Hydrogenation of Zr–Cu–Al–Ni–Pd metallic glasses by electrochemical means

Amorphous materials of Zr–Cu–Ni–Al systems have shown attractive electrochemical hydrogen absorption properties. A comparison between Zr₆₀Cu₁₅Al₁₀Ni₁₀Pd₅ and Zr₆₅Cu_17.5Al_7.5Ni₁₀ reveals that the palladium (Pd) increases the hydrogen absorption capacity. Charging melt-spun Zr₆₀Cu₁₅Al₁₀Ni₁₀Pd₅ ribbons electrochemically to different hydrogen-to-metal (H/M) ratios and following the effusion of hydrogen by thermal desorption analysis (TDA) reveals hydrogen desorption from interstitial sites of high energy levels at temperatures below 630 K. Zirconium hydrides are formed above 630 K. At higher temperatures partial desorption of hydrogen occurs. The thermal stability observed with differential scanning calorimetry (DSC) of the amorphous phase has been significantly deteriorated by hydrogen absorption. After hydrogenation, the crystallization behaviour shows suppression of the characteristic quasicrystalline phase and depends on the hydrogen content. Therefore, at low hydrogen concentrations H/M = 0.3, Cu and/or Cu-rich phases are primarily formed while at high hydrogen concentrations H/M ≥ 0.9 Zr-hydride phase(s) are mainly formed.     

Thermal stability and crystallisation of a Zr55Cu30Al10Ni5 bulk metallic glass studied by in situ ultrasonic echography

An original in situ ultrasonic echography technique was used to study the thermal stability and crystallisation of a Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glass between RT and 630 °C. Changes in Young's modulus with temperature were reported allowing to study the supercooled-liquid state and the crystallisation process. Investigations of viscoelastic properties gave information on the correlation factor (hierarchically correlated motion theory) and three distinct crystallisation stages were observed. Their kinetics were studied using Voigt's and Reuss' approximations for a two-phase material and comparisons with the Johnson–Mehl–Avrami–Kolmogorov theory allowed us to consider a mixed surface/internal nucleation for the first stage and a surface nucleation for the two last stages.     

The corrosion behaviour of Zr-based bulk metallic glasses in 0.5 M NaCl solution

The corrosion behaviour of eutectic Zr₅₀Cu₄₀Al₁₀ and hypoeutectic Zr₇₀Cu₆Al₈Ni₁₆ bulk metallic glasses (BMGs) was studied by electrochemical measurements, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Zr₅₀Cu₄₀Al₁₀ BMG was highly susceptible to pitting corrosion in naturally aerated 0.5 M NaCl solution at 30 °C. In contrast, Zr₇₀Cu₆Al₈Ni₁₆ BMG passivated spontaneously under the same condition. EDX results for Zr₅₀Cu₄₀Al₁₀ indicated that enrichment of Cu, Cl and O occurred in the pitted region, while for Zr₇₀Cu₆Al₈Ni₁₆ BMG, no significant difference was found in the surface composition from the specimens before and after immersion in the solution. XPS analysis including angle-resolved measurements for Zr₇₀Cu₆Al₈Ni₁₆ BMG revealed that zirconium cation (Zr⁴⁺) was highly concentrated in both air-formed and passive films. Furthermore, the concentration of Zr⁴⁺ ions after immersion for 24 h or more showed tendency to increase with decreasing take-off angle, indicating that the exterior part of the passive film consisted exclusively of zirconium oxyhydroxide. The high corrosion resistance of Zr₇₀Cu₆Al₈Ni₁₆ BMG was attributed to the formation of homogeneous and stable passive film enriched with zirconium.     

Peculiarities of ball-milling induced crystalline–amorphous transformation in Cu–Zr–Al–Ni–Ti alloys

An amorphization process in (Cu₄₉Zr_45−xAl_6+x)_100−y−zNi_yTi_z (x = 1, y, z = 0; 5; 10) induced by ball-milling is reported in the present work. The aim was investigation of the effect of Ni and Ti addition to Cu₄₉Zr₄₅Al₆ and Cu₄₉Zr₄₄Al₇ based alloys as well as type of initial phases on the amorphization processes. Also the milling time sufficient for obtaining fully amorphous state was determined. The entire milling process lasted 25 h. Drastic structural changes were observed in each alloy after first 5 h of milling. In most cases, after 15 h of milling the powders had fully amorphous structure according to XRD except for those ones, where TEM revealed a few nanosized crystalline particles in the amorphous matrix. In (Cu₄₉Zr₄₅Al₆)₈₀Ni₁₀Ti₁₀ alloy the amorphization process took place after 12 h of milling and the amorphous state was stable up to 25 h of milling. In the case of (Cu₄₉Zr₄₄Al₇)₈₀Ni₁₀Ti₁₀ alloy the powders have fully amorphous structure between 12 h and 15 h of milling.     

Zr-based bulk glassy alloy with improved resistance to stress corrosion cracking in sodium chloride solutions

In order to evaluate stress corrosion cracking (SCC) susceptibility of Zr-based bulk glassy alloys and develop the BGAs with low susceptibility to SCC, the SCC behaviour of Zr₅₀Cu₄₀Al₁₀, Zr₅₀Cu₃₀Al₁₀Ni₁₀ and hypoeutectic Zr₇₀Cu₆Al₈Ni₁₆ BGAs in various environments including sodium chloride solution has been investigated using a slow strain rate technique at an initial strain rate of 5 × 10⁻⁶ s⁻¹. It is found, for the first time, that the Zr₇₀Cu₆Al₈Ni₁₆ BGA has no susceptibility to SCC in a 0.5 M NaCl solution. On the other hand, Zr₅₀Cu₄₀Al₁₀ and Zr₅₀Cu₃₀Al₁₀Ni₁₀ BGAs are highly susceptible to SCC in the NaCl solution, although they are not susceptible to SCC in de-ionized water, phosphate buffer, 0.5 M Na₂SO₄ and 0.5 M NaNO₃ solutions. The possible cause of the high susceptibility to SCC in the NaCl solution for the Zr₅₀Cu₄₀Al₁₀ and Zr₅₀Cu₃₀Al₁₀Ni₁₀ BGAs is discussed.     

Conditions for quasicrystal formation from mechanically alloyed Zr-based glassy powders

The absence of quasicrystalline phase formation in mechanically alloyed Zr₅₇Ti₈Nb_2.5Cu_13.9Ni_11.1Al_7.5 glassy powders with respect to melt-spun ribbon having the same composition has been investigated by analyzing the influence of mechanical deformation, oxygen contamination and chemical composition on quasicrystal (QC) formation. The results suggest that oxygen affects the formation of QCs by selectively reacting with zirconium thus driving the chemical composition of the glassy phase out of the range suitable for QC formation. This hypothesis has been verified by appropriately adjusting the composition of the powders. The addition of an adequate amount of zirconium changes the crystallization behavior inducing the formation of the quasicrystalline phase as the first crystallization product. This implies that if a particular short-range order is necessary for the formation of QCs, it can be achieved also by solid-state processing and therefore the presence of a specific quenched-in short-range order is not a mandatory prerequisite for QC formation.     

Microstructure and electrochemical behavior of Ti-coatedZr55Al10Ni5Cu30 bulk metallic glass

In this study, pure Ti was coated on Zr₅₅Al₁₀Ni₅Cu₃₀ bulk metallic glass (BMG) using a physical vapour deposition (PVD) technique with magnetron sputtering. Microstructures of Ti coating, BMG substrate and interface were investigated by conventional and high-resolution transmission electron microscopy (TEM and HREM). The electrochemical behavior of Ti-coated Zr₅₅Al₁₀Ni₅Cu₃₀ BMG was studied by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) in Hanks' solution. Scanning electron microscopy (SEM) was used to characterize the surface morphology of the coating after electrochemical testing. HRTEM observation reveals that the sputtering Ti coating consists of α-Ti nano-scale particles with the size about 10 nm. The polarization curves revealed that the open-circuit potential shifted to a more positive potential and the passive current density was lower after Ti coating was applied in comparison with that of the monolithic Zr₅₅Al₁₀Ni₅Cu₃₀ BMG. Electrochemical impedance spectroscopy (EIS) measurements showed that the Bode plots of Ti-coated Zr₅₅Al₁₀Ni₅Cu₃₀ BMG presented one time constant for 1 h and 12 h immersion and two time constants after 24 h immersion. The good bonding condition between Ti coating and Zr₅₅Al₁₀Ni₅Cu₃₀ BMG substrate may be responsible for the high corrosion resistance of Ti-coated Zr₅₅Al₁₀Ni₅Cu₃₀ BMG.     

Calorimetric and structural analysis of the new phase Al33Ni16Zr51 produced by direct synthesis and mechanical alloying

A new phase has been synthesized in the ternary phase diagram Al–Ni–Zr: its nominal composition is Al₃₃Ni₁₆Zr₅₁. For the Al₃₃Ni₁₆Zr₅₁ compound obtained by mixing the three components in suitable proportions, a study has been carried out by direct synthesis (calorimetry) and mechanical alloying in our laboratory. With the first method we know directly the enthalpy of formation of this alloy. For the amorphous alloys prepared by mechanical alloying we can determine the crystallisation enthalpy with the differential scanning calorimetry (DSC) method. So it is possible to determine a fundamental piece of information: the amorphous alloy formation enthalpy.     

Intrinsic and extrinsic size effects in the deformation of metallic glass nanopillars

Nanosized pillars with diameters ranging from 90 to 600 nm of four amorphous alloys, Cu₄₇Ti₃₃Zr₁₁Ni₆Sn₂Si₁, Zr₅₀Ti_16.5Cu₁₅Ni_18.5, Zr_61.8Cu₁₈Ni_10.2Al₁₀ and Al₈₆Ni₉Y_5, were fabricated and tested in situ in a transmission electron microscope. The major consideration when varying the composition was the change in bulk modulus and Poisson’s ratio, which may affect the deformation mode and ductility of metallic glasses (MGs) at the nanoscale. Differences between the deformation behavior of tapered (1.5-3°) and taper-free systems were also investigated. The yield stress of all the MGs measured through the in situ experiments is found to be essentially size independent, irrespective of tapering. With increasing size, all the MGs examined show a ductile-to-brittle transition under compression; the transition point, however, depends on the chemical composition of the specific MG investigated. The lower the μ/B ratio, the larger the pillar diameter above which more brittle behavior occurs. Al₈₆Ni₉Y₅ taper-free MG showed a transition threshold to brittle behavior at the largest pillar diameter of 300 nm. A micromechanical model is presented to explain the various dependencies.     

Studying fatigue behavior and Poisson's ratio of bulk-metallic glasses

High-cycle fatigue (HCF) experiments were conducted on zirconium (Zr)-based bulk-metallic glasses (BMGs): Zr₅₀Cu₄₀Al₁₀, Zr₅₀Cu₃₀Al₁₀Ni₁₀, Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5, and Zr₅₀Cu₃₇Al₁₀Pd₃ (in atomic percent) in air. The fatigue-endurance limit of Zr₅₀Cu₃₇Al₁₀Pd₃ was significantly greater than those of Zr₅₀Cu₄₀Al₁₀, Zr₅₀Cu₃₀Al₁₀Ni₁₀, and Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5. The resonant ultrasound spectroscopy technique was employed to determine the Poisson's ratios, shear moduli, and bulk moduli of these BMGs. The ratio of the fatigue-endurance limit to the tensile strength increases with increasing Poisson's ratio. A possible relationship between the ratio of the fatigue-endurance limit to the tensile strength and the ratio of the shear modulus to the bulk modulus or Poisson's ratio will be discussed.     

Nanostructure controlling in Zr-based metallic glasses using icosahedral local structure

Glass-nano(quasi)crystal composite materials based on Zr–Al–Ni–Cu metallic glasses have been synthesized by controlling the nucleation and growth rates of the precipitated phase correlated with a unique icosahedral local structure. It is well known that the Zr₆₅Al_7.5Ni₁₀Cu_17.5 metallic glass has a high glass-forming ability (GFA), which enables us to produce the glassy sample with a bulky shape. Controlling a substitution of QC-forming elements such as noble metals with Cu and annealing condition, the nanoscale icosahedral quasicrystalline phase (I-phase) with various grain sizes and nucleus densities can be formed. Moreover, we have succeeded to control the nano-QC phase nucleation by changing the atmosphere pressure during casting, which results in the formation of new bulk metallic glasses (BMGs). These nanoscale structure and nucleation controlling techniques in BMGs bring a significant improvement of mechanical properties such as high strength and good ductility.     

Quasicrystals and nano-quasicrystals in annealed ZrAlNiCuAg metallic glasses

Quasicrystals and nano-quasicrystals are observed in annealed Zr₆₅Al_7.5Ni₁₀Cu_12.5Ag₅ and Zr₆₅Al_7.5Ni₁₀Cu_7.5Ag₁₀ metallic glasses. Through systematic transmission electron microscopy (TEM) analyses, selected area electron diffraction (SAED) and nano-beam electron diffraction (NBED) patterns corresponding to five-, three- and twofold rotational symmetry are obtained, demonstrating that the precipitated phases in the first stage devitrification of the alloys are icosahedral quasicrystalline phases. The discovery of the quasicrystals directly reflects the intrinsic relationship between the Zr-based bulk metallic glasses and the icosahedral structure.     

Thermal and mechanical properties of Cu60−xZr25Ti15Nix bulk metallic glasses with x=0, 1, 3, 5, 7, 9 and 11 at.%

Systematic investigation on thermal and mechanical properties of Cu_60?xZr₂₅Ti₁₅Ni_x bulk metallic glasses with x = 0, 1, 3, 5, 7, 9 and 11 at.% points out that monolithic Cu₅₃Zr₂₅Ti₁₅Ni₇ and Cu₅₁Zr₂₅Ti₁₅Ni₉ bulk metallic glasses containing optimum Ni content of 7 and 9 at.% are effective to enhance both thermal stability of amorphous structure up to 716 K and plastic strain of 2.4% at room temperature. This indicates that a selection of additional elements such as Ni by considering a mixing enthalpy to the constituent elements is very important to control the thermal stability and plasticity. Moreover, it is believed that the addition of minor Ni can be a trigger to form the chemical heterogeneity upon solidification. Such chemical heterogeneity formed by the selection of the minor elements has a strong influence to cause the oscillation of the shear stress by wavy propagation of the shear bands thus leading the improvement of macroscopic plasticity of the bulk metallic glasses.     

Change in primary phase from icosahedral quasicrystal to fcc Zr2Ni by mechanical disordering in Zr–Al–Ni–Cu–Pd glassy alloy

Change in the primary crystallization from a single icosahedral quasicrystalline phase into the fcc Zr₂Ni phase by mechanical disordering was investigated in a melt-spun Zr₆₅Al_7.5Ni₁₀Cu_12.5Pd₅ glassy alloy. The transition of the primary phase is attributed to the mechanical strain induced in the icosahedral local structure in the glassy state.     

Investigation of glass forming ability and crystallization kinetics of Zr63.5Al10.7Cu10.7Ni15.1 bulk metallic glass

The glass forming ability, thermal stability and non-isothermal crystallization kinetics of Zr_63.5Al_10.7Cu_10.7Ni_15.1 glass forming alloy were investigated. Its maximum glass forming dimension is up to 6 mm and its critical cooling rate is less than 40 K s⁻¹. It manifests two crystallization procedures and the second crystallization peak is more sensitive to heating rate than the first crystallization peak. The glass transition and crystallization both have remarkable kinetics effects. The ms fitted by Arrhenius and VF equations are consistent with each other. Small m value about 17 indicates better thermodynamic stability and GFA of Zr_63.5Al_10.7Cu_10.7Ni_15.1.     

Microstructural analysis of Zr55Cu30Al10Ni5 bulk metallic glasses by laser surface remelting and laser solid forming

The crystallization behavior of Zr₅₅Cu₃₀Al₁₀Ni₅ bulk amorphous alloy during laser solid forming (LSF) was analyzed. Since laser surface remelting (LSM) is a key process for the LSF, the crystallization behavior of as-cast Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glasses (BMGs) during LSM was also investigated. It was found that the amorphous state of the as-cast BMGs was maintained when they were repeatedly remelted four times in a single-trace LSM, and as for the LSF of Zr₅₅Cu₃₀Al₁₀Ni₅ bulk amorphous alloy, the crystallization primarily occurred in the HAZ between the adjacent traces and layers after the two layers were deposited. The as-deposited microstructure exhibited a series of phase evolutions from the molten pool to the HAZ as follows: the amorphous → NiZr₂–type nanocrystal + amorphous → NiZr₂–type equiaxed dendrite + amorphous → Cu₁₀Zr₇–type dendrite + NiZr₂–type nanocrystal. Among these microstructural patterns, the NiZr₂–type nanocrystals and equiaxed dendrites primarily formed from the rapid solidification of the remelted liquid in the laser processing process, and the Cu₁₀Zr₇–type dendrites in the HAZ primarily formed by the crystallization of pre-existed nuclei in the already-deposited amorphous substrate.     

Thermal stability and primary phase of Al–Ni(Cu)–La amorphous alloys

Thermal stability and primary phase of Al_85+xNi_9−xLa₆ (x = 0–6) and Al₈₅Ni_9−xCu_xLa₆ (x = 0–9) amorphous alloys were investigated by X-ray diffraction and differential scanning calorimeter. It is revealed that replacing Ni in the Al₈₅Ni₉La₆ alloy by Cu decreases the thermal stability and makes the primary phase change from intermetallic compounds to single fcc-Al as the Cu content reaches and exceeds 4 at.%. When the Ni and La contents are fixed, replacing Al by Cu increases the thermal stability but also promotes the precipitation of single fcc-Al as the primary phase.     

Formation and properties of the Zr75−xAlxNi10Cu10Ti5 bulk metallic glasses

Zr-based bulk metallic glasses (BMGs) exhibit interesting mechanical properties since they combine high fracture stress, elastic strain (up to 2%), significant fracture toughness and good corrosion resistance. Quaternary systems with general composition Zr–Ni–Cu–Ti show wide composition ranges in which BMG can be obtained. The addition of the another element to the quaternary alloys often increases the glass forming ability (GFA). The aim of this work was to study the influence of aluminium content on the GFA and on the mechanical properties of the Zr–Ni–Cu–Ti alloys. Multicomponent Zr_75−xAl_xNi₁₀Cu₁₀Ti₅ (x = 15, 20 at%) alloys were produced by melt spinning method obtaining ribbons, and by casting technique into a copper mould, manufacturing rod shape samples with maximum diameter of 2 mm. Supercooled liquid region depends on chemical composition and exceeds 45 °C. Vickers microhardness of studied alloys is comparable to the highest ones for other Zr-based BMG.     

Nanoindentation analysis of the mechanical behavior of Zr-based metallic glasses with Sn, Ta and W additions

Zr₅₇Cu₂₀Al₁₀Ti₈Ni₅ and modified composition by adding Sn, W or Ta are studied using standard mechanical test and nanoindentation. Addition of refractory elements with a Sn micro-addition increases clearly the Young's modulus and the hardness of basic BMG. However, Sn reduces plasticity. Moreover these experiments allowed, in confine plasticity conditions, estimating an apparent activation volume associated to a plastic deformation (≈150 Å³).