Fabrication and characterization of metallic glasses with a specific microstructure for micro-electro-mechanical system applications

Metallic glass microstructures with high aspect ratios for micro-electro-mechanical system applications have been fabricated by micro-electro-discharge machining and selective electrochemical dissolution methods. Micro-holes and three-dimensional microstructures machined on the La₆₂Al₁₄Ni₁₂Cu₁₂, Zr₅₅Al₁₀Ni₅Cu₃₀ and Cu₄₆Zr₄₄Al₇Y₃ bulk metallic glasses by micro-electro-discharge machining are evaluated by using X-ray diffraction, scanning electron microscopy, and nanoindentation. The experimental results demonstrate that the machined samples kept their amorphous structure without devitrification, and their machining characteristics are related to the thermo-physical properties of the alloys and the electrode diameters. Porous, single-pore and thin-walled Zr-based metallic glass tubes with micro-pore structures can be prepared by selective electrochemical dissolution method. The high aspect ratio microstructures fabricated by the two methods have the potential applications as micro-nozzles, polymer micro-injection molding tools, micro-channels or micro-flow meters in micro-electro-mechanical system devices.     

The effect of annealing on the mechanical properties of a ZrAlNiCu metallic glass

In this paper, the effects of annealing and nanocrystallization on the mechanical properties of a Zr₅₅Al₁₀Ni₅Cu₃₀ metallic glass have been studied. It has been shown that the high volume fractioned nanocrystals facilitate the formation of shear bands and thus decrease the yield stress. At the same time, the nanocrystals also facilitate the formation of interfacial voids during compression, resulting in substantial decrease in the plasticity of the metallic glass.     

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.     

Crystallization kinetics in Cu46Zr45Al7Y2 bulk metallic glass by differential scanning calorimetry (DSC)

Crystallization transformation kinetics in isothermal and non-isothermal (continuous heating) modes were investigated in Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass by differential scanning calorimetry (DSC). In isochronal heating process, activation energy for crystallization at different crystallized volume fraction is analyzed by Kissinger method. Average value for crystallization in Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass is 361 kJ/mol in isochronal process. Isothermal transformation kinetics was described by the Johnson-Mehl-Avrami (JMA) model. Avrami exponent n ranges from 2.4 to 2.8. The average value, around 2.5, indicates that crystallization mechanism is mainly three-dimensional diffusion-controlled. Activation energy is 484 kJ/mol in isothermal transformation for Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass. These different results were discussed using kinetic models. In addition, average activation energy of Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass calculated using Arrhenius equation is larger than the value calculated by the Kissinger method in non-isothermal conditions. The reason lies in the nucleation determinant in the non-isothermal mode, since crystallization begins at low temperature. Moreover, both nucleation and growth are involved with the same significance during isothermal crystallization. Therefore, the energy barrier in isothermal annealing mode is higher than that of isochronal conditions.     

Inhomogeneous structure and glass-forming ability in Zr-based bulk metallic glasses

Recently, a series of quaternary Zr-based bulk metallic glasses (BMGs), i.e., Zr₅₃Cu_18.7Ni₁₂Al_16.3, Zr_51.9Cu_23.3Ni_10.5Al_14.3 and Zr_50.7Cu₂₈Ni₉Al_12.3, have been developed and their glass-forming ability (GFA) increases with Cu concentration. In this work, atomic structures of the three BMGs were rebuilt by reverse Monte Carlo simulations based on the reduced pair distribution functions measured by high energy X-ray diffraction. The results show that a certain amount of substitution of short Zr-Cu, Cu-Cu pairs with long Zr-Zr and Zr-Al pairs enhances the local denser packing of Kasper polyhedral centered by Zr and Al atoms. A cell sub-divided method is proposed to describe the fluctuation of local number density which is found to follow the normal distribution function. The largest possible free volume in the three alloys is found to approaches to 3.8 Å. For the three alloys, the enhancement of GFA with increasing Cu content is due to the increase in the fluctuation degree of local density instead of the dense packing.     

Investigation of viscosity and crystallization in supercooled-liquid region of Zr-based glassy alloys

Using viscosity measurement method and in-situ heating synchrotron radiation, the viscosity of the (Zr_0.55Al_0.1Ni_0.05Cu_0.3)_100 ? xY_x (x = 0, 0.5, 1, 2) bulk metallic glasses (BMGs) in their supercooled liquid regions (SLRs) and the in-situ heating nucleation were investigated, respectively. In the SLR, the (Zr_0.55Al_0.1Ni_0.05Cu_0.3)₉₉Y₁ metallic glass which shows distinct plastic strain in compression exhibits higher viscosity than the other three BMGs, however their Poisson's ratios are almost the same. The synchrotron diffraction results show that crystallization happened in the SLR of the (Zr_0.55Al_0.1Ni_0.05Cu_0.3)₉₉Y₁ glassy alloy, which could be the reason for the higher viscosity and larger plastic strain in compression compared to the other three alloys. The fracture surfaces of the glassy alloys were observed and analyzed.     

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.     

Determination of activation energies for nucleation and growth from isothermal differential scanning calorimetry data

Differential scanning calorimetry (DSC) is usually adopted to analyze solid-state phase transformation incorporating nucleation, growth and impingement. Then, for isothermal transformation, time-dependent Avrami exponent and overall effective activation energy can always be deduced using recipes, which are derived from an analytical phase transformation model. On this basis, a concise and reliable approach to determine time-independent activation energies for nucleation and growth is proposed. Numerical calculations have demonstrated that the new approach is sufficiently precise under different conditions of transformation (e.g. nucleation: mixed nucleation and Avrami nucleation; growth: interface-controlled growth and diffusion-controlled growth; impingement: randomly nuclei dispersed, anisotropic growth and non-random nuclei distributions). Application of the approach in crystallization of Zr₅₅Cu₃₀Al₁₀Ni₅, Zr₅₀Al₁₀Ni₄₀ and Cu₄₆Zr₄₅Al₇Y₂ bulk amorphous alloys as measured by isothermal DSC was performed.     

Atomic dynamics in different regions of the potential energy surface

The atomic dynamics in two (bulk) metallic glasses, Ni₄₀Pd₄₀P₂₀ and Zr₅₅Cu₃₀Al₁₀Ni₅, were investigated by neutron inelastic scattering in different regions of the potential energy landscape, which are reached by slow cooling the bulk glasses and by hyper-quenching the same alloys. The results prove that the atomic dynamics depends also on the fictive temperature, i.e. the region of the potential energy surface, in which the glass is frozen in. Obviously the shapes of the basins or inherent structures are not the same everywhere on the potential energy surface, and the glass with a higher fictive temperature has more low energy modes than has the same glass with a lower fictive temperature. As results from computer simulation have suggested already, on moving to regions of lower mean potential energy (aging), part of theses low energy modes are transferred to the energy region of the calculated Debye cut-off energy. The difference between the vibrational entropies, calculated from the generalized vibrational density-of-states, which have been determined for both fictive temperatures, shows that the contribution from the vibrational entropy to the total entropy change, when moving through the potential energy landscape, is small for the two metallic glasses investigated. Structural relaxation of the hyper-quenched glass removes part of the additional low energy modes, but quantitatively possibly only at the low and perhaps also at the high-energy limit of the density-of-states. The wavelength dependence of the dynamics suggests that the additional low energy modes in the glass with the higher fictive temperature are not dominated by extended but more likely by localized modes.     

Crystallization kinetics of the Zr61Al7.5Cu17.5Ni10Si4 alloy using isothermal DSC and TEM observation

The crystallization behavior and microstructure development of the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy during annealing were investigated by isothermal differential scanning calorimetry, X-ray diffractometry and transmission electron microscopy. During isothermal annealing of the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy at 703 K, Zr₂Cu crystals with an average size of about 5 nm were first observed during the early stages (30% crystallization) of crystallization by TEM. The Zr₂Cu crystal size increased with annealing time and attained an average size of 20 nm corresponding to the stage of 80% crystallization. In addition, the change in particle size with increasing annealing time exhibited a linear relationship between grain growth time and the cube of the particle size for the Zr₂Cu type crystalline phase. This indicates that the crystal growth of the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy belongs to a thermal activated process of the Arrhenius type. The activation energy for the grain growth of Zr₂Cu is 155 ± 20 kJ/mol in the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ amorphous alloy. The lower activation energy for grain growth in compared to that for crystallization in Zr₆₅Cu₃₅ 440 kJ/mol crystal corresponds to the rearrangement of smaller atoms in the metallic glass, Al or Si (compare to Zr).     

Kinetic analysis of the isochronal crystallization of Ti40Zr25Ni8Cu9Be18 metallic glass

The isochronal crystallization kinetics of the Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ metallic glass has been investigated by differential scanning calorimetry (DSC). Results indicate that the two crystallization events of this metallic glass cannot be well-described by the classic Johnson-Mehl-Avrami (JMA) kinetic equation. The kinetic equation considering the impingement effect has been found more applicable for describing the isochronal crystallization kinetics of this amorphous alloy. Accurate values of kinetic parameters were determined by fitting the theoretical DSC data to experimental curves. The kinetic parameters change in different crystallization stages and show strong heating rate dependence. Reasons of the deviation from the JMA kinetics for the isochronal crystallization of Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ metallic glass were discussed.     

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.     

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.     

Bulk metallic glass formation: The positive effect of hydrogen

Zr₅₅Cu₃₀Ni₅Al₁₀ alloys with different amounts of hydrogen have been prepared by arc melting under the gaseous mixture of hydrogen and argon. Proper additions of hydrogen have been proved to effectively increase the glass-forming ability (GFA) of this alloy. Positive effect of hydrogen on GFA has been interpreted from the thermodynamic and structural points of view. Proper additions of hydrogen can decrease the liquidus temperature, which leads to more stable glass-forming liquid. Structure analysis by positron annihilation lifetime spectroscopy shows that proper additions of hydrogen can increase the concentration of shortest open volume and decrease the concentration of intermediate and largest open volumes. This leads to formation of a denser random packed structure, and thus increases the GFA of Zr₅₅Cu₃₀Ni₅Al₁₀ alloys.     

Enhanced plasticity of a Zr50Cu48Al2 bulk metallic glass

In this work, a Zr₅₀Cu₄₈Al₂ bulk metallic glass (BMG) with improved glass forming ability and mechanical properties was synthesized by adding a small amount of Al to a Zr-Cu binary glass-forming alloy. The as-cast Zr₅₀Cu₄₈Al₂ glassy rod does not exhibit macroscopic failure under compression at room temperature indicating the excellent plasticity of the BMG. The deformation behavior of the BMG was studied intensively by examining the true stress-strain curve and the surface morphology of the post-deformation sample. The effect of minor Al addition on the mechanical properties including the fracture stress, the modulus, and the kinetic characteristics of the BMG was also investigated. The enhanced plasticity of the BMG is considered to be related to its reduced fragility parameter m value.     

Influence of structural relaxation on atomic mobility in a Zr41.2Ti13.8Cu125Ni10.0Be22.5 (Vit1) bulk metallic glass

The effect of an annealing at a temperature above or below the glass transition temperature in a Zr_41.2Ti_13.8Cu₁₂₅Ni_10.0Be_22.5 bulk metallic glass was investigated using dynamic mechanical analysis. Structural relaxation influences both the storage modulus (elastic component) and the loss modulus (viscoelastic component). Kinetics can be captured by a stretched exponential relaxation function. Experimental results are correctly described using a physical model based on the concept of defects for the mechanical response of amorphous materials and especially for the characteristic time relative to atomic mobility.     

Effect of Ni-addition on the crystallization behavior and the oxidation resistance of Zr-based metallic glasses below the crystallization temperature

The crystallization behavior of Zr_65.0Al_7.5Ni_10.0Cu_17.5 metallic glasses by addition of Ni with 753 K annealing treatment and its effect on the oxidation resistance around the supercooled liquid region at 663 K were studied. By annealing at 753 K, the nanocrystalline phase of bct-Zr₂Cu precipitates was observed in the Zr_65.0Al_7.5Cu_27.5 specimen, while microstructures consisting of finer nanocrystalline bct-Zr₂Cu, fcc-Zr₂Ni and Zr₆Al₂Ni formed in the Zr_65.0Al_7.5Ni_10.0Cu_17.5 specimen. The oxidation resistance of the melt-spun Zr_65.0Al_7.5Ni_10.0Cu_17.5 specimen was improved by addition of Ni, which is evidenced by less mass gain and thin oxide scale. The microstructural refinement by the formation of numerous nanocrystalline phases of bct-Zr₂Cu, fcc-Zr₂Ni and Zr₆Al₂Ni from the matrix resulted in an improvement of the oxidation resistance, whereas a relative coarse nanocrystalline phase consisting of bct-Zr₂Cu exhibited fast oxidation along grain boundaries. Although the oxide species for both specimens were composed of a large amount of CuO/Cu₂O, some tetragonal and monoclinic-ZrO₂ as well as a minor amount of the oxide state of Cu³⁺, the amount of oxides especially for ZrO₂ in the Zr_65.0Al_7.5Ni_10.0Cu_17.5 specimen was lower, which was probably due to suppressed oxygen diffusion in ZrO₂.     

Precipitation of icosahedral quasicrystalline and crystalline approximant phases in Zr-Cu-(Co, Rh or Ir) metallic glasses

Zr₇₀Cu₃₀, Zr₇₀Cu₂₀Co₁₀, Zr₇₀Cu₂₀Rh₁₀ and Zr₇₀Cu₂₀Ir₁₀ glassy alloys were prepared by the single roller melt-spinning method and the crystallization process was studied by differential scanning calorimetry, X-ray diffraction and transmission electron microscopy, with an emphasis on the initial stage. The Zr₇₀Cu₃₀ metallic glass crystallizes through the direct precipitation of the stable crystalline phase from the matrix. The addition of Co, Rh and Ir to the Zr₇₀Cu₃₀ metallic glass induces the precipitation of metastable phases prior to the formation of the stable ones. The metastable phases are a Ti₂Ni-type compound for Zr₇₀Cu₂₀Co₁₀, a mixture of the Ti₂Ni-type compound and an icosahedral quasicrystalline phase (I-phase) for Zr₇₀Cu₂₀Rh₁₀, and the I-phase for Zr₇₀Cu₂₀Ir₁₀ metallic glasses. The different effects of Co, Rh and Ir addition are explained based on their difference in atomic radius.     

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.     

Ductility improvement of Zr-Cu-Ni-Al glassy alloy

We studied the relationship between the phase diagrams and formation of crystalline inclusions in cast samples in order to improve the ductility of Zr-Cu-Ni-Al bulk amorphous alloy. A Zr₅₀Cu₄₀Al₁₀ bulk amorphous alloy with a composition close to the ternary eutectic point has no crystalline inclusions and possesses superior mechanical properties of tensile strength σ_B=2000 MPa, Young’s modulus E=107 GPa and Vickers hardness HV=580. Oxygen embrittlement of Zr-Cu-Ni-Al bulk amorphous alloys can be avoided in the Zr₅₀Cu₃₀Ni₁₀Al₁₀ bulk amorphous alloy, which exhibits superior ductility and resistance to oxidation. Furthermore, we tried to improve ductility by cold rolling by making use of the apparent work-softening phenomenon of Zr-Cu-Ni-Al bulk amorphous alloys. Preliminary studies indicated that cold rolling to produce a fine slip-band structure, which enabled uniform deformation and superior deformability, is an important procedure for improving the ductility of Zr-Cu-Ni-Al bulk amorphous alloys.