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.     

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.     

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.     

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.     

Effect of crystallization behavior on the oxidation resistance of a Zr–Al–Cu metallic glass below the crystallization temperature

The crystallization behavior of Zr_65.0Al_7.5Cu_27.5 (at.%) metallic glass with 753 and 1053 K annealing treatment and its effect on oxidation resistance around the supercooled liquid region at 623 and 663 K was studied. The crystalline phase of bct-Zr₂Cu precipitates for the specimen annealed at 753 K was observed, while duplex structures of bct-Zr₂Cu and Zr₂Al formed in the specimen annealed at 1053 K. The oxidation resistance of the specimen depended on the amount of crystalline precipitates. Regardless of the exposure temperature, the annealed specimens showed higher oxidation resistance than the melt-spun one, especially for the specimen annealed at 1053 K. The formation of numerous crystalline phases of bct-Zr₂Cu and Zr₂Al from the matrix was responsible for improving the oxidation resistance due to their higher oxidation resistance and promotion of the development of Al₂O₃ and oxides of copper. The oxide constituents of the amorphous alloy after long exposure depended on the temperature. The oxide was composed of a large amount of CuO, some tetragonal and monoclinic-ZrO₂ and a minor amount of Cu₂O as well as a slight amount of Al₂O₃ for the melt-spun specimen during exposure at 623 K. Under the 663 K exposure, however, the oxide state of Cu³⁺ in the scale was also detected.     

Static and dynamic crystallization in Mg–Cu–Y bulk metallic glass

The static and dynamic crystallization behavior of Mg₆₅Cu₂₅Y₁₀ bulk metallic glass was investigated by X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. It was found that the kinetics of both anisothermal and isothermal crystallization were adequately represented by the Kissinger and KJMA relations, respectively. The apparent activation energy for crystallization was calculated to be 139 kJ/mol; this value is close to the self diffusion of Mg in both a crystalline and non-crystalline matrix. The Avrami exponent was found to vary from 2.2 to 2.5 with increasing annealing temperature which implies that, at high annealing temperatures, nucleation occurs at a constant rate accompanied by diffusion-controlled growth of spherical grains. Tensile straining in the supercooled liquid region indicated that crystallization is slightly accelerated compared with static crystallization; this phenomenon was found to adversely affect the ductility of the alloy.     

Change in environmental structure around Al in Zr60Ni25Al15 metallic glass upon crystallization studied by nuclear magnetic resonance

In order to investigate the structural evolution around Al, pulse NMR experiments were carried out on ²⁷Al in the Zr₆₀Ni₂₅Al₁₅ metallic glass and the related crystalline compound, Zr₆NiAl₂. Different chemical shift peaks were observed around 2750 and 3000 ppm in the as-quenched Zr₆₀Ni₂₅Al₁₅ and crystalline compound, Zr₆NiAl₂, respectively. Considering that the capped triangular prism of Zr₉Al₃ is formed around Al in the Zr₆NiAl₂ crystal, chemical correlation pairs of Al–Zr and/or Al–Al are fairly faint while that of Al–Ni may be dominant instead in the as-quenched state. These results suggest an inhomogenous chemical bonding nature in the Zr₆₀Ni₂₅Al₁₅ metallic glass. The resonant peaks around 3000 ppm, which were distinctive in the Zr₆NiAl₂ crystal, appeared and became stronger upon crystallization through the relaxed state. Thus, drastic change in the local atomic configuration around Al was confirmed so as to form the unlike chemical correlation pairs of Al–Zr upon crystallization. The high glass-forming ability of the Zr₆₀Ni₂₅Al₁₅ metallic glass should be attributed to the difficulties of significant atomic redistribution of the constituents around Al.     

Enthalpy recovery and its effect on homogeneous flow stress during supercooled liquid region for Ti40Zr25Ni8Cu9Be18 bulk metallic glass

The homogeneous flow exhibits strain softening for Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ bulk metallic glass, which is related to increase in free volume concentration based on free volume model. When metallic glass is pre-annealed above T_g before deformation, the trend of strain softening becomes slow with pre-annealing time, indicating that the enthalpy recovery contributes to strain softening, because the enthalpy of metallic glass will recover towards equilibrium value above T_g, and leads to increase in free volume concentration. So the strain softening for Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ bulk metallic glass is related to enthalpy recovery.     

Study of the kinetics of free volume in Zr45.0Cu39.3Al7.0Ag8.7 bulk metallic glasses during isothermal relaxation by enthalpy relaxation experiments

Enthalpy relaxation experiments were conducted to study the kinetics of free volume in Zr_45.0Cu_39.3Al_7.0Ag_8.7 bulk metallic glasses (BMGs) during isothermal relaxation using differential scanning calorimetry (DSC) at the temperature within the range from 648 to 684 K. Stretched exponential relaxation functions and the ?esták–Berggren SB (m, n) model were employed to analyze the kinetics of free volume. It is found that the relaxation time decreases from 2643.5 to 242.8 s while the Kohlrausch exponents increase from 0.717 to 0.892 in the investigated temperature range. The activation energy fluctuates slightly in the course of the relaxation process and its mean value is determined to be 239.7 kJ/mol. By fitting the first derivative of the conversion degree as a function of annealing time using the ?esták–Berggren SB (m, n) model, it is found that the values of m at all annealing temperatures are very small and can be approximated as zero. The values of the pre-exponential factors Z and the kinetic parameter n are found to be slightly varying with the annealing temperature, indicating the complexity of the kinetics of the isothermal relaxation. Finally, an approximate rate equation describing the kinetics of free volume during isothermal relaxation is proposed.     

Microstructure and stored energy evolutions during rolling of Cu60Zr20Ti20 bulk metallic glass

The microstructure and stored energy of Cu₆₀Zr₂₀Ti₂₀ bulk metallic glass rolled at cryogenic temperature in a wide strain rate range 1.0 × 10^?4 ? 5.0 × 10^?1 s^?1 have been investigated. As the specimen is rolled to be thinner, the stored energy first increases linearly, and then saturates above a critical thickness reduction at lower strain rates, or decreases at high strain rates. At the initial stage of rolling, no phase transformation except shear bands appears in the glass. Phase transformation occurs only when the specimen is severely deformed at strain rates higher than 1.0 × 10^?4 s^?1. As strain rate increases, the critical strain for the stored energy to saturate increases, but the critical strain for phase separation to occur decreases, and meanwhile the type of the phase transformation changes from phase separation to nanocrystallization. The stored energy does not change with the occurrence of phase separation, but decreases due to nanocrystallization. It is proposed that coalescence of more free volume in shear bands into nano-voids should be principally responsible for the saturation of the stored energy, which balances the results from the increase in shear band number at higher strains.     

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.     

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.     

Mechanical properties and ion irradiation of bulk amorphous Zr55Cu30Al10Ni5 alloy

The bulk amorphous Zr₅₅Cu₃₀Al₁₀Ni₅ alloy is one of the widely studied Zr-based alloys due to very attractive thermal and mechanical properties. Alloy ribbons and bulk samples were synthesized by Cu mould casting and characterized by SEM/EDS, DSC and XRD. Mechanical properties like Vicker’s hardness, nanohardness and elastic modulus etc. were measured. Ion irradiation of the samples was carried out to enhance the surface properties without altering the amorphous nature of the bulk material. Ion irradiation by singly charged Ar⁺ enhanced the hardness as well as elastic modulus considerably.     

Thermal stability and crystallization phenomena of low cost Ti-based bulk metallic glass

The thermal stability, kinetics of crystallization, and glass forming ability of a Ti₄₈Ni₃₂Cu₈Si₈Sn₄ bulk amorphous alloy have been studied by differential scanning calorimetry using both isothermal and non-isothermal experiments. The activation energy, frequency factor, and reaction rate for the crystallization cascade were determined via the Kissinger method. X-ray diffractometry and transmission electron microscopy studies revealed that crystallization starts with the primary precipitation of Ti(Ni,Cu), followed by the nucleation of Cu₃Ti from the amorphous precursor. The kinetics of nucleation of the primary crystalline phase was also investigated using the Johnson–Mehl-Avrami method and the Avrami exponent, n, was determined. This new alloy possesses a significantly larger supercooled liquid region than any other non beryllium- or non rare earth – containing titanium-based bulk metallic glass to date.     

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.     

The thermal stability and activation energy of crystallization of (Cu61.8Zr38.2)1−xAlx bulk metallic glasses

The present paper reports on the thermal stability and activation energy of crystallization of bulk metallic glasses (BMGs) (Cu_61.8Zr_38.2)_1−xAl_x. The (Cu_61.8Zr_38.2)_1−xAl_x composition series, prepared by copper mould suction casting into bars with a diameter of 3 mm, form BMGs with an e/a range of 1.24–1.3. These BMGs manifest increased thermal stability with increased e/a ratios. The activation energies (ΔE) of crystallization as derived from thermal analysis at different heating rates follow a similar tendency to that of the thermal stability, indicating stronger short-range ordering with increasing e/a ratios. The optimum BMG Cu_58.1Zr_35.9Al₆ exhibits the highest thermal stability and the largest ΔE.     

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.     

Thickness dependence of the crystallization process of wedge-shaped Zr70Cu27.5Rh2.5 metallic glass prepared by ion milling

Crystallization happens when metallic glass is annealed at elevated temperature. It has been believed that the crystallization process is primarily determined by the metallic glass composition with no report and expectation that it may depend on the physical dimension. In the present work, in situ annealing experiment was performed on a Zr₇₀Cu_27.5Rh_2.5 metallic glass under transmission electron microscopy observation. The specimen is prepared by ion milling and is wedge-shaped with its edge thin enough to be transparent for 200 keV electrons. At annealing condition well beyond that required for full crystallization for the regular ribbons, crystallization was not observed in areas with thickness of approximately 250 nm while crystalline grains in the thin and thick neighboring areas were observed.     

On the curious structural phases in Al-deficient (Al62Cu23Cr15) and Al-rich (Al68Cu17Cr15) quasicrystalline alloys

In the present work investigations on the effect of stoichiometric variations on the Occurrence and stabilization of quasicrystalline (qc) and related phases have been carried out. Based on the explorations of several Al-deficient and Al-rich versions of the ideal nominal composition i.e. Al₆₅Cu₂₀Cr₁₅, it has been found that the alloy compositions corresponding to Al₆₂Cu₂₃Cr₁₅ (Al-deficient) and Al₆₈Cu₁₇Cr₁₅ (Al-rich) exhibit several structural subtleties. The Al-deficient alloy, has been found to exhibit crystalline bcc and fcc phases with a = 8.90 Å and a = 17.98 Å respectively. In addition to these phases, a new crystalline bcc variant (a = 15.42 Å) originating from the bcc (a = 8.90 Å) phase has been found. Also a curious superstructure of the fcc (a = 17.98 Å) has been observed. The Al-rich alloy typified by Al₆₈Cu₁₇Cr₁₅, on the other hand, does not exhibit any structural variants, instead it shows nearly pure i-phase.