Flow and fracture in Zr-based bulk metallic glasses

Effects of atomic-scale open-volume regions in metallic glass structure on the flow and fracture behavior of a Zr-Ti-Ni-Cu-Be bulk metallic glass were examined. Metallic glass structure was changed by annealing at 300 °C. Studies of relaxation time scales showed that atomic arrangement processes for viscous flow were significantly retarded with annealing. Plane strain fracture toughness was significantly decreased and fatigue crack-growth rates were dramatically increased, indicating degradation of resistance to crack extension as a result of annealing. Fracture morphology completely changed from vein patterns to cleavage-like features with little evidence of plasticity with annealing. The positron lifetime and Doppler broadening experiments revealed decreased open-volume regions as a result of annealing. The observed variation of the viscoelastic relaxation time scales with annealing was well described in terms of the anneal-out of open-volume regions. The current metallic glass was found to posses a low fractional free volume of the order of 0.1% indicating dense-packed structure which contributes to the excellent glass forming ability. The loss of stress relief ability by retarded crack tip viscous flow as a result of the anneal-out of open-volume regions is believed to contribute to subsequent embrittlement.     

On non-isothermal kinetics of two Cu-based bulk metallic glasses

In this paper, two Cu-based bulk metallic glasses, Cu₅₅Zr₃₇Ti₈ and Cu₆₁Zr₃₄Ti₅, have been evaluated in thermodynamics and kinetics. The activation energies with the constant values were generalized by different theoretical models. The E _x of Cu₅₅Zr₃₇Ti₈ and Cu₆₁Zr₃₄Ti₅ are 319 ± 12 and 359 ± 12 kJ mol^?1, respectively, implying that the as-cast alloys have a good stability in thermodynamics. On the other hand, variable activation energies were also determined using Kissinger–Akahira–Sunose method, Ozawa–Flynn–Wall method, and Friedman’s method. The results showed that the Ea(x) at the beginning of the crystallization are higher than that at the end of the crystallization in the first exothermic peak. By introducing the local Avrami exponent, n(x), the growth and nucleation mechanisms were discussed. Furthermore, the effects of different activation energies on local Avrami exponent were also given a discussion.     

Effect of reversible structural relaxation on diffusion in bulk metallic glasses

Long time relaxation at temperatures below the calorimetric glass transition causes reversible structural changes in metallic glasses. The resulting enthalpy recovery was measured by means of DSC in Zr_46.8Ti_8.2Cu_7.5Ni₁₀Be_27.5- and Pd₄₀Cu₃₀Ni₁₀P₂₀-bulk glass annealed for different times at 553 K and 542 K, respectively. Relaxation times of about 10⁶ s and 10⁴ s, respectively, were determined. The diffusion coefficients of B, Fe and Co were measured above and below the calorimetric glass transition temperature. Whereas the temperature dependence of these diffusion coefficients in the non-relaxed glasses shows “non-linear” Arrhenius behaviour with a break near the glass transition, the diffusion in the long time relaxed glasses follows a uniform temperature dependence over the entire temperature range with considerably reduced diffusion coefficients below the glass transition. This behaviour can be reversed by annealing the relaxed glasses again at higher temperatures indicating the strong effect of the reversible structural relaxation on the diffusion coefficients.     

A new correlation between the characteristics temperature and glass-forming ability for bulk metallic glasses

Journal of Thermal Analysis and Calorimetry - A new criterion or parameter χ given by $$\left( {\frac{{T_{\text{x}} - T_{\text{g}} }}{{T_{\text{l}} - T_{\text{x}} }}} \right) \times \left(...     

Effects of crystals on the mechanical properties of Zr52.5Ti5Cu17.9Ni14.6Al10 bulk metallic glasses

In this paper the mechanical properties of fully amorphous and partially cystalline Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ alloys were investigated. The changes in hardness and ductiity caused by the presence of crystals in the amorphous matrix is presented.     

Processing,microstructure and properties of bulk metallic glass composites

A Zr-Nb-Cu-Ni-Al bulk metallic glass was reinforced with up to 80 volume-percent (% V_f) continuous fibers, short fibers or particles. Characterization based on X-ray diffraction, differential scanning calorimetry, electron microprobe and scanning electron microscopy is presented. The metallic glass matrix remains amorphous after adding reinforcements. Reactions at the matrix/reinforcement interfaces were examined using transmission electron microscopy. A narrow band of crystalline particles typically forms adjacent to the reinforcement. The composites were tested in compression. Compressive strain-to-failure increased by up to factor of 12 compared to the unreinforced bulk metallic glass. The increase in compressive strain-to-failure is due to the particles restricting shear band propagation, promoting the generation of multiple shear bands and additional fracture surface area.     

Crystallization kinetics of metallic glasses

In this study, the temperature-heating rate diagram of the main crystallization process of two metallic glasses, Fe₇₄Ni_3.5Mo₃B₁₆Si_3.5 and Fe₄₁Ni₃₈Mo₃B_18, was obtained from one experimental differential scanning calorimetry (DSC) scan and the knowledge of their activation energy as determined by an isoconversional method. A good concordance was observed between the diagram curves obtained by calculation (isoconversional approach) and the experimental data, which verifies the reliability of the method and the validity of the kinetic approach in these alloys.     

Biocorrosion and biocompatibility of Zr–Cu–Fe–Al bulk metallic glasses

Owing to their unique chemo‐physical and structural characteristics, amorphous bulk metallic glasses (BMGs) are of great demand for fabrication of variety of advanced and innovative products including surgical and biomedical tools and devices. In this study, a series of Ni‐free Zr‐based BMGs in Zr–Cu–Fe–Al system are fabricated using copper‐mold casting technique, and their biocorrosion and biocompatibility are evaluated with respect to their corrosion behavior in the phosphate buffered saline (pH = 7.4) solution. Anodic polarization curves, scanning electron microscopy combined with energy‐dispersive X‐ray, and wettability analyses are conducted to characterize the surfaces of BMG samples. The biocompatibility of the BMG and control samples is studied by comparing cell–substrate interactions among different samples. It is found that Zr₆₀Cu₂₀Fe₁₀Al₁₀ displays a higher passive region compared with that of Zr₆₀Cu_22.5Fe_7.5Al₁₀, but both BMGs exhibit lower corrosion resistance compared with Ti–6Al–4V alloy. By addition of titanium to Zr–Cu–Fe–Al system (Zr₆₀Ti₆Cu₁₉Fe₅Al₁₀), a significant increase in the passive region of the polarization curve is detected. The cell culture experiments reveal that the number of attached and grown cells is significantly higher on the surface of the treated BMGs as compared with Ti–6Al–4V substrates and the culture plate as controls. There is no noticeable difference in cellular morphology among the BMG samples, and no cytotoxicity is detected. We speculate that the interaction of water molecules and matrix proteins with the surfaces of BMGs plays an important role in cell–substrate interactions and improved cell response. Copyright © 2013 John Wiley & Sons, Ltd.     

Investigations of the solid state reaction process in mechanically alloyed Zr-Al-Cu-Ni bulk metallic glasses by analytical transmission electron microscopy

Starting with elemental powders, the progress of amorphization in mechanically alloyed Zr₆₅Al_7.5Cu_17.5Ni₁₀ has been investigated by x-ray diffractometry and transmission electron microscopy (TEM). Detailed investigations of the microstructural evolution during milling indicate that the amorphization proceeds by solid state reaction, similar to other well known binary or multicomponent systems.     

Thermal evolution of ferromagnetic metallic glasses

A traditional TG apparatus was modified by placing two permanent magnets producing a controlled magnetic field (TG(M): Magneto Thermogravimetry). This technique proved to be useful to study both structural relaxation and crystallisation of ferromagnetic metallic glasses. Results obtained for the amorphous alloys Fe₄₀Ni₄₀P₁₄B₆ and Fe_62.5Co₆Ni_7.5Zr₆Nb₂Cu₁B₁₅, are reported in this paper. Structural relaxation can be evaluated by measuring changes in Curie temperature induced by thermal treatments. Crystallisation in TG(M) is detected through a change in the measured apparent mass (difference between the sample mass and magnetic force driving it upward). These results were confirmed by DSC analysis. Whether the obtained crystalline phase is ferromagnetic, it can be identified through its Curie temperature, measured by TG(M). In fact the value of 770°C measured as Curie temperature of crystallised Fe_62.5Co₆Ni_7.5Zr₆Nb₂Cu₁B₁₅led to conclude that the only ferromagnetic crystalline phase is a-Fe. These hypothesis was confirmed by XRD analysis, showing that the first crystallisation yields to a-Fe nanocrystals. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evaluation of microstructures and properties of bulk mesoporous silica

Synthesis of mesoporous MCM-type bulks prepared by hydrothermal hot-pressing (HHP) method using MCM-type mesoporous powder was attempted. Scanning electron microscopy (SEM), bulk density measurement, N₂ adsorption-desorption isotherms and formaldehyde adsorption test have been employed to characterize the bulky products. As a result, we succeeded in preparing a dense and strong mesoporous bulks with high BET over 1000 m²/g through the hydrothermal hot-pressing method under appropriate conditions.     

Structural instability of metallic glasses under radio-frequency-ultrasonic perturbation and its correlation with glass-to-crystal transition of less-stable metallic glasses

It has been reported that the structural stability is significantly deteriorated under radio-frequency-ultrasonic perturbation at relatively low temperatures, e.g., near/below the glass transition temperature T(g), even for thermally stable metallic glasses. Here, we consider an underlying mechanism of the ultrasound-induced instability, i.e., crystallization, of a glass structure to grasp the nature of the glass-to-liquid transition of metallic glasses. Mechanical spectroscopy analysis indicates that the instability is caused by atomic motions resonant with the dynamic ultrasonic-strain field, i.e., atomic jumps associated with the beta relaxation that is usually observed for low frequencies of the order of 1 Hz at temperatures far below T(g). Such atomic motions at temperatures lower than the so-called kinetic freezing temperature T(g) originate from relatively weakly bonded (and/or low-density) regions in a nanoscale inhomogeneous microstructure of glass, which can be straightforwardly inferred from a partially crystallized microstructure obtained by annealing of a Pd-based metallic glass just below T(g) under ultrasonic perturbation. According to this nanoscale inhomogeneity concept, we can reasonably understand an intriguing characteristic feature of less-stable metallic glasses (fabricated only by rapid melt quenching) that the crystallization precedes the glass transition upon standard heating but the glass transition is observable at extremely high rates. Namely, in such less-stable metallic glasses, atomic motions are considerably active at some local regions even below the kinetic freezing temperature. Thus, the glass-to-crystal transition of less-stable metallic glasses is, in part, explained with the present nanoscale inhomogeneity concept.     

XRF and PIXE analysis of metallic glasses

Radionuclide X-ray fluorescence (XRF) and particle induced X-ray emission (PIXE) methods have been used for a rapid and nondestructive analysis of metallic glasses. The methods are compared in accuracy and precision with the atomic absorption method. Some results of analyses of Fe_xNi₈₀-_xB₂₀ materials are briefly reviewed. The distribution of elements along the width as well as length and a qualitative analysis of the composition of material surfaces are considered.     

Nuclear techniques characterization of short-range order in Zr-TM-Cu-Al-Ni (TM=Hf,Ti, Fe) bulk metallic glasses

In this paper an overview of recent and new perturbed angular correlation (PAC), Mössbauer (MS) and positron annihilation (PAS) experiments on Zr- and Hf-based bulk metallic glasses is presented. PAC results showed that all samples in their as-prepared amorphous state, irrespective of the preparation method used, are well described by a dense random packing of ions (DRP). Hf-based samples showed a greater electric field gradient (EFG) value than Zr-based ones for same composition. A general observation is that the relaxed amorphous and supercooled liquid has the same local arrangements than the initial amorphous state. The crystallization behaviour is not preparation-method dependant but composition and impurities have a major influence on it. Preliminary positron annihilation lifetime measurements on amorphous samples yielded one characteristic lifetime, with a value intermediate between those for the well-annealed and defected metallic constituents of the alloys. Mössbauer spectra of as-prepared amorphous alloys consist of a broad asymmetric doublet, originating from a distribution of electric field gradients characteristic for amorphous samples.     

Photoluminescent coordination polymer bulk glasses and laser-induced crystallization

We synthesized luminescent coordination polymer glasses composed of d¹⁰ metal cyanides and triphenylphosphine through melt-quenching and mechanical milling protocols. Synchrotron X-ray total scattering measurements and solid-state NMR revealed their one-dimensional chain structures and high structural dynamics. Thermodynamic and photoluminescence properties were tunable by the combination of heterometallic ions (Ag⁺, Au⁺, and Cu⁺) in the structures. The glasses are moldable and thermally stable, and over centimeter-sized glass monoliths were fabricated by the hot-press technique. They showed high transparency over 80% from the visible to near-infrared region and strong green emission at room temperature. Furthermore, the glass-to-crystal transformation was demonstrated by laser irradiation through the photothermal effect of the glasses.

Over centimeter-sized luminescent coordination polymer glasses were fabricated. They showed high transparency (over 80%) and strong green emission at room temperature. The glass-to-crystal transformation by laser irradiation was demonstrated.     

Phase transformations in bulk nanostructured potassium niobiosilicate glasses

In potassium niobiosilicate (KNS) glasses, nanostructuring can be driven and controlled by thermal treatments at the glass transition temperature and/or by modulation of the chemical composition. The tight relationship between nanostructure and nonlinear optical properties suggests these bulk nanomaterials as an appealing route to nanophotonics. The focus of this paper is placed on assessing the phase transformations which occur in these materials upon annealing at the glass transition temperature and subsequent heating. High-temperature resolved X-ray diffraction (HTXRD) and high-resolution transmission electron microscopy (HRTEM) experiments are integrated with previously published results for in-depth insight. It will be shown that nanostructuring evolves from nucleation of niobium-rich nanocrystals, which are up to 20 nm large, uniformly distributed in the matrix bulk, and metastable. Formation kinetics as well as phase transformation of the nanocrystals are determined by the glass composition. Depending on it, nanocrystal nucleation can be preceded or not by phase separation, and the nanocrystals' phase transition can be of first or second order.