Correlation between the thermal change of resistance and the process of devitrification of glassy titanium based alloys

Investigations were carried out on the temperature dependence of electrical resistance of glassy alloys based on titanium: 52% Ti, 39% Cu, 8% Ni and 1 wt.% Si (A); 39% Ti, 51% Cu, 10 wt.% Ni (B); 60% Ti, 13% Cu and 27 wt.% Ni (C); 50% Ti, 45% Cu and 5 at.% Ni (D), from room temperature up to 30–40 K above the crystallization temperature. It was shown that the temperature coefficients of resistance in the glassy state region are negative for alloys A, C and D and positive for alloy B. After crystallization, with all alloys except C, coefficients are positive. A clear correlation exists between the stepwise process of crystallization of the glassy state and the change of electrical resistance. Each step of devitrification is followed by a discontinuous change (decrease) of the resistance which is caused by the increase of the mean free path of electrons because of the growth of the ordering of the alloy structure.     

Nano structure of rapidly quenched Cu–(Zr or Hf) – Ti alloys and their devitrification process

The structure and primary devitrification process of the melt-spun Cu₆₀(Zr or Hf)₃₀Ti₁₀ alloys were investigated. It was confirmed that the compositional segregation in the diameter range of 5–10 nm exists in the as-quenched state. The nanocrystalline particles with cubic structure are observed in the glassy matrix in thehigh-resolution transmission electron microscopy images, of which size is corresponding to the scale of compositional segregation. Small-angle X-ray scattering measurement also indicates the development of nanoscale inhomogeneity with the same size as that of nanocrystalline particles. The nanocrystalline region has high Cu content. In contrast, Zr or Hf and Ti elements are enriched in the glassy region. These results are recognized as the formation of novel structure consisting of the glassy and nanocrystalline phases. It is suggested that the precipitation of bcc CuZr phase as a primary crystallization phase proceeds in the glassy phase remaining the nanocrystalline phase in the Cu–Zr–Ti alloy. Meanwhile, the glassy and nanocrystalline phases are transformed to an orthorhombic Cu₈Hf₃ phase at the initial crystallization stage in the Cu–Hf–Ti alloy. These differences of crystallization process are consistent with the results of thermodynamic and kinetic analyses of the transformation mode.     

Bonding thermosprayed silicon to glass-ceramic

Many materials, among them silicon, fail to form adherent coatings when thermosprayed onto smooth oxide glasses and glass-ceramics. Mechanical bonding is usually possible if the substrate surface is abraded before spraying, but this process also weakens the crack-sensitive substrate. This paper gives the results of examining alternate surface preparations which create a roughened substrate without the usual weakening of the finished article. The methods rely on the ability of mechanically damaged glass articles to heal, i.e. to decrease the flaw population during crystallization. The surface roughness essential to the mechanical bond is produced when the article is in the green or glassy state, and in a subsequent processing step the glass is converted to glass-ceramic by a crystallization heat treatment. The simplest application is the fabrication of roughened durable ceramic substrates for spraying after the crystallization step. Profilometry and measurements of silicon metal coating adhesion to a lithium aluminosilicate glass-ceramic indicate that for hard glasses the crystallization step does not reduce the surface roughness. When high temperature materials are to be used as coatings, the spraying step can be performed while the substrate in the glassy state, and then the entire article is subjected to the crystallization heat treatment. For a silicon-coated glass-ceramic, this method produces significantly improved adhesion over simple mechanical bonding.     

Crystallization kinetics of Zr60Al15Ni25 bulk glassy alloy

The crystallization kinetics of Zr₆₀Al₁₅Ni₂₅ bulk glassy alloy under isochronal and isothermal conditions has been investigated by differential scanning calorimetry (DSC). The microstructure of as-cast Zr₆₀Al₁₅Ni₂₅ bulk glassy alloy is observed by high-resolution electron microscopy (HREM). It is found that there exist nanocrystals with a size of about 7 nm in the glassy matrix, which are not observed in the XRD image. The results of Kissinger analysis show that the effective activation energies for glass transition (457 kJ/mol) and crystallization (345 kJ/mol) are high, indicating that it has large thermal stability against crystallization. The crystallization of Zr₆₀Al₁₅Ni₂₅ bulk glassy alloy under isothermal annealing can be modeled by the Johnson-Mehl-Avami equation. The crystallization kinetics parameters show that the isothermal crystallization starts from the growth of the pre-existing nanocrystals and the crystallization process is diffusion-controlled.     

Toughening of polyester resins through blending with polyolefins

The effect of blending low concentrations (1 to 10%) of polyolefins on the crystallization and irreversible deformation behaviour of polyester (PET) resins was investigated. The olefin particles did not nucleate crystallization of PET from the melt but did depress the rate of crystallization. Decreases in the cold crystallization temperature of PET during heating from the glassy state in blends with linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) were attributed to stress induced crystallization created by large volume expansions associated with melting of the olefin particles. A sharp transition in fracture strain for PET and the PET-olefin blends was observed with strain rate, where at low strain rates fracture occurred during work hardening, and as the strain rate increased, fracture occurred during cold drawing. The transition corresponded with a decrease in draw stress, a decrease in draw ratio and an increase in density of the neck. A shift in the transition to higher strain rates for the blend compositions was attributed to increased rates of crystallization and orientation due to strain induced crystallization at the stress fields surrounding the olefin particles. As the strain rate increased and the cold drawing process became more adiabatic, the mechanical behaviour was controlled by the kinetics of crystallization and orientation.     

Study of crystallization kinetics and structural relaxation behavior in phase separated Ag33Ge17Se50 glassy alloys

We report on the crystallization processes and structure (crystal phases) of Ag₃₃Ge₁₇Se₅₀ glassy alloy using differential scanning calorimetry and x-ray diffraction techniques, respectively. The devitrification that gives rise to the first exothermic peak results in the crystallization of Ag₂Se and Ag₈GeSe₆ phases, while the growth of GeSe₂ accompanied by the transformation of Ag₈GeSe₆ to Ag₂Se phase occurs during the second crystallization process. Different theoretical models are used to elucidate various kinetic parameters for the crystallization transformation process in this phase separated system. With annealing below the glass transition temperature, an inverse behavior between the variation of the optical gap and the band tailing parameter is observed for the thermally evaporated films. These results are explained as the mixing of different clusters/species in the amorphous state and/or changes caused by structural relaxation of the glassy network for the thermally evaporated films.     

Effect of Ag addition on crystallization in As2Se3 glasses

Differential scanning calorimetry (DSC) behaviours of glassy As₂Se₃ containing Ag up to 5 at % were measured at various heating rates. The effect of addition of Ag on crystallization of glassy As₂Se₃ was investigated. The glass transition temperature decreased with increasing Ag content. Crystallization kinetics were analysed on the basis of the two-step process model, where crystal growth takes place after nucleation. For non-doped As₂Se₃ two-dimensional growth of crystal was predominant, while for Ag-doped ones three-dimensional growth was very likely. It was supposed for all glassy Ag-doped samples that crystal nuclei exist, though micro-crystallites were not observed on X-ray diffraction traces. The activation energy for crystallization and the glass-forming tendency decreased by the addition of Ag.     

A study on the crystallization of amorphous arsenic

The results of DTA analysis of amorphous arsenic in combination with electron microscopical study of the samples through the whole temperature range reveal that no changes of the initial state of the sample occur on heating up to crystallization temperature. Close above this temperature the spontaneous crystallization starts. It differs markedly from the case of glassy materials, like As₂Te₃, in which case at temperatures as low as the transition region T_g the crystallization nuclei of geometrically defined shapes appear and grow into crystalline grains on further heating.     

Characterization of crystal phases,morphology and crystallization processes in lithium aluminosilicate glass-ceramic

The nucleation and crystallization processes of Li₂O-Al₂O₃-SiO₂ glass-ceramics were investigated by differential thermal analysis. The crystalline phases produced during thermal treatment at different temperatures and the residual glassy phase were characterized by X-ray diffraction, SEM and image analysis techniques. The activation energy of the crystallization process was calculated as E=380±20 kJ mol^–1. The influence of nucleating agents (TiO₂, ZrO₂) was evaluated to obtain glass-ceramics transparent to visible light. The stability of the glassy phase in cooling was determined by means of temperature-time-transformation curves.     

PC／PET共混物的非等温结晶动力学

采用等速变温ＤＳＣ法对ＰＣ／ＰＥＴ共混体系的非等温结晶动力学进行了研究，结果表明，从玻璃态结晶时，随着ＰＣ含量的增加，ＰＥＴ组分的结晶速率先增加后降低。耐从熔体结晶时，体系的结晶速率随着ＰＣ含量的增加而增加，讨论了ＰＣ对ＰＥＴ组分结晶过程的影响。     

High formability of glass plus fcc-Al phases in rapidly solidified Al-based multicomponent alloy

A multicomponent Al₈₄Y₉Ni₄Co_1.5Fe_0.5Pd₁ alloy was found to keep a mixed glassy + Al phases in the relatively large ribbon thickness range up to about 200 μm for the melt-spun ribbon and in the diameter range up to about 1100 μm for the wedge-shaped cone rod prepared by injection copper mold casting. The glassy phase in the Al-based alloy has a unique crystallization process of glass transition, followed by supercooled liquid region, fcc-Al + glass, and then Al + Al₃Y + Al₉ (Co, Fe)₂ + unknown phase. It is also noticed that the primary precipitation phase from supercooled liquid is composed of an Al phase instead of coexistent Al + compound phases, being different from the crystallization mode from supercooled liquid for ordinary Al-based glassy alloys. In addition, it is noticed that the mixed Al and glassy phases are extended in a wide heating temperature range of 588–703 K, which is favorable for the development of high-strength nanostructure Al-based bulk alloys obtained by warm extrusion of mixed Al + amorphous phases. The Vickers hardness is about 415 for the glassy phase and increases significantly to about 580 for the mixed Al and glassy phases. The knowledge of forming Al + glassy phases with high hardness in the wide solidification and annealing conditions through high stability up to complete crystallization for the multicomponent alloy is promising for future development of a high-strength Al-based bulk alloy.     

Surface nucleated crystallization in Ge15Te80As5 semiconducting glasses

The crystalline morphology of electrically conductive surface layers in thermally annealed bulk samples of Ge₁₅Te₈₀As₅ glass has been studied using a scanning electron microscope. Results confirm previous suggestions that crystallization is surface nucleated. Two regions of different crystalline morphology are observed and result from a two-stage crystallization process. Selected-area electron diffraction on extracted particles was used to identify the crystalline phases. The crystallites near the inner glassy material are Te, the first phase to segregate upon annealing. The crystalline material near the sample surface is more dense and contains two phases: GeTe (second-stage crystallization product) and crystalline Te. Energy Dispersive X-Ray Analysis with spacial resolution of 2–3 μm and accuracy of ±10% has shown that the “average” composition is the same for these crystalline regions and for the glassy material.     

Crystallization kinetics of J-1 polymer under different thermal treatments

A bis-para-amino cyclohexylmethane (PACM)-based polyamide homopolymer (J-1 polymer produced by Du Pont), utilized as a matrix for composites, was subjected to different thermal treatments in order to investigate its crystallization thermodynamics and crystallization kinetics. Various J-1 samples, quenched, annealed from the glassy state, isothermally crystallized from the melt and slowly cooled, were studied by differential scanning calorimetry (DSC). A thermodynamic melting temperature of 352.6 °C was determined from a Hoffman-Weeks diagram of polymer samples annealed at different temperatures between the glass transition and melting temperature. By using DSC isothermal crystallization data from the melt, the existence of two crystallization regimes, already found in a previous investigation, was confirmed, and a transition temperature between the two regimes, equal to 262.2 °C was determined, in good agreement with 260.5 °C, obtained by depolarized light measurements, reported elsewhere. Moreover, the ratio between the crystallization kinetics factor of two crystallization regimes is 1.87, very close to the value of 2 predicted by the Huffman theory. Crystallization of samples from the melt, at different cooling rates, was also performed. The Arrhenius plot of data indicated that the crystallization process proceeds with two distinct activation energies (589 and 244 kJ mol^–1), below or above a cooling rate of 2.67 °C min^–1, corresponding to a temperature of 253.9 °C. This result is in good agreement with the two crystallization regimes reported above.     

Physical and thermodynamic aspects of the glassy state,and intrinsic non-linear behaviour of creep and stress relaxation

Rheology in the highly viscous liquid and the glassy state is reviewed and discussed. A distribution relaxation time due to the co-operative molecular motion is exhibited both in the highly viscous liquid and in the glassy states. However, only in the glassy state has the structure been frozen-in at some particular internal state resulting from the incomplete establishment of a thermodynamic equilibrium state. Therefore, the intrinsic non-linear rheological behaviour of the glassy state is explained from the physical and thermodynamic aspects in the glass transformation region. The volume relaxation of soda-lime-silica glass and the thermal history of glass during forming process have been studied. Finally, the role of rheology in thermal stress and fracture mechanics is also mentioned, which subsequently will allow us to re-evaluate the mechanisms of toughening and weakening of composite materials.     

用差示扫描量热法研究Mg65Cu25Gd10块体非晶合金的晶化动力学

通过铜模吸铸法得到Mg65Cu25Gd10块体非晶舍金，用差示扫描量热仪（DSC）研究其晶化动力学和玻璃转变行为，玻璃转变温度Tg，晶化起始温度Tx，晶化峰值温度Tp都与加热速率有关，通过Kissinger方程可以得到表面激活能，发现晶化初始激活能Ex小于峰值激活能Ep，表明形核过程比生长过程容易，讨论了此非晶舍金的玻璃形成能力，根据JMA方程非等温模型研究了晶化动力学，Avrami参数表明在不同温度下的晶化机制是不同的．     

金属间化合物与大块玻璃合金的形成

综述了大块玻璃合金液态结构,结晶动力学行为,玻璃态形成机制以及其与金属间化合的关系,应用二元合金液态结构的理论模型,分析与推测,分析与推测大块玻璃合金的液相结构,解释了大块玻璃合金的特殊热力学性质和大块玻璃态形成机制,介绍了Ｚｒ基大块玻璃合金的一些研究结果。     

Kinetics of thermal devitrification (crystallization) of FexCryBz glassy alloys

Devitrification process of four glassy Fe _x Cr _y B _z alloys of different composition was investigated by both methods, differential-scanning-calorimetry and X-ray diffraction. It is shown that the crystallization process proceeds via one or two stages depending on the alloy composition. Kinetic parameters for the crystallization process (activation energy, rate constant, frequency factor) as well as the crystallization enthalpy are determined for all four alloys that are used. It is shown that exothermal maxima on DSC thermograms correspond to the crystallization process when polycrystalline Fe₂B and FeB phases are formed.     

Influence of seeding on crystallization behaviour of BaNaB<Subscript>9</Subscript>O<Subscript>15</Subscript> glasses

Transparent BaNaB₉O₁₅ (BNBO) glasses were fabricated via the conventional melt-quenching technique. X-ray powder diffraction (XRD) followed by differential scanning calorimetric (DSC) studies confirmed the amorphous and glassy nature of the as-quenched samples, respectively. The effect of seeding on the crystallization of BNBO glasses was studied by non-isothermal DSC method and was modeled using the Johnson-Mehl-Avrami and Ozawa equations. The activation energy for seeded glasses decreased with the increase in fraction of crystallization. The values for the onset of crystallization and Avrami exponent were found to be lower for seeded samples which were associated with the heterogeneous nucleation and epitaxial processes.     

Influence of seeding on crystallization behaviour of BaNaB<Subscript>9</Subscript>O<Subscript>15</Subscript> glasses

Transparent BaNaB₉O₁₅ (BNBO) glasses were fabricated via the conventional melt-quenching technique. X-ray powder diffraction (XRD) followed by differential scanning calorimetric (DSC) studies confirmed the amorphous and glassy nature of the as-quenched samples, respectively. The effect of seeding on the crystallization of BNBO glasses was studied by non-isothermal DSC method and was modeled using the Johnson-Mehl-Avrami and Ozawa equations. The activation energy for seeded glasses decreased with the increase in fraction of crystallization. The values for the onset of crystallization and Avrami exponent were found to be lower for seeded samples which were associated with the heterogeneous nucleation and epitaxial processes.     

Preparation,crystallization and properties of rapidly solidified YBa2Cu3O7−δ

YBa₂Cu₃O_7−δ was prepared in the glassy state by rapid solidification from the melt. The quenched material was paramagnetic down to liquid-He temperature. Diamagnetic behavior developed after crystallization of the glass at high temperatures (> 800°C). A well-defined superconducting transition occurred at 90 K for material heat treated and crystallized above 900 °C.