Effects of electrochemical hydrogenation of Zr-based alloys with high glass-forming ability

Zr–(Ti)–Cu–Al–Ni metallic glasses exhibit a high thermal stability corresponding to a wide undercooled liquid region. Depending on their composition, the formation of metastable intermediate phases, e.g. a quasicrystalline phase is possible. The combination of early and late transition metals makes these alloys very interesting regarding their interaction with hydrogen. Amorphous Zr₅₅Cu₃₀Al₁₀Ni₅, Zr₆₅Cu_17.5Al_7.5Ni₁₀ and Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ ribbons were prepared by melt spinning and their microstructure and thermal behaviour was checked by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The cathodic reactivity of alloy samples at different microstructural states and after pre-etching in 1 vol.-% HF was investigated in 0.1 M NaOH by applying potentiodynamic polarisation techniques. Galvanostatically hydrogenated samples were characterised by XRD, DSC, TEM and thermal desorption analysis (TDA). For amorphous Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ samples an increase in electrochemical surface capacity by two orders of magnitude is observed after pre-etching. Compared to the quasicrystalline and crystalline alloy, the hydrogen reduction takes place at significantly lower overpotentials. Zr-based alloys cathodically absorb hydrogen up to H/M=1.65 while keeping the amorphous structure. Already small amounts of hydrogen cause a significant decrease of the thermal stability and changes in the crystallisation sequence. The hydrogen desorption is a two-stage process: (T<623 K) hydrogen desorption from high interstitial-site energy levels and (T>623 K) zirconium hydride formation and subsequent transformation under hydrogen effusion. Hydrogen suppresses the oxygen-triggered formation of metastable phases upon heating and supports primary copper segregation. At very high H/M ratios, severe zirconium hydride formation causes the crystallisation of new compounds.     

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 thermal stability of new Zr–Cu-based bulk glassy alloys with unusual glass-forming ability

The simultaneous addition of Al and Ag to Zr–Cu binary alloys increased in the stabilization of supercooled liquid, the reduced glass transition temperature and γ value, leading to greatly enhance the glass-forming ability (GFA). The Zr–Cu–Ag–Al glassy alloy samples with diameters above 15 mm were obtained in the wide composition range of 42–50 at% Zr, 32–42 at% Cu, 5–10 at% Ag, and 5–12 at% Al. The best GFA was obtained for Zr₄₈Cu₃₆Ag₈Al₈ alloy, and the glassy samples with diameters up to 25 mm were fabricated by an injection copper mold casting. The Zr₄₈Cu₃₆Ag₈Al₈ glassy alloy exhibited high tensile and compressive fracture strength of over 1800 MPa.     

Heating rate dependence of Tg and Tx in Zr-based BMGs with characteristic structures

Heating rate dependence of glass transition and crystallization temperatures is applicable for evaluation of glass structural stability and also for discussion on the apparent activation energies for glass transition and crystallization. The glass-structure stabilities of the Zr-based BMGs (Zr₅₀Cu₄₀Al₁₀, Zr₇₀Cu₂₀Al₁₀ and Zr₇₀Ni₂₀Al₁₀) and the conventional amorphous alloys (Zr₇₀Cu₃₀ and Zr₇₀Ni₃₀) are assessed by the densely packed glass structure as well as the complicated crystallization process. By the reverse Monte Carlo (RMC) simulation with X-ray and neutron diffraction data, it is shown that the densely packed structure is built by icosahedron-like clusters. In Zr₆₅Al_7.5Ni₁₀Pd_17.5 and Zr₆₅Al_7.5Ni₁₀Cu_17.5, the effect of Pd atoms on the glass structure is also described.     

Effects of loading modes on the fatigue behavior of Zr-based bulk-metallic glasses

The fatigue behavior of a Zr-based bulk-metallic glass (BMG), Zr₅₀Cu₃₇Al₁₀Pd₃ (in atomic percent), was systematically investigated under uniaxial tension–tension, uniaxial compression–compression, and bending loading. The fatigue results show that the fatigue limits of Zr₅₀Cu₃₇Al₁₀Pd₃ under tension–tension and compression–compression loading are comparable. However, the fatigue lifetimes above the fatigue limit under compression–compression loading are longer than that under tension–tension loading. Similarly, Zr₅₀Cu₃₇Al₁₀Pd₃ demonstrates the comparable fatigue limits and lifetimes under three- and four-point bending loading. In addition, the fatigue-endurance limits of Zr₅₀Cu₃₇Al₁₀Pd₃ under tension–tension and compression–compression loading were found to be greater than those under three- and four-point bend loading. The influence of the loading mode on the fatigue behavior of Zr-based BMGs is clarified.     

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.     

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.     

Wetting behavior and interfacial characteristics of In–Sn alloy on CuZr-based bulk metallic glass

The wettabilities of In–Sn alloy on Cu₄₀Zr₄₄Al₈Ag₈ BMG substrate were investigated using the sessile-drop method at different temperature. The result shows that the equilibrium contact angle decreased with increasing temperature. The interfacial reaction of the active Sn atoms in molten In–Sn alloy and the active Zr atoms in Cu₄₀Zr₄₄Al₈Ag₈ BMG caused crystallization reaction. Ion beam sputtering profiling in combination with AES technique was employed to investigate the Sn diffusion in Cu₄₀Zr₄₄Al₈Ag₈ BMG. Between 473 K and 673 K, the diffusion coefficients vary from 0.7 × 10⁻¹⁶ m²/s to 12.9 × 10⁻¹⁶ m²/s. It is concluded that the interfacial reaction is favorable to the crystallization and then the crystallization also promotes Sn diffusion.     

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.     

Electron irradiation induced phase transformation in Zr66.7Cu33.3 and Zr66.7Pd33.3 metallic glass

Crystallization and phase selection in Zr_66.7Cu_33.3 and Zr_66.7Pd_33.3 metallic glass during thermal annealing and electron irradiation were examined. During thermal annealing an equilibrium C11_b–Zr₂Cu phase directly precipitated in the amorphous phase of Zr_66.7Cu_33.3 metallic glass while a thermal equilibrium C11_b–Zr₂Pd phase formed after icosahedral quasi-crystalline phase precipitation in Zr_66.7Pd_33.3 metallic glass. The amorphous phase was not stable under electron irradiation and metastable crystalline phases with face-centered cubic-based structure formed in both kinds of metallic glass by electron irradiation induced crystallization. The unique phase selection in electron irradiation induced crystallization is due to a change in the phase stability of crystal, quasi-crystal and amorphous phase under electron irradiation.     

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.     

A Ni-free Zr-based bulk metallic glass with remarkable plasticity

The effect of Nb and Pd combination on the glass forming ability (GFA) and mechanical properties of Zr₅₃Cu₃₀Nb_xPd_9?xAl₈ (x = 3.5–6.0) bulk metallic glasses (BMGs) were systematically investigated by X-ray diffractometry (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and compression test. TEM observation revealed that a nanocrystalline phase embeds in the amorphous matrix of the as-cast Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ alloy. A tiny nano-crystalline phase (with size about 5–20 nm) embedded uniformly in the amorphous matrix of the Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ alloy was observed and identified to be the tetragonal structured NbPd₃ phase based on the analyses of nano beam electron diffraction. According to the results of thermal analyses, the composition of Zr₅₃Cu₃₀Nb₅Pd₄Al₈ and Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ present the optimum GFA as well as thermal stability in the Zr₅₃Cu₃₀Nb_xPd_9?xAl₈ (x = 3.5–6.0) alloy system. In addition, the result of compression test shows that the yield strength significantly increases from 1700 MPa (Zr₅₃Cu₃₀Nb₅Pd₄Al₈) to 1900 MPa (Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈). A remarkable compression plastic strain (11.2%) occurs at Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ BMG rod with 2 mm in diameter. This significant increase in plasticity is presumably due to the restriction on shear banding by the nano-size second phase.     

Effects of the addition of Pd on the hydrogen absorption–desorption characteristics of Ti33V33Cr34 alloys

The hydrogen absorption–desorption performance of the body-centered-cubic (bcc) Ti–V–Cr–Pd alloys have been investigated. Ti₃₃V₃₃Cr₃₄ ingots with 0, 0.05, 0.5 at.% Pd were prepared by arc melting. X-ray diffraction (XRD) revealed that all of these alloys were homogeneous bcc solid solutions. Pd-containing (0.05, 0.5 at.% Pd) Ti–V–Cr alloys have better initial activation properties than those without Pd, and the desorption plateau pressure of the (Ti₃₃V₃₃Cr₃₄)_99.5Pd_0.5 alloy was substantially higher than that of the alloy without Pd. It is also found that the hysteresis difference is smaller in these alloys and degradation of hydrogen absorption capacity becomes steady after the 25th cycling test. (Ti₃₃V₃₃Cr₃₄)_99.5Pd_0.5 alloy exhibits large hydrogen absorption and desorption capacity of up to 3.42 and 2.07 mass% at 353 K, respectively.     

Constitution of the high-Al region of Al–Cu–Rh

Phase equilibria between 540 and 1010 °C were studied in Al–Cu–Rh alloys containing more than 50 at.% Al. Congruent equiatomic AlRh dissolves more than 40 at.% Cu and extends up to 58 at.% Al at the high-Cu part of its compositional range. High-temperature cubic C-Al₅Rh₂ (C-phase) dissolves up to 13 at.% Cu, “Al₃Rh” (₆-phase) up to 15 at.% Cu and Al₉Rh₂ up to 1.5 at.% Cu. The solubility of the third element in other binary Al–Rh and Al–Cu phases is below 0.5 at.%. Close to the high-Cu limit of the C-phase region the fcc C₂-phase structurally related to the C-phase is formed. Stable decagonal phase (D₁-phase) is formed below 1005 °C in a compositional range extending from Al₆₅Cu₁₆Rh₁₉ to Al₆₂Cu₂₃Rh₁₅, which shifts to higher Cu concentrations with decreasing temperature. An additional ternary phase forming around the Al₇₀Cu₂₀Rh₁₀ composition below 660 °C was revealed. Partial 1010, 990, 900, 800, 700, 600 and 540 °C isothermal sections were determined.     

Thermodynamic and kinetic studies of Mg–Fe–H after mechanical milling followed by sintering

The transition metal complex hydride Mg₂FeH₆ has been successfully synthesized utilizing mechanical milling of a 2Mg–Fe mixture followed by heating at 673 K under 6 MPa of hydrogen pressure, without pressing step. The obtained yield of Mg₂FeH₆ was about 50%. Hydrogen storage properties of the Mg–Fe–H system, i.e. capacities, absorption/desorption kinetics and thermodynamic parameters, were examined. The pressure-composition isotherm (PCI) measurements of the samples at 548–673 K showed that the alloys possessed good cyclic stability and reversibility. Enthalpies and entropies of decomposition of the Mg–Fe–H system were evaluated by van’t Hoff plots. The absorption/desorption rates at 573 K were very fast in comparison with the reported data. The non-isothermal desorption of hydrogen was found greatly dependent on the thermal history of the sample.     

Microstructure identification of devitrified Cu–Ti–Zr–Ni bulk amorphous alloy

The microstructures of devitrified Cu–Ti–Zr–Ni bulk amorphous alloy were identified by X-ray diffractometry (XRD) and transmission electron microscope (TEM). XRD and TEM examinations show that the deep eutectic structures of the tested alloy consist of CuTi₂–Cu₁₀Zr₇, Cu₃Ti–CuZr, Cu₃Ti–Cu₁₀Zr₇–CuZr low-order eutectics. Moreover, short-range ordering clusters in the melt with configuration similar to that of Cu₁₀Zr₇ compound may contribute to the glass forming ability of bulk amorphous Cu–Ti–Zr–Ni alloy.     

Room-temperature mechanically induced solid-state devitrifications of glassy Zr65Al7.5Ni10Cu12.5Pd5 alloy powders

The high-energy ball milling technique was employed for synthesizing a single phase of glassy Zr₆₅Al_7.5Ni₁₀Cu_12.5Pd₅ alloy powder, using a room-temperature mechanical alloying method. Whereas the glass transition temperature of the obtained glassy alloy is 683 K, the crystallization temperature is 783 K. The mechanically alloyed Zr₆₅Al_7.5Ni₁₀Cu_12.5Pd₅ glassy powders maintain their unique disordered structure through a large supercooled liquid region (100 K). The possibility of devitrification of the synthetic glassy phase upon increasing the ball milling time was investigated. The results have shown that the glassy powder that is obtained after 173 ks of milling is subjected to numerous lattice imperfections and tends to transform into a metastable big-cube phase after further ball milling (259–432 ks). After 540 ks of milling, a complete glassy–metastable phase transformation is achieved and the end-product of this stage of milling is nanocrystalline big-cube powder that has a lattice constant of 1.2293 nm. As the milling time increases (720 ks), the obtained big-cube phase can no longer withstand the shear and impact stresses that are generated by the milling media and is transformed into a new metastable phase of nanocrystalline fcc-Zr₆₅Al_7.5Ni₁₀Cu_12.5Pd₅. The fcc-metastable phase transforms into a mixture of Zr₂Cu and Zr₆NiAl₂ crystalline phases at rather high temperature, as high as 993 K.     

Hydrogen and Zr-based metallic glasses: Gas/solid absorption process and structure evolution

The absorption of hydrogen by means of gas-solid reaction and its consequence on the structure have been studied for fully amorphous alloys as well as quasicrystals/glassy composite alloys based on the composition Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈. The process of hydrogen absorption has been performed and monitored under 20 bar of H₂ using high pressure-differential scanning calorimetry (HP-DSC). The structure evolution of the samples has been followed by X-ray diffractometry (XRD). Results show that the nature of the surface oxide layer strongly affects the process of hydrogen absorption, especially its starting temperature. The structure evolves nevertheless along the same basic sequence, regardless of the sample: (i) the alloys keep a global amorphous structure up to roughly H/M = 0.8 and T = 350 °C; (ii) then ZrH₂ and at higher temperature Cu₂AlZr are formed. The stability of the glass is weakened and the formation of quasicrystals is inhibited under 20 bar of H₂. An heterogeneous distribution of hydrogen atoms inside the amorphous matrix has been inferred from the results.     

A study of the corrosion behaviour of Zr50Cu(40−X)Al10PdX bulk metallic glasses with scanning Auger microanalysis

The anodic-polarization behaviours of Zr₅₀Cu₄₀Al₁₀ and Zr₅₀Cu₃₃Al₁₀Pd₇ (atomic, at., %) bulk metallic glasses were investigated in aerated and deaerated 0.6 M NaCl. Polarization in both environments yielded similar results. Initial anodic-polarization of both alloys demonstrated that small increases in the applied potential resulted in significant increases in the current density. Continued polarization produced diffusion controlled polarization. Scanning Auger microanalysis was used to analyze the corrosion pits. Pits formed on both BMGs were enriched with Cu and Cl. Palladium enrichment was also observed in pits on Zr₅₀Cu₃₃Al₁₀Pd₇. A corrosion mechanism related to the formation of CuCl and Cu₂O is proposed.     

Glass formation and mechanical properties of (Cu50Zr50)100−xAlx (x = 0, 4, 5, 7) bulk metallic glasses

The effect of Al content on the glass formation and mechanical properties was studied for (Cu₅₀Zr₅₀)_100−xAl_x (x = 0, 4, 5, 7) bulk metallic glasses. The crystallization temperatures of fully amorphous Cu₅₀Zr₅₀, Cu₄₈Zr₄₈Al₄, Cu_47.5Zr_47.5Al₅ and Cu_46.5Zr_46.5Al₇ as-cast rods are 724 K, 753 K, 758 K and 782 K, respectively. The mechanical properties were investigated under compression at room temperature. As-cast Cu_46.5Zr_46.5Al₇ shows the highest yield strength (1867 MPa), whereas Cu_47.5Zr_47.5Al₅ shows the largest fracture strain of 11.2%. The fracture surfaces of the compressed samples were investigated by scanning electron microscopy and their morphology has been correlated with the compressive plasticity.