Phonon dispersion in bulk metallic glass Zr55Cu30Al10Ni15

The expressions for the phonon frequencies of Zr₅₅Cu₃₀Al₁₀Ni₁₅ bulk metallic glass employing a simple model given by Bhatia and Singh for a hypothetical one-component metallic glass are derived both for longitudinal and transverse modes of excitations. The model assumes a central force, effective between the nearest neighbours, and a volume dependent force. Both types of excitations of phonons are computed for the Zr₅₅Cu₃₀Al₁₀Ni₁₅ bulk metallic glass for the first time both for self-consistent screening of conduction electrons with and without the inclusion of correlation effects. Phonon frequency expressions reproduce the main characteristic features of the dispersion curves. The theoretical results predicted are in a good agreement with available experimental data of different quaternary bulk metallic glasses having same constituents. These are also compared to the theoretical results of a quaternary glass to have an insight of the structural behavior of the glass under consideration.     

Toughness of Zr55Cu30Al10Ni5 bulk metallic glass for two oxygen levels

This paper demonstrates the embrittlement of a Zr-based bulk metallic glass caused by the presence of very low oxygen traces during its synthesis. Toughness studies are conducted both at room and low (77 K) temperatures. Samples with 2000 appm of oxygen contain crystalline defects (trapping all oxygen) while those with 300 appm are completely glassy. Toughness results clearly indicate that these defects embrittle the glass. Therefore, careful attention must be paid during synthesis with respect to the oxygen content to get a completely glassy alloy. This issue is of paramount importance when dealing with mechanical properties such as toughness.     

Synthesis and characterization of bulk amorphous Zr65Cu18Ni9Al8 and [Zr0.65Cu0.18Ni0.09Al0.08]98Er2 alloys

Bulk metallic glasses (BMGs), especially Zr-based BMGs, have attracted lot of attention of materials scientists because of their very attractive physical, thermal and mechanical properties and a few unique applications. In the present study, Zr₆₅Cu₁₈Ni₉Al₈ alloy was designed according to the criterion of conduction electron/atom (e/a ratio) ∼1.395 and average atomic size of alloy (R_a) ∼0.1498 nm. Addition of 2 at.% Er was carried out in the base alloy to investigate its effect on thermal and mechanical properties. Characterization of alloys was performed using the techniques of XRD, DSC, and SEM/EDS. Mechanical properties like Vicker’s microhardness, nanohardness, elastic modulus, density and fracture strength were measured. Average shear angle was found to be ∼35 ± 1° for base alloy and about 31 ± 1° for alloy containing 2 at.% Er. Wide supercooled liquid regions of 129 K and 119 K were found for the base alloy and the alloy containing 2 at.% Er.     

Preparation and mechanical properties of a bulk icosahedral quasicrystalline Ti45Zr35Ni17Cu3 alloy

A bulk Ti₄₅Zr₃₅Ni₁₇Cu₃ alloy, which consisted of the icosahedral quasicrystalline phase, was prepared by mechanical alloying(MA) and subsequent pulse discharge sintering. Ti₄₅Zr₃₅Ni₁₇Cu₃ amorphous powders (with particle size <50 μm) were obtained after mechanical alloying for more than 150 h from the mixture of the elemental powder. The transformation temperature range from amorphous phase to the quasicrystalline phase was from 400 K to 900 K. The mechanical properties of the bulk quasicrystalline alloy have been examined at room temperature. The Vickers hardness and compressive fracture strength were 620 ± 40 and 1030 ± 60 MPa, respectively. The bulk quasicrystalline alloy exhibited the elastic deformation by the compressive test. The fracture mode was brittle cleavage fracture.     

Atomic structure and hydrogen storage properties of amorphous–quasicrystalline Zr–Cu–Ni–Al melt-spun ribbons

A structural state of the Zr–Cu–Ni–Al melt-spun ribbons has been investigated by means of X-ray diffraction analysis. It was established that conditions of ribbon production have a very strong effect on their structural state. The Zr_69.3Cu_9.7Ni_15.1Al_5.9 ribbons produced at the surface wheel velocity of 44 m/s have an amorphous state at the contact side and mixed amorphous–quasicrystalline state at the free side. At the same time the Zr_67.5Cu_12.5Ni₁₂Al₈ ribbons are fully amorphous in case of producing velocity of 44 m/s and have only small features of quasicrystalline peaks on the amorphous halo on the free side in case of producing velocity of 30 m/s in contrast to an amorphous structure on the contact side. The uncoated by Pd amorphous and amorphous–quasicrystalline Zr–Cu–Ni–Al ribbons have a property of absorbing a large amount of hydrogen. Parameters of the amorphous state were calculated for the ribbons as-prepared and hydrogenated. The temperature coefficient of resistivity for all ribbons is negative in range of 20–380 °C, what as well as the high resistivity values is typical to the Zr–Cu–Ni–Al based systems in amorphous and quasicrystalline states.     

Structure changes in Al80Ni15Y5 amorphous alloy

The structure of Al₈₀Ni₁₅Y₅ amorphous alloy at various temperatures have been studied with X-ray diffraction methods. The obtained scattered intensities, structure factors, pair correlation functions and main structure parameters have been analyzed. Temperature dependences of the parameters suggest formation of Al, Al₃Ni and Al₂₃Ni₆Y₄ phases upon crystallization of an amorphous alloy.     

Reversible structural relaxation and crystallization of Zr62Al8Ni13Cu17 bulk metallic glass

The effect of pre-treatment above glass transition temperature (T_g) on the reversible structural relaxation in a Zr₆₂Al₈Ni₁₃Cu₁₇ bulk metallic glass at sub-T_g annealing has been investigated. It is found that the enthalpy relaxation can be well described by a stretched exponential function. A free-volume model can only qualitatively simulate the process of enthalpy recovery. Sub-T_g heat treatments after enthalpy recovery have almost no effect on the following crystallization, which can be accelerated when critical clusters are formed for a longer time annealing.     

Phase formation and physical properties of mechanically alloyed amorphous 55Mg35Ni10Si

Amorphous 55Mg35Ni10Si alloy powder has been synthesized by mechanical alloying technique using pure Mg, Ni and Si elemental powders. The transformation of the crystalline powders into an amorphous one has been investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and differential scanning calorimetry. The new material produced has a higher thermal stability than reported results, which is beneficial to the fabrication of Mg–Ni–Si bulk amorphous components through powder metallurgy.     

Effect of microalloying on the glass-forming ability of Cu60Zr30Ti10 bulk metallic glass

One atomic percent of Sn and Si each was added (replacing Zr) to the bulk-glass-forming Cu₆₀Zr₃₀Ti₁₀ alloy. Sn improves the glass-forming ability (GFA) of the alloy, while Si triggers nanocrystalline phase formation in the glassy matrix, as resolved by high-resolution transmission electron microscopy. The observed variation does not originate from atomic size and/or heats of mixing effects. The results described here indicate, rather, that Sn improves GFA because it reduces the liquidus temperature and shifts the composition toward the off-eutectic reaction during melting.     

Rheology and nano-crystallization of a Zr41.25Ti13.75Ni10Cu12.5Be22.5 bulk metallic glass

Creep of Zr-based bulk metallic glass (BMG) was carried out under hydrostatic pressure of 270 MPa superimposed on uniaxial compressive loads of 30–300 MPa. The temperature of testing was maintained at 633 K, which is close to the glass transition temperature of the BMG. The samples had heat treatment at 676 K for different lengths of time applied prior to testing, which resulted in microstructural changes. Apparent viscosity during flow was derived from the creep measurements. The effect of crystalline volume fraction, temperature, and strain rate are considered. Finally, we have constructed a TTT diagram for this BMG spanning almost three decades of time and 650–690 K temperature range, showing transformation from glassy to crystalline solid as a function of volume fraction.     

Ni59Zr20Ti16Sn5 amorphous alloy obtained by melt spinning and mechanical alloying

Amorphous Ni₅₉Zr₂₀Ti₁₆Sn₅ alloys were fabricated by melt spinning and by mechanical alloying (MA) techniques. Additionally the melt spun ribbon was powdered by ball milling. Differential scanning calorimetry (DSC) measurements revealed two or three-steps crystallization of the amorphous alloys. MA powders exhibited the lowest crystallization temperature of the first DSC peak, the melt spun ribbon – the highest one. On the other side, the lowest value of activation energy of crystallization was calculated for ball milled ribbon. Further studies are needed to clarify the differences in thermal stability and crystallization behavior of the alloys.     

Effect of liquid state on the crystallization of Zr65Al7.5Ni10Cu17.5 metallic glass

The correlation between the quenching temperature and the crystallization of the Zr₆₅Al_7.5Ni₁₀Cu_17.5 metallic glass was examined. The electrical resistivity and the thermal property were measured to monitor the structural change of the samples quenched at the temperature of 1473 K, 1573 K, 1673 K and 1773 K, respectively. The consistent results of DSC and d(ρ_(T)/ρ₀)/dT‐T curves indicated different crystallization behaviors of the samples. For the samples quenched from 1773 K, the increase in ΔT_x, T_rg and γ imply higher glass forming ability. Moreover, according to the XRD patterns of samples annealed at different temperatures, the melt temperature influences the formation of crystallized phases of amorphous Zr₆₅Al_7.5Ni₁₀Cu_17.5 alloy.     

Phase transformation and crystallization kinetics of melt-spun Ni60Nb20Zr20 amorphous alloy

The glass transition and crystallization kinetics of melt-spun Ni₆₀Nb₂₀Zr₂₀ amorphous alloy ribbons have been studied under non-isothermal and isothermal conditions using differential scanning calorimetry (DSC). The dependence of glass transition and crystallization temperatures on heating rates was analyzed by Lasocka's relationship. The activation energies of crystallization, E_x, were determined to be 499.5 kJ/mol and 488.6 kJ/mol using the Kissinger and Ozawa equations, respectively. The Johnson–Mehl–Avrami equation has also been applied to the isothermal kinetics and the Avrami exponents are in the range of 1.92–2.47 indicating a diffusion-controlled three-dimensional growth mechanism. The activation energy obtained from the Arrhenius equation in the isothermal process was calculated to be E_x = 419.5 kJ/mol. The corresponding three dimensional (3D) time–temperature–transformation (TTT) diagram of crystallization for the alloy has been drawn which provides the information about transformation at a particular temperature. In addition, the intermetallic phases and morphology after thermal treatment have been identified by X-ray diffraction (XRD) and scanning electron microscope (SEM).     

Mechanical properties of NiFe based alloy glasses

The ductility and tensile strength of NiFe based alloy glasses prepared by the ‘roller quenching’ technique were investigated. These alloy glasses are intrinsically ductile and possess a high tensile strength. However, the mechanical properties of glasses which are predominantly Fe are very susceptible to the quenching conditions, and such glasses tend to be brittle. It is suggested that the hot rolling of the alloy during quenching is responsible for the brittle behavior. Tensile strengths as high as 230 000 psi were obtained for the NiFe based alloy glasses.     

Electron transport properties of Al–Sm and Al–Sm–Ni amorphous and nanocrystalline alloys

The calculated temperature variation of electrical resistivity and thermoelectric power for composite Al–Sm alloys, composed of the X fraction of fcc-Al nanocrystals embedded in amorphous phase, are in good agreement with experimental data. No interphase layer was detected on nanograin surface. Correlation between a value of electrical resistivity, temperature coefficient of resistivity and thermoelectric power of amorphous Al–Sm and Al–Sm–Ni alloys is qualitatively consistent with Ziman theory. The attracting interaction between Al and Ni atoms, which stabilizes the amorphous structure, is detected.     

Consolidation and mechanical properties of Cu46Zr42Al7Y5 metallic glass by spark plasma sintering

Cu₄₆Zr₄₂Al₇Y₅ metallic glass with nearly 100% relative density was obtained by spark plasma sintering (SPS) with a diameter of 15 mm, which was larger than the largest size of 10 mm for the as-cast specimen. The fracture strength of the sintered specimen reached 2044 MPa, which was 15% higher than that of the as-cast Cu₄₆Zr₄₂Al₇Y₅ glassy specimen. The densification and compressive properties of the sintered specimens were related to sintering temperature. Structural changes of the specimens sintered at various sintering temperatures resulted in the difference of macro-mechanical properties.     

Phase separation and crystallization in a melt-spun Ti45Zr35Ni17Cu3 alloy

Structure and crystallization behavior of amorphous and quasicrystalline Ti₄₅Zr₃₅Ni₁₇Cu₃ alloy have been studied. DSC trace of the amorphous alloy obtained during continuous heating to 1300 K shows distinctly an exothermic peak and two endothermic peaks. The amorphous alloy has different structures depending on annealing temperature. The first exothermic reaction at low temperature region from 400 K to 900 K is due to the precipitation of an icosahedral quasicrystalline phase, and the second endothermic reaction at higher temperature region from 950 K to 990 K results from the transformation of the I-phase to C14 Laves and α-(Ti, Zr) phases.     

Structural relaxation and glass transition behavior of novel (Ti33Zr33Hf33)50(Ni50Cu50)40Al10 alloy developed by equiatomic substitution

A series of glass forming alloys (Ti₃₃Zr₃₃Hf₃₃)_100−x−y(Ni₅₀Cu₅₀)_xAl_y (x = 20–70 at.% and y = 0–30 at.%) have been developed by equiatomic substitution of similar elements. Of these alloys (Ti₃₃Zr₃₃Hf₃₃)₅₀(Ni₅₀Cu₅₀)₄₀Al₁₀ was chosen in this study to investigate the structural relaxation and glass transition behavior. The as-quenched (Ti₃₃Zr₃₃Hf₃₃)₅₀(Ni₅₀Cu₅₀)₄₀Al₁₀ alloy was fully amorphous and had a wide supercooled liquid region ΔT = T_x(503 °C) − T_g(433 °C) = 70 °C, where T_g and T_x are the glass transition and crystallization temperatures, respectively. Low temperature pre-heat treatments of the (Ti₃₃Zr₃₃Hf₃₃)₅₀(Ni₅₀Cu₅₀)₄₀Al₁₀ alloy for 10 min at 310 °C, 370 °C and 390 °C caused structural relaxation accompanied by the formation of very fine scale lattice ordering. After these heat treatments, the glass transition became hard to observe in the (Ti₃₃Zr₃₃Hf₃₃)₅₀(Ni₅₀Cu₅₀)₄₀Al₁₀ alloy. Increasing the pre-heat treatment temperatures and holding times caused the glass transition to become more clearly detectable with increasing endothermic heat release.