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.     

Study of kinetics of glass transition of metallic glasses

In this study, the kinetics of glass transitions of Ti₅₀Cu₂₀Ni₃₀ and Fe₆₇Co₁₈B₁₄Si₁ metallic glasses are studied using thermal analysis technique, i.e., differential scanning calorimetry, by means of continuous heating of the sample at various heating rates. In the present study, based on the heating rate dependence of glass transition temperature (T _g), the activation energy (E) of the glass transition region is determined by two most frequently used approaches, i.e., Moynihan's method and Kissinger's equation. The fragility index, m, is also calculated using T _g, which is a measure of glass-forming ability of the given system. The result shows that the fragility index, m, of the given systems is <16. This indicates that the given systems are strong liquids with excellent glass-forming ability.     

Study of formation of nano-quasicrystals and crystallization kinetics of Zr-Al-Ni-Cu metallic glass

The formation of nano-quasicrystals on isothermal annealing of melt-spun ribbons of Zr_69.5Al_7.5Ni₁₁Cu₁₂ metallic glass has been investigated using transmission electron microscopy (TEM). The crystallization study of this metallic glass has been carried out using differential scanning calorimetry (DSC) in non-isothermal (linear heating) mode. It exhibits two-stage crystallization where the first stage corresponds to the precipitation of icosohedral nano-quasicrystalline phase. This has been confirmed with the help of TEM investigations. The crystallization parameters like the activation energy (E _c) and frequency factor (k ₀) have been derived using the Kissinger peak shift analysis. The activation energies for the first and second crystallization peak are found to be 278 and 295 kJ mol^–1, respectively. The frequency factors obtained for the two peaks are respectively 7.16·10¹⁹ and 1.42·10²⁰ s^–1. E _c, k ₀ and the Avrami exponent (n) have also been derived by fitting the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation for the transformed volume fraction (x) to the crystallization data. JMAK results of E _c for the first and second crystallization peak turn out to be 270 and 290 kJ mol^–1 respectively. However, k ₀ and n are found to be heating rate dependent as reported in similar studies. The values of n for the first crystallization stage ranges between 1.66 and 2.57 indicating diffusion-controlled transformation in agreement with earlier reports.     

Effect of polydispersity on the crystallization kinetics of suspensions of colloidal hard spheres when approaching the glass transition

We present a comprehensive study of the solidification scenario in suspensions of colloidal hard spheres for three polydispersities between 4.8% and 5.8%, over a range of volume fractions from near freezing to near the glass transition. From these results, we identify four stages in the crystallization process: (i) an induction stage where large numbers of precursor structures are observed, (ii) a conversion stage as precursors are converted to close packed structures, (iii) a nucleation stage, and (iv) a ripening stage. It is found that the behavior is qualitatively different for volume fractions below or above the melting volume fraction. The main effect of increasing polydispersity is to increase the duration of the induction stage, due to the requirement for local fractionation of particles of larger or smaller than average size. Near the glass transition, the nucleation process is entirely frustrated, and the sample is locked into a compressed crystal precursor structure. Interestingly, neither polydispersity nor volume fraction significantly influences the precursor stage, suggesting that the crystal precursors are present in all solidifying samples. We speculate that these precursors are related to the dynamical heterogeneities observed in a number of dynamical studies.     

Thermal analysis and crystallization kinetics of polyurethane

Journal of Thermal Analysis and Calorimetry - Non-isothermal crystallization kinetics of polyurethane was investigated by differential scanning calorimetry at different cooling rates. Various...     

Nanofiller effect on the glass transition of a polyurethane

The effect of silicananofiller on the glass transition of a polyurethane was studied by DSC. Thepristine polymer exhibits a single glass transition at about –10°C.Uniform SiO2 spheres with different average sizes and narrow size distributionswere synthesized in solution by the Stöber method [1]. Both the effectsof silica content within the polymer and particle size were investigated,as well as two different surface treatments. Scanning electron microscopy(SEM) clearly confirms the presence of the particles within the polymer matrix,showing uniform distribution and no agglomeration. While shifting of the glasstransition has been reported by many authors, we have not seen any noticeableshift in this polymer. Surprisingly, we found no relevant effects when eitherincreasing the filler content or changing the particle size. Different amountsof particles with average diameters of 175, 395 and 730 nm did not affectthe glass transition temperature of the pristine polymer.     

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.     

Preparation of polymer nanoparticles, and the effect of nanoconfinement on glass transition, structural relaxation and crystallization

In this review the preparation methods of polymer nanoparticles from chemical microemulsion polymerization to physical methods such as spray-drying, freeze-drying, freeze-extracting, fast evaporation and spreading evaporation have been summarized. The influence of nanoconfinement on glass transition temperature (T _g) variation from significant or slight decrease, no evident T _g deviation, to even T _g increase, as well as possible explanations of T _g deviations were discussed. The influences of nanoconfinement or entanglement on the other properties such as structural relaxation, crystallization in polymer nanoparticle samples were also reviewed in this article.     

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.     

The glass transition and crystallization of ball milled cellulose

Samples of ball milled cellulose were prepared by ball milling pulps from eucalyptus and softwood (spruce/pine). Water sorption isotherms were obtained by both dynamic vapor sorption and equilibration over saturated salt solutions, in the water content range of 5–42% db (db = dry basis; water as a % age of total solids). Dynamic mechanical analysis using a pocket technique showed a water content dependent thermal transition occurring at the same temperature for the two pulp samples, which was interpreted as a glass transition. Fitting the data to a Couchman–Karasz relationship predicted a value for T _g of the dry cellulose of approximately 478 K, which was similar to values previously reported for other dry polysaccharides. No clear glass transition could be observed calorimetrically, although an endotherm at approximately 333 K was measured, which in polymers is normally attributed to enthalpic relaxation, however the lack of dependence of this endotherm on water content suggests that the melting of some weak associations, such as residual hydrogen bonds, could be a more credible explanation. An exotherm was also observed on heating, which was dependent on water content and which was attributed to partial crystallization of the cellulose. This was confirmed by Wide angle X-ray diffraction and cross polarization magic angle spinning ¹³C NMR (CPMAS NMR). The recrystallisation was predominantly to form I of cellulose. This was thought to be caused by a small amount of residual form I (probably less than 5%) acting as a template for the crystallizing material. Differential scanning calorimetry reheat curves showed the appearance of freezable water for water contents higher than 20%, as a result of a transfer of water to the amorphous phase following crystallization. The increase in cellulose rigidity following crystallization was also confirmed by CPMAS NMR relaxation. Low resolution proton NMR T ₂ relaxation suggested the presence of proton water/cellulose exchange, which was active at water contents of 20% and above.     

Analysis of thermodynamic behaviour of bulk metallic glass forming melts and glass forming ability

The thermodynamic behaviour of bulk metallic glass (BMG) forming melts have been studied by analyzing the temperature dependence of the Gibbs free energy difference (∆G), entropy difference (∆S) and enthalpy difference (∆H)between the undercooled melt and the corresponding equilibrium solid phases. The study is made by calculating∆G, ∆S and ∆H in the entire temperature range T _m (melting temperature) to T _g (glass transition temperature) using the expressions obtained on the basis of Taylor’s series expansion. The entire analysis is made for La-based five samples of BMGs; La₅₅Al₂₅Ni_20, La₅₅Al₂₅Ni₁₅Cu₅, La₅₅Al₂₅Ni₁₀Cu₁₀, La₅₅Al₂₅Ni₅Cu_15, and La₅₅Al₂₅Ni₅Cu₁₀Co₅ and a comparative study is also performed between present results and results obtained in the framework of expressions proposed by earlier workers. An attempt has also been made to study the glass forming ability for BMGs.     

Corrosion performances of a Nickel-free Fe-based bulk metallic glass in simulated body fluids

The corrosion performance of a Nickel-free Fe-based bulk metallic glass (BMG), Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ alloy, in Hank’s solution with pH value 7.4 and artificial saliva solution with pH value 6.3, was investigated by electrochemical techniques, aiming to assess the feasibility of Fe-based BMG as potential biomaterial. It was found that Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ BMG shows superior corrosion resistance in both simulated body fluids (SBF). The EIS analysis and cyclic polarization measurements indicated that the Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ BMG has larger polarization resistance value than that of 316L SS. The pitting corrosion potentials of Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ BMG are much higher than that of the 316L SS, resulting in very few ions releasing into the SBFs while a significant amount of Ni and Fe ions release was found for 316L SS under the same condition.     

The influence of grain size on the overall kinetics of surface-induced glass crystallization

A simple kinetic model of surface-induced glass crystallization is proposed. The grain size of glass powders, a constant density of surface nuclei and a steadily increasing temperature throughout the reaction are taken into account. The crystal growth rate and the density of surface nuclei can be estimated easily from overall kinetic curves (e.g. DTA curves) of powder of different grain size.The usefulness of the model is demonstrated in the case of the primary surface-induced crystallization of cordierite glass powders.     

Generalized kinetics of overall phase transition explicit to crystallization

Modeling of process for reaction kinetics is a fashionable subject of publications. This paper tests various phenomenological models starting with the Verhulst logistic functions, Kolmogorov adaptation up to the renowned KJMA and SB variants. The meaning of both the mortality (α) and fertility (1 ? α) terms are mathematically analyzed in detail involving variation of their power exponents, their characteristic points, and shape examination of the rate curves. Kolmogorov derivation of nucleation-growth process is revised.     

Nonisothermal crystallization kinetics and microstructure evolution of calcium lanthanum metaborate glass

This paper reports results on the crystallization kinetics of 35.5CaO–7.25La₂O₃–57.25B₂O₃ glass under nonisothermal conditions based on the studies carried out from the differential thermal analysis upon using various well-established models. The crystalline phases formed during the optimized ceramization process have been confirmed from the X-ray diffraction. The activation energies of the first (formation of CaLaB₇O₁₃) and second (formation of LaBO₃) crystallization events have been estimated using the conventional methods of Kissinger, Augis–Bennett, Ozawa, and Matusita, and the results are found to be in good agreement with each other. The Avrami exponents that are determined by these models for the crystallization of CaLaB₇O₁₃ and LaBO₃ are found to be in the range of (1.81–2.35) and (4.03–4.65), respectively. This indicates that the formation of CaLaB₇O₁₃ is dominated by a surface crystallization, whereas LaBO₃ is formed by three-dimensional bulk crystallization with an increased rate of nucleation. This observation is further validated by microstructural investigation, which shows the formation of CaLaB₇O₁₃ phase as a surface layer and a bulk crystallization of LaBO₃ in optimally ceramized samples.     

Effects of chain microstructure of butadiene-isoprene copolymers on their glass transition and crystallization

<正>A series of butadiene-isoprene copolymers(BIR) with various compositions were synthesized with a neodymiumbased catalyst system.The microstructure,composition and sequence of copolymers were characterized by FTIR and ~(13)C-NMR spectroscopy.The crystallization behavior of the BIR copolymers was investigated by DSC analysis.The results demonstrate that the content of cis-1,4 configuration in both butadiene(Bd) and isoprene(Ip) units are around 98%when Bd content in feed(f_(Bd)) covering the range from 55.7 mol%to 96.0 mol%.The reactivity ratios of Bd and Ip were determined to be 1.40 and 0.48 respectively.The random copolymers of Bd and Ip show only one glass transition temperature(T_g) from -107.4℃to -80.5℃,which is dependent on the composition and fits nicely with Fox equation.The sequence distribution followed the first-order Markov statistical model.It is found that the copolymer chains with higher Bd content contain longer polybutadiene(PBd) segments,and the sequence length of PBd segments(N_(Bd)) exhibits great influence on the crystallization behavior of the copolymer.The copolymers with N_(Bd)≥11.8 could crystallize at low temperatures(-71℃to-43℃).The crystallization temperature and enthalpy values decreased gradually with decreasing N_(Nd).The copolymers with N_(Bd)≤7.9 are amorphous even at very low temperatures(0℃to-150℃) due to the short PBd segments.