Low expansion in [(Fe0.9Co0.1)0.72B0.24Nb0.04]95.5Y4.5 bulk metallic glass

The thermal expansion behavior of an iron-based bulk metallic glass (BMG) [(Fe_0.9Co_0.1)_0.72B_0.24Nb_0.04]_95.5Y_4.5 is studied. The expansion coefficient in the wide temperature range of 100–300 K keeps in a constant, which is close to that of the Kovar alloy. Furthermore, the expansion coefficient of the BMG can be tuned by annealing below glass transition temperature. The results have implication for understanding the origin of low expansion, and the BMG may have a potential for commercial low expansion material.     

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.     

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.     

Electron microscopy of bulk metallic glass machining chips

Machined Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ bulk metallic glass (BMG) chips were characterized using high resolution electron microscopy. Compared to conventional processing techniques, machining produces very high heating/cooling, and deformation rates. It is therefore of interest to compare structural changes in machining chips with those produced by conventional processing. Large (～1 μm) crystalline grain, residual amorphous region, and phase separation in the amorphous–crystalline transition region were detected in bright field TEM images. Three equilibrium phases, Zr₂Cu, ZrAl₂, and Zr₂Ni, which have been identified in samples undergoing conventional annealing, were revealed from selected area electron diffraction patterns of the chips. High magnification TEM micrographs showed nanocrystallites, about 10 nm in size, in the amorphous–crystalline transition region.     

Surface and bulk residual stresses in Li2O · 2SiO2 glass–ceramics

In the present work surface and bulk residual stresses generated in partially crystallized Li₂O · 2SiO₂ glass–ceramics are analyzed after different heat treatments. The phase specific residual stresses in the crystalline Li₂Si₂O₅-phase are evaluated for the first time in the near-surface zone and the bulk of the samples using both medium and high energy synchrotron radiation. The results reveal that in the crystals within the bulk of the samples micro residual stresses generated by the thermal anisotropy of the isolated individual crystallites depend on the crystallographic direction. In contrast, the residual stress state in the near-surface zone is isotropic due to the superposition of thermal residual stresses in and around the crystals of the near-surface area. Residual stress calculations using a modified Selsing’s model yield a good estimate of the anisotropic residual stresses in the bulk crystallites, whereas the isotropic residual stress state in the crystallized surface layer can be described by an elastic stress model for thin films.     

Elasticity, thermal expansion and compressive behavior of Mg65Cu25Tb10 bulk metallic glass

The existence of special covalently bonded short-range ordering structures in a Mg₆₅Cu₂₅Tb₁₀ bulk metallic glass (BMG) is confirmed by thermal expansion and compression behavior. Under ambient conditions the linear thermal expansion coefficient obtained is almost constant in the glassy state with a value of 4.0 × 10⁻⁵ K⁻¹. By fitting the static equation of state at room temperature under ambient conditions we find the value for bulk modulus B of 48.7 GPa, which is in excellent agreement with the experimental study by pulse-echo techniques of 44.7 GPa. Unlike many bulk metallic glasses, such as Zr- and Pd-based, which bulk modulus is much larger than 100 GPa, the value B of Mg₆₅Cu₂₅Tb₁₀ BMG falls into the range of SiO₂ and fluorozirconate glass ZBLAN. Moreover, the elastic constant of the Mg₆₅Cu₂₅Tb₁₀ BMG is almost the same as those of ZBLAN. No evidence for the high-pressure phase transitions of the Mg₆₅Cu₂₅Tb₁₀ BMG has been found up to 31.19 GPa at room temperature.     

The use of flow behavior and thermal expansion to monitor structural change of amorphous Fe78Si9B13 ribbon

Samples of the amorphous Fe₇₈Si₉B₁₃ ribbon were annealed isothermally and isochronally under different tensile stresses. These samples were also investigated using thermal expansion tests, DSC and XRD methods. The results show that a structural transition similar to the glass transition was found using thermal expansion method. Comparing the XRD patterns of the tensile stress annealed samples with those of stress free annealed samples, it is suggested that tensile stress promotes the crystallization.     

Simple correlation between mechanical and thermal properties in TE-TL (TE = Ti,Zr,Hf;TL = Ni,Cu) amorphous alloys

We provide new data for the stiffness, hardness, Debye temperatures and crystallization and glass transition temperatures for fourteen Ti-TL and Hf-TL amorphous alloys (TL = Ni,Cu) covering wide composition range (x(TL) ≤ 70). These data increase linearly with x, as was previously observed for Zr-TL glassy alloys. Thus, the strength of interatomic bonding (reflected in stiffness) in all TE-TL amorphous alloys (TE = Ti,Zr,Hf) increases with x for x ≤ 65. The actual variations of studied properties depend on TE, being feeble for TE = Ti. The approximately linear variation of mechanical, vibrational and thermal parameters with x enables one to estimate these parameters for pure amorphous TEs, but does not indicate compositions at which bulk metallic glasses (BMG) form in these alloy systems. The reduced glass transition temperature T_rg is the property which roughly correlates with the formation of BMGs in glassy TE-TL alloys.     

Thermal stress calculations based on a linear viscoelastic deviatoric response and a fictive temperature component for the volumetric response

A temperature dependent thermal expansion coefficient is introduced into the Lee, Rogers and Woo linear viscoelastic model for calculating thermal stresses in glass. Volumetric strain, is calculated as follows: ? = α_liq (T_f - T_O) + α_gl (T - T_f) where α_liq and α_gl are the thermal expansion coefficients for the liquid and glassy states respectively, T is the glass temperature, T_O is the initial temperature, and T_f is the fictive temperature as defined by Tool. Fictive temperatures are calculatedfrom theory and measurements independent of thermal stress experiments. Comparison of calculated with experimentally measured stresses shows that the modified model is useful for predicting thermal tempering propensity for different glass compositions as well as different processing parameters.     

Superplasticity and superplastic forming ability of a Zr-Ti-Ni-Cu-Be bulk metallic glass in the supercooled liquid region

The superplastic deformation behavior and superplastic forming ability of the Zr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5 (at.%) bulk metallic glass (BMG) in the supercooled liquid region were investigated. The isothermal tensile results indicate that the BMG exhibits a Newtonian behavior at low strain rates but a non-Newtonian behavior at high-strain rates in the initial deformation stage. The maximum elongation reaches as high as 1624% at 656 K, and nanocrystallization was found to occur during the deformation process. Based on the analysis on tensile deformation, a gear-like micropart is successfully die-forged via a superplastic forging process, demonstrating that the BMG has excellent workability in the supercooled liquid region.     

Alloying effect on the viscous flow in metallic glasses

The temperature dependence of viscosities near the glass transition is measured from the rates of thermal transformation for metallic glasses PtNiP, PdNiP, NiPBA1 and (Fe, Co)PBA1. Alloying among metallic elements which lowers the glass transition temperature T_g lowers the ideal glass transition temperature T₀, but raises the residual configurational entropy S_g and the activation energies for “diffusive” rearrangement, $\text{Δμ}{}^1$, of the alloying glasses, while compositional ordering associated with the addition of metalloids raises the T_g and T₀ and lowers the S_g and $\text{Δμ}{}^1$. Results are correlated to the atomic ordering and stability of the glasses. The extracted free volume and the critical diffusive volume are much smaller, by a factor of 4, for metallic glasses than for many other glasses.     

Mg based bulk metallic glasses: Glass transition temperature and elastic properties versus toughness

In this work, optimal compositions for bulk metallic glasses (BMGs) formation in the ternary Mg-Cu-Nd and Mg-Ni-Nd systems are located at the Mg₅₇Cu₃₄Nd₉ and Mg₆₄Ni₂₁Nd₁₅, respectively, with the critical diameter of 4 mm for the rods fabricated by copper mold casting. As indicated by notch toughness testing, Mg₆₄Ni₂₁Nd₁₅ BMG (K_Q = 5.1 MPa√m) manifests higher toughness with respect to the Mg₅₇Cu₃₄Nd₉ (K_Q = 3.6 MPa√m), even though both BMGs have similar compressive fracture strength of 870-880 MPa. Such an improvement in toughness for Mg BMGs correlates with the reduction of shear modulus and the enhancement of thermal stability to resist to the structural relaxation at room temperature, which is indicated by the elevated glass transition temperature T_g. Under the Mode I loading condition, morphology in fracture surface of the Mg₆₄Ni₂₁Nd₁₅ BMG varies along the crack propagation path. Fractographic evolution of the fracture surface follows the Taylor's meniscus instability criterion. For the Mg-based BMGs, shear modulus scales with the glass transition temperature, and can be expressed as μ = 4.7 + 625T_g/V_m[1-4/9(T/T_g)^2/3]. Meanwhile, correlation between the calorimetric T_g and elastic properties at T_g can be rationalized with Egami's model.     

Kinetics of ion exchange in glasses

The kinetics of $\text{K}{}^{\mathrm{+}}\text{?}\text{Na}{}^{\mathrm{+}}$ exchange in two glass systems, 20Na₂O·(60?x)B₂O₃· (20 + x)Si₂ (where x = 0, 15, 30 and 45 mol%) and Na₂O·3SiO₂, were studied as a function of glass composition, salt bath composition, exchange temperature and time The distribution of K in the glass specimens after exchange in molten KNO₃ was determined with an electron probe. Stresses in these speciments were measured photoelastically. The interdiffusion coefficient $\text{D}$ for ion exchange was calculated as a function of local composition in the glass using the Boltzmann-Matano method. The strong variation of $\text{D}$ in any particular glass approximated that predicted by a mixed alkali model (as advanced by Lacharme), where the glass in the ion-exchanged region approximates a composite of stacked layers of mixed alkali glasses with a gradually varying alkali ratio. The small discrepancy between the experiment and the mixed alkali model was partly, but not fully, reconciled by considering the strains in the glasses. The observation which remained unexplained was that the calculated stress profiles did not show perfect agreement, both in magnitude and in shape, with the experimentally measured stress profiles. It appeared that the kinetics of ion exchange in the glasses were also influenced by a network relaxation process which may have occurred well below the glass transition temperature.     

Electrical conductivity of new zinc phosphate glass/metal composites

Zinc phosphate–glass/metal composites have been successfully prepared. Glass with composition of 45 mol% ZnO–55 mol% P₂O₅ (ZP) has been filled with metallic powders (nickel and cobalt). The glass matrix thermal stability has been assessed by differential thermal analysis technique. The morphology has been examined by scanning electronic microscopy, showing almost homogenous composites. Comparison between the measured and calculated densities as a function of metallic content exhibits a good coherence and allows the estimation of porosity inside the composites. X-ray diffraction analysis has revealed that the ZP-matrix phase is amorphous when the temperature treatment is below the glass transition temperature T_g. However, the principal peaks observed in the case of the composites have been assigned to the metallic crystals of nickel or cobalt fillers. It has been found that the phosphate glass phase is not affected by the growing of the metallic network. The electronic conductivity measurements versus filler volume fraction have been investigated for the first time on phosphate–glass/metal composites. This study has shown the occurrence of a conducting transition at around 30% filler volume fraction. The obtained result has been interpreted on the basis of the statistical percolation theory frame.     

Strengthening of soda-lime-silica glass by surface treatment with sol–gel silica

In this study, the failure resistance of soda-lime-silica glass was increased by surface treatment with sol–gel silica. Samples annealed and ion-exchanged in KNO₃ for 24 h at 450 °C were considered. Sol–gel silica coating was carried out by dipping the glass samples into a sol suspension prepared by hydrolysis of Si(OEt)₄ in ethanol/water solution. The deposited layer was consolidated in air for 24 h and subjected to mild thermal treatment at 300 °C for 1 h. The surface treatment increased the fracture resistance of annealed glass of about 35 MPa; conversely, ion-exchanged specimens showed an average increase of about 90 MPa. The strengthening effect induced by the surface treatment was attributed to the reduction of the effective crack length generated by the silica coating. The different strength increase between annealed and ion-exchanged samples is discussed in terms of fracture toughness which, for ion-exchanged glass, is not constant, due to the presence of the surface residual stresses and thus the reduction of the crack length due to the silica coating determines a higher strength increase than for annealed glass.     

Thermophysical properties of Zr-Cu-Al metallic glasses during crystallization

The crystallization behavior of Zr-Cu-Al metallic glasses was studied using thermophysical property measurements. When the Zr content of Zr-Cu-Al metallic glass decreased from 65 at.% to 45 at.%, the thermal conductivity gradually increased and the maximum value obtained was the composition of Zr:Cu:Al = 50:39.3:10.7(at.%). These metallic glasses were not crystallized upon heat treatment below the glass transition temperature T_g, and the thermophysical properties of these metallic glasses were almost constant. In contrast, these metallic glasses started to crystallize upon heat treatment above T_g after a certain derived time, and their thermal conductivity increased with the crystallinity of the metallic glass.     

Structural behavior of amorphous and liquid metallic alloys at elevated temperatures

The thermal behavior of the short-range order of Pd₄₀Cu₃₀Ni₁₀P₂₀ bulk metallic glasses has been investigated in situ by means of high-temperature X-ray synchrotron diffraction. The dependence of the X-ray structure factor S(q) of the glassy state on temperature follows the Debye theory up to the glass transition. Above the glass transition temperature T_g, the temperature dependence of S(q) is altered toward a continuous development of structural changes in the liquid state with temperature. The behavior of the structure factor during heating and cooling through the glass transition gives experimental evidence for melting the glass, and for freezing the liquid, respectively at the caloric glass temperature.     

Effect of microalloying of Nb on corrosion resistance and thermal stability of ZrCu-based bulk metallic glasses

Bulk metallic glasses (BMGs) of Zr_46−xNb_xCu_37.6Ag_8.4Al₈ with x = 0, 0.5, 1, 2 and 4 at.% were prepared by copper mould casting. The corrosion resistance of the ZrCu-based BMGs with different Nb contents was carefully examined by weight loss measurements and potentiodynamic polarization tests in 3 mass% NaCl, 1 N HCl and 1 N H₂SO₄ solutions, respectively. Nb addition improves the newly developed BMGs’ corrosion resistance in chloride-containing solutions and the alloys all exhibit excellent corrosion resistance in 1 N H₂SO₄. The corrosion behavior of the alloy containing 0 and 4 at.% Nb in phosphate-buffered solution was examined by electrochemical polarization tests. The influence of Nb addition on glass forming ability (GFA), thermal stability and mechanical property was investigated by X-ray diffraction, differential scanning calorimetry and compression tests, respectively. It is found that the addition of Nb can deteriorate the GFA and thermal stability of the base system, but little effect is observed on the mechanical properties, e.g., yielding strength and plasticity, of the ZrCu-based BMG alloys.     

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.     

Effects of Zn on the glass forming ability and mechanical properties of MgLi-based bulk metallic glasses

Through the addition of Zn element, Mg-Li-Cu-Zn-(Y, Gd) bulk metallic glasses (BMGs) with diameter of 2 mm have been successfully fabricated by conventional copper mold injection casting method. The X-ray diffractometer (XRD) patterns and differential scanning calorimeter (DSC) traces demonstrated that the as-cast Mg₆₀Li₅Cu₂₀Zn₅(Y, Gd)₁₀ BMGs were fully amorphous phase. By the minor addition of 5 at.% Zn, the supercooled liquid regions (SLR) of Mg₆₀Li₅Cu₂₀Zn₅(Y, Gd)₁₀ BMGs increased 6 K and 3 K respectively compared with that of Zn-free samples, which implied that their glass forming ability (GFA) and thermal stability were improved. It was also proved that the addition of Zn effectively enhanced the alloys’ fracture strength (453 MPa and 456 MPa) and elastic strain (0.75% and 0.92%), which were much higher than that of Mg₆₀Li₅Cu₂₅Y₁₀ BMG (403 MPa, 0.62%) and Mg₆₀Li₅Cu₂₅Gd₁₀ BMG (412 MPa, 0.82%).