Formation of bulk Pt–Pd–Ni–P glassy alloys

The thermal behavior of (Pt_0.4Pd_0.3Ni_0.3)_100−xP_x (x = 16–25 at.%) glassy alloys has been investigated. It is found that the crystallization behavior of the (Pt_0.4Pd_0.3Ni_0.3)_100−xP_x glassy alloys changes from a single-stage exothermic reaction to a two-stage exothermic reaction depending on phosphorous content. When the phosphorous content is 23 at.%, the glassy alloy exhibits the largest supercooled liquid region (ΔT_x) and a sharp single exothermic peak. Fixing the phosphorous content at 23 at.%, the Pt_77−x−yPd_xNi_yP₂₃ (x = 7.7–61.6 at.%, y = 7.7–61.6 at.%) glassy alloys have a wide composition range in which the glassy alloys exhibit a large supercooled liquid region (ΔT_x beyond 60 K). In this range, the Pt_30.8Pd_23.1Ni_23.1P₂₃ glass has the largest ΔT_x (77 K) and a high reduced glass transition temperature (T_rg) of 0.60. This alloy can be cast into fully glassy rods with a diameter of 3 mm. Under uni-axial compression, bulk Pt_30.8Pd_23.1Ni_23.1P₂₃ glassy alloy has an elastic strain of ∼2%, an ultimate strain (to fracture) of ∼6.4%, a Young’s modulus of ∼106 GPa and a failure strength of ∼1390 MPa.     

Binary Ni–P bulk amorphous glass prepared by electrodeposition method

Compact and uniform bulk Ni–P binary amorphous alloys (BAAs) with thickness of up to 1 mm were prepared using electrodeposition method. This is the first finding that amorphous alloy can be obtained up to ‘bulk’ size not by rapid solidification but by electrochemical method. By improving the electrodeposition techniques, selecting proper plating solution, electrodeposition is probably a new method for preparing bulk amorphous alloys. The formation mechanism of the bulk amorphous was discussed within the concept of ‘disorder solid’.     

Heterogeneous structure of the quaternary metallic glass Fe40Ni40P14B6: An EXAFS investigation

The problems encountered in the analysis of EXAFS data from Fe₄₀Ni₄₀P₁₄B₆ are solved by assuming a micro-heterogeneous structure from chemically and structurally different micro-phases. A special Bayesian-type least-squares algorithm with general constraints was developed to reduce the effective number of free parameters, which fits simultaneously the EXAFS spectra at the NiK and the FeK edges and maximum prior knowledge as available. As a result of the analysis, the decomposition into micro-phase regions with Fe₃B-type short-range order (52%) as well as random dense packed FeNiPB solid solution regions (48%) is proposed.     

Hydrogen permeation in the metallic glass Fe40Ni40P14B6

Hydrogen permeation from the gas phase through the metallic glass Fe₄₀Ni₄₀P₁₄B₆ (Metglas 2826) was measured with the electrochemical technique over 313 to 353K and 10⁵ to 10³ Pa. Specimen surfaces were coated with a thin layer of electrodeposited palladium to eleminate surface impedances to hydrogen entry and exit. Sievert's law behavior was observed over the temperature and pressure ranges studied. An excellent fit to the permeation transients as obtained from Fick's diffusion theory. Diffusivities derived from least squares fitting of permeation trasients were in good agreement with those obtained from time lag measurements. Within the concentration range studied, diffusivity was found to be independent of input hydrogen concentration. The expressions for the permeability coefficient, 2.24 × 10¹⁶ exp{[(48.99 ± 2.93) kJ/mol]/RT} atom H/ms $\text{Pa}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, the diffusivity 8.52 × 10^?7 exp{[(?47.07 ± 1.63) kJ/mol]/RT} m²/s, and the solubility, 2.62 × 10²²exp{[(?1.92 ± 4.56) kJ/mol]/RT} atom H/m³$\text{Pa}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, were determined.An annealing treatment at 573K for one hour decreases bot the permeability and diffusivity of hydrogen in the metallic glass.     

Direct imaging of phase separation in Pd41.25Ni41.25P17.5 bulk metallic glasses

The alloy system, Pd_41.25Ni_41.25P_17.5, has a negative heat of mixing among its constituent elements. By using high resolution transmission electron microscopy, high angle annular dark field in scanning transmission mode, and energy dispersive X-ray spectroscopy, it was found that phase separation occurs in Pd_41.25Ni_41.25P_17.5 glassy alloys. For a clear exhibition of the amorphous phase separation reaction, it is desirable to introduce intermediate thermal annealing before an undercooled Pd_41.25Ni_41.25P_17.5 melt is cooled down to become a solid amorphous specimen. The results suggest that there may be unique short range orders in amorphous/liquid Pd_41.25Ni_41.25P_17.5, which are responsible for the phase separation.     

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.     

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.     

Real-time dissolution of P40Na20Ca16Mg24 phosphate glass fibers

This study characterizes the dissolution of quaternary P₄₀Na₂₀Ca₁₆Mg₂₄ phosphate glass fibers. Mass loss rate, the evolution of the surface morphology and the pH of solution were monitored. The effects of sizing and annealing on these processes were also studied. Silane coating did not slow down the fiber dissolution rate whereas annealing did. Scanning electron microscopy showed that pitting and hydration led to the peeling of surface layers from the fibers.     

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.     

Diffusion in bulk-metallic glass-forming Pd–Cu–Ni–P alloys: From the glass to the equilibrium melt

Since the discovery of bulk-metallic glasses there has been considerable research effort on these systems, in particular with respect to mass transport. Now the undercooled melt between the melting temperature and the caloric glass transition temperature, which has not been accessible before due to the rapid onset of crystallization, can be investigated and theories can be tested. Here we report on radiotracer diffusion measurements in metallic bulk-glass-forming Pd–Cu–Ni–P alloys. Serial sectioning was performed by grinding and ion-beam sputtering. The time, temperature as well as the mass dependence, expressed in terms of the isotope effect E, of Co-diffusion were investigated. In the glassy state as well as in the deeply supercooled state below the critical temperature T_c, where the mode coupling theory predicts a freezing-in of liquid-like motion, the experimentally determined very small isotope effects indicate a highly collective hopping mechanism involving some ten atoms. Below T_c the temperature dependence shows Arrhenius-type behavior with an effective activation enthalpy of 3.2 eV. Above T_c the onset of liquid-like motion is evidenced by a gradual drop of the effective activation energy and by the validity of the Stokes–Einstein equation, which is found to break down below T_c. This strongly supports the mode coupling scenario. The Stokes–Einstein equation is presently tested for other constituents of the alloy. The Co isotope effect measurements, which have never been carried out near T_c in any material, show atomic transport up to the equilibrium melt to be far away from the hydrodynamic regime of uncorrelated binary collisions.     

First-principles study of the binary Ni60Ta40 metallic glass: The atomic structure and elastic properties

Ni–Ta bulk metallic glass (BMG) with compositions around Ni₆₀Ta₄₀ is a newly found binary BMG with high glass forming ability and extraordinary mechanical strength. Using ab initio molecular dynamics, the local atomic structure, elastic properties and electronic structures of Ni₆₀Ta₄₀ glass have been explored. The pair-correlation functions, coordination numbers, and chemical compositions of the most abundant local clusters have been analyzed. We demonstrated the existence of icosahedral Ni₇Ta₆ clusters as the major Ni-centered clusters, while the most popular Ta-centered cluster is Ta₇Ni₈. These findings agree with our previous cluster model of Ni–Ta binary BMG. The elastic moduli of Ni₆₀Ta₄₀ glass were also computed and the experimental Young's modulus is well reproduced. Analysis of electronic structures further revealed that the interaction between d electrons of Ni and Ta atoms is responsible for the experimentally observed ultrahigh mechanical strength for the Ni–Ta BMGs.     

The effect of structural relaxation on the local structure of Cu40Zr60 amorphous alloys

Structural relaxation phenomena associated with heat treatments near the glass transition temperature range were investigated in liquid-quenched amorphous Cu₄₀Zr₆₀ alloys by means of differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS) and EXAFS. A kinetic study was performed by DSC in order to determine the evolution of the configurational enthalpy ($\text{3 H}{}_{\mathrm{\sigma }}$) against temperature (upon continuous heating or during isothermal annealing treatments) and to define the heat treatment suitable to obtain highly relaxed samples.SAXS results show that the structure of the relaxed samples remains homogeneous at the intermediate range of order up to at least 100 Å, whatever be the previous heat treatment. EXAFS detects a slight change in the local structure only for the most highly relaxed samples; this change could be interpreted by a variation of 0.5 atom in the coordination numbers (probably of the ZrZr pairs) or by a change of the disorder parameter of about 0.01 Å.No phase separation is detected in the relaxed amorphous Cu₄₀Zr₆₀ specimens.     

Structural relaxation of Ti40Zr25Ni8Cu9Be18 bulk metallic glass

Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ bulk metallic glass has a unique quenched-in nuclei/amorphous matrix structure. The crystallization of quenched-in nuclei, when the experimental isothermal annealing time is within its incubation time, may not disturb the enthalpy relaxation, which makes it have the accordingly common enthalpy relaxation behavior with amorphous materials. The alloy's annealing time dependence of recovery enthalpy follows a stretched exponential function with the mean relaxation time obeying an Arrhenius law. The equilibrium recovery enthalpy ΔH_T^eq, mean relaxation time τ and stretching exponent β are all dependent on the annealing temperature, and generally, a higher annealing temperature comes with a lower value of ΔH_T^eq, τ and a higher value of β. Two parameters, β_g and τ_g, representing the stretching exponent and the mean structural relaxation time at the calorimetric glass transition temperature, respectively, are correlated with glass forming ability and thermal stability, respectively. For Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ BMG, the high value of β_g, which is much higher than 0.84 and approaches unity, reveals its good glass forming ability, while, on the other hand, the low value of τ_g indicates a worse thermal stability compared with typical BMGs.     

Isochronal crystallization kinetics of Cu60Zr20Ti20 bulk metallic glass

Isochronal crystallization kinetics of Cu₆₀Zr₂₀Ti₂₀ bulk metallic glass has been investigated by differential scanning calorimetry. By means of the Kissinger, Ozawa, Kempen, Matusita and Gao methods, average effective activation energies for the first and second crystallization reactions in Cu₆₀Zr₂₀Ti₂₀ are calculated to be about 375 ± 9 and 312 ± 11 kJ mol⁻¹, respectively, which are smaller than the values deduced from isothermal experiments. Meanwhile, average Avrami exponents, 3.0 ± 0.1 and 3.4 ± 0.2, for two crystallization reactions in isochronal anneals, differ from the value about 2.0 in isothermal anneals. The nonidentity of the Avrami exponents and effective activation energies may be contributed to different crystallization mechanisms and the nature of non-isokinetic between isochronal and isothermal experiments. The values of frequency factor k₀ for the first and second crystallization reactions of Cu₆₀Zr₂₀Ti₂₀ are (1.7 ± 0.3) × 10²⁴ and (7.0 ± 0.8) × 10¹⁸ s⁻¹, respectively, and the large value of k₀ has been discussed in terms of the atomic configuration and interaction.     

Heat capacity of Pd40Ni40P20 in the deeply undercooled liquid state determined by relaxation experiments in the glass transition region

An indirect method is presented which allows for the heat capacity determination of deeply undercooled melts at temperatures in the glass transition region, which are inaccessible to continuous calorimetric measurements. The method is based only on the unique relaxation behavior of glass-forming liquids in the transition region, and thus is generally applicable for vitreous systems. Here it is illustrated by measurements of the specific heat of deeply undercooled Pd₄₀Ni₄₀P₂₀, a metallic bulk-glass former, resulting in a considerable approach of the measurements towards the isentropic limit. Additionally, the consequent application of this method allowed for the determination of the dependence of the glass temperature on the cooling rate during vitrification, yielding a linear relation between the glass temperature and the logarithm of the cooling rate, as observed for several non-metallic glass-forming systems.