alloys can be explained by the percolation theory. The (Zr0.58Cu0.42)95Al5 composite containing ∼70 vol.% crystalline phase possesses the largest plastic strain of ∼6%, and fracture strength of over 1900 MPa under compressive condition. The superior plastic deformation capability under compression is related to the following factors: (1) The formation of three types of shear bands with distinct morphological characteristics, (2) the plastic deformation of B2–CuZr phase itself, together with stress-induced martensitic transformation from B2–CuZr phase to B19’ phase, and (3) the interaction between crystals and shear bands. The present results have implications for better understanding the deformation mechanisms of the Zr-rich Zr–Cu–Al BMGCs and for designing high-performance BMGCs with enhanced plasticity.

Enhanced heterogeneity and plasticity in a Zr - Cu – Al bulk metallic glass with micro-addition of oxygen

In this work, the effect of the addition of oxygen on the plasticity of a Zr60Cu25Al15 bulk metallic glass (BMG) was discussed. It has been shown that the plasticity of Zr60Cu25Al15 BMG can be enhanced through the addition of 0.25% of O. The glass forming ability and plasticity will deteriorate with an addition of 0.5% and 0.75% of O. The underlying mechanism of the improved plasticity has been clarified as a heterogeneous Cu-rich amorphous phase and the decrease of plasticity is caused by partial crystallization when there is an excess addition of O.

In Situ Scattering Studies of Crystallization Kinetics in a Phase-Separated Zr–Cu–Fe–Al Bulk Metallic Glass

Thermal stability and the crystallization kinetics of a phase-separated Zr–Cu–Fe–Al bulk metallic glass were investigated using in situ high-energy synchrotron X-ray and neutron diffraction, as well as small-angle synchrotron X-ray scattering. It was revealed that this glass with excellent glass-forming ability possesses a two-step crystallization behavior. The crystalline products and their evolution sequence are more complicated than a homogeneous Zr–Cu–Al glass with average glass-forming ability. The experimental results indicate that a finely distributed nanometer-sized cubic Zr2Cu phase forms first and then transforms to a tetragonal Zr2Cu phase, while the matrix transforms to an orthorhombic Zr3Fe phase. The strength of the Zr–Cu–Fe–Al composite containing cubic Zr2Cu phase and glass matrix increases, and the plasticity also improves compared to the as-cast Zr–Cu–Fe–Al bulk metallic glass. Our results suggest that the formation of multiple and complex crystalline products would be the characteristics of the Zr–Cu–Fe–Al glass with better glass-forming ability. Our study may shed light on the synthesis of bulk-sized glass–nanocrystals composites of high strength and good plasticity.

Cryo-Casting for Controlled Decomposition of Cu–Zr–Al Bulk Metallic Glass into Nanomaterials: Implications for Design Optimization

Cryo-Casting for Controlled Decomposition of Cu–Zr–Al Bulk Metallic Glass into Nanomaterials: Implications for Design Optimization

Crystallization kinetics of Cu–Zr–Al bulk metallic glass with Nd addition

AbstractThe crystallization kinetics of (Cu48Zr48Al4)100−xNdx (x = 0, 2 at.%) bulk metallic glasses in nonisothermal and isothermal conditions were studied by differential scanning calorimetry and X‐ray diffraction (XRD). XRD analysis shows that the crystallization product Cu10Zr7 changes to Cu10Zr7 and Cu2Nd with Nd presence in nonisothermal and isothermal conditions. Crystallization activation energy was calculated in nonisothermal conditions using Kissinger and Ozawa methods and in isothermal conditions using Arrhenius model. The results show that the activation energy harbored by (Cu48Zr48Al4)98Nd2 expresses larger value compared with Cu48Zr48Al4, which indicate that the energy barrier for crystallization has enhanced along the Nd addition. The Johnson–Mehl–Avrami model was employed for analyzing the crystallization kinetics in isothermal condition. The Avrami exponent n for Cu48Zr48Al4 is more than 2.5, demonstrating that the crystallization is mainly governed by diffusion‐controlled three‐dimensional growth with increasing nucleation rate. Comparably, the Avrami exponent n for (Cu48Zr48Al4)98Nd2 is 1.5 < n < 2.5, suggesting that the crystallization is mainly determined by diffusion‐controlled three‐dimensional growth with decreasing nucleation rate. The evident enhancement in crystallization activation energy based on Nd addition is rationally made up of an influential factor commonly harbored on the enhanced glass forming ability for the rare‐earth comprising Cu–Zr–Al metallic glasses.

Mechanical and tribological properties of Zr–Cu–Ni–Al bulk metallic glasses with dual-phase structure

The effect of Fe addition on the structure, mechanical and wear properties of Zr–Cu–Ni–Al bulk metallic glasses (BMGs) was systematically investigated by thermal analysis, transmission electron microscopy (TEM), the compression test, the pin-on-disk macroscopic wear test and the microscopic nanoscratch test. TEM observation revealed the formation of a second glassy phase with a few nanometers in diameter in the Zr63.6-xCu18Ni10.4Al8Fex (x = 1, 3, 5) BMGs. The results of the mechanical tests showed that although compression strength and hardness do not change much, a remarkable enhancement in plasticity occurs in the Zr58.6Cu18Ni10.4Al8Fe5 BMG, which is presumably due to the restriction of the nano-scale second phase on shear banding. Phase separation was also found to promote multiple shear bands, enhanced toughness and oxidative wear, which work synergistically during the tribological test to improve the wear resistance of Zr–Cu–Ni–Al BMGs. These findings provide some insights into the wear mechanisms of dual-phase BMGs, which may offer new strategies for design and optimization of BMGs with desired wear performance.

Negative Strain Rate Sensitivity Induced by Structure Heterogeneity in Zr64.13Cu15.75Ni10.12Al10 Bulk Metallic Glass

The negative strain rate sensitivity (SRS) of metallic glasses is frequently observed. However, the physical essence involved is still not well understood. In the present work, small-angle X-ray scattering (SAXS) and high-resolution transmission electron microscopy (HRTEM) reveal the strong structure heterogeneity at nanometer and tens of nanometer scales, respectively, in bulk metallic glass (BMG) Zr64.13Cu15.75Ni10.12Al10 subjected to fully confined compression processing. A transition of SRS of stress, from 0.012 in the as-cast specimen to −0.005 in compression processed specimen, was observed through nanoindentation. A qualitative formulation clarifies the critical role of internal stress induced by structural heterogeneity in this transition. It reveals the physical origin of this negative SRS frequently reported in structurally heterogeneous BMG alloys and its composites.

Rejuvenation of a naturally aged bulk metallic glass by elastostatic loading

Abstract Rejuvenation of a naturally aged Zr64.13Cu15.75Ni10.12Al10 bulk metallic glass (BMG) can be achieved by laterally elastostatic pre-loading. The serration flow of the rejuvenated BMG shows a bimodal fashion, in which smaller serrations and larger serrations emerge alternately. Both coarse and a large number of fine shear bands, with strong interactions among themselves, coexist in the rejuvenated BMG. More pronounced β relaxation of the rejuvenated BMG indicates that the bimodal patterns of serration flows and shear bands can be attributed to elastostatically induced a more heterogeneous structure.