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The activation energies of heterogeneous equilibrium phase transformations have been frequently determined by Kissinger(-like) analysis. This work demonstrates that the applicability of Kissinger(-like) method(s) to determine activation energies of heterogeneous phase transformations is justified if the phase transformation occurs far away from the equilibrium phase-transformation temperature but not if the phase transformation is investigated in the vicinity of the equilibrium phase-transformation temperature. Experimental results on the kinetics of the order-disorder transformation in NiSn, obtained both near the equilibrium phase-transformation temperature and considerably below it (using quenched samples), expose the non-validity and confirm the validity, respectively, of activation-energy values obtained by Kissinger(-like) analyses applied in the corresponding temperature ranges. Application of the Kissinger(-like) analysis in a temperature range close to the equilibrium phase-transformation temperature leads to erroneous, abnormally high values for the activation energy. [ABSTRACT FROM AUTHOR]

Polycrystalline samples of Al modified lead lanthanum zirconate titanate [PLZT] with Zr = 55% and Ti = 45%, have been synthesized by a high-temperature solid-state reaction technique. X-ray diffraction analysis of the compounds suggests the formation of a single-phase compound with tetragonal structure. Dielectric studies of the compounds as a function of temperature (from 30 to 350°C) at frequency (1, 10 and 100 kHz) show that the compounds undergo a phase transition of diffuse type.The transition temperature shifts towards higher side with increase in frequency a typical characteristic of a relaxor material. Diffusivity (γ) study of phase transition of these compounds provided its value between 1 and 2, indicating the variation of degree of disordering in the system. Measurement of dc resistivity (ρ) as a function of temperature (Room temperature, RT to 350°C) at a constant biasing field suggests the compounds have negative temperature coefficient of resistance (NTCR). [ABSTRACT FROM AUTHOR]

This paper focuses on a novel syntactic foam formulation based on a model diepoxy-diamine matrix with a controlled architecture, discusses the factors governing the long-term performance of these materials and gives a predictive model to assist in the design of efficient and safe insulating systems. Hygrothermal ageing in deionized water at 20°C, 60°C, 100°C and 120°C over 18 months (with no additional pressure) is followed by both gravimetric and impedance measurements. This original protocol provides the evolution with time of both mass gain and intrinsic material conductivity. Attention is paid to the degradation phenomenon observed after the matrix has reached saturation and the corresponding increase of both mass gain and conductivity. The latter suggests the occurrence of ionic extraction from the microsphere glass undergoing water leaching. A model of mass gain is proposed to explain the gravimetric data and predict the long-term mass gain that governs the mechanical and thermal performances. The temperature dependence of fitting parameters follows an Arrhenius law and activation energies calculated support the scheme of glass corrosion by water during hygrothermal ageing, with respect to the literature. [ABSTRACT FROM AUTHOR]

Single crystals of the B2 CoTi intermetallic compound were deformed in compression at temperatures between room temperature and 1000 K. The orientation was about 10° off the [001] cube axis. For B2 compounds with `strong’ AB bonding, the slip dislocations are often $$\langle 100\rangle$$ type gliding on $$\{110\}$$ planes although $$\langle 110\rangle$$ and $$\langle 111\rangle$$ slip cannot be ruled out especially when, as in this case, the Schmid factors for $$\langle 100\rangle$$ glide are very small. In the lower half of the temperature range, deformation progressed by a series of load drops which manifests itself as coarse bands formed at intervals along the gauge length. The bands are roughly parallel to the (001) plane but it is concluded that the slip planes are $$\{110\}$$ and that the kink-like behaviour is due to the formation of twist boundaries. A model for this behaviour is presented which is based on a pole mechanism and the glide of $$\langle 100\rangle$$ dislocations. [ABSTRACT FROM AUTHOR]

Creep behavior of the lead-free Sn-5%Sb solder alloy was studied by long time Vickers indentation testing at room temperature. Based on the steady-state power law creep relationship, the stress exponents were determined for the cast and wrought materials in the homogenized and unhomogenized conditions. The stress exponent values of 4.5 and 12, depending on the processing route of the material, are in good agreement with those reported for the same material in conventional creep testing at room temperature. The results are discussed on the basis of the microstructural features developed during different processing routes of the material. [ABSTRACT FROM AUTHOR]

This article reports that in ceramic multilayered composites, consisting of layers of different composition, residual stresses are generated by different sintering strain rate between layers and thermal expansion mismatch between the layers during cooling down from sintering temperature. In Al2O3/Al2O3+3Y-TZP multilayered composites, layers with the lower thermal expansion coefficient (CTE), that is Al2O3, undergo compressive residual stress while tensile stress appears in Al2O3+3Y-TZP (AZ) layers with higher CTE. This stress field directly affects the mechanical properties of the composites and it is responsible for cracking and/or delamination. In addition, spontaneous cracking upon cooling is sometime observed as a result of different sintering strain rate and excessive mismatch in CTE between layers.

The knowledge of the Seebeck coefficient of materials is essential in determining the actual solid/liquid interface temperature during the melt growth of crystals. In this study, the Seebeck coefficient of the solid and liquid germanium was measured employing the small ?Ttechnique. The Seebeck coefficient increased from -107 µ V/°C at 857°C to -54 µ V/°C at 931°C in the solid. However, an average constant value of -0.6 µ V/°C was obtained in the liquid from 938 to 960°C, above which a value of -18 µ V/°C was recorded. Also, when the liquid samples were cooled below the melting point, a melt supercooling was observed, which seemed to be larger with higher melt temperatures. [ABSTRACT FROM AUTHOR]

The high solar energy density achieved in our simple and cheap Fresnel installation has been used for several surface modifications of metallic materials. This equipment is a very useful tool to apply concentrated solar energy in the field of high and very high temperatures (1500-2000 K). These temperatures are achieved in a few seconds and usually the materials treatments are completed in minutes. Fresnel lens installation is a serious alternative to the conventional equipment for material treatment and even to the large solar installations. In this work we review the surface modifications produced by concentration of solar energy with a Fresnel lens. [ABSTRACT FROM AUTHOR]

Crack growth at elevated temperature has been examined in a new fine-grained nickel-based superalloy under triangular, fast-slow, slow-fast, dwell and sustained loading conditions at 650 and 725°C. The effect of loading waveform seems to be minimal for base frequencies over 0.01 Hz with a mixture of time and cycle dependent crack growth observed for all but the fast-slow waveform, where the crack growth remained cycle-dependent and the crack growth rate mostly constant. For base frequencies less than 0.01 Hz, crack growth under dwell load clearly accelerated and the crack growth rates were comparable with those under sustained load. Creep contribution was found to be negligible while crack tip constraint may be relevant to the out-of-plane crack growth observed under predominantly sustained load conditions. [ABSTRACT FROM AUTHOR]

Reports on the effect of strain rate and temperature on the dynamic tensile properties of unidirectional glass fiber reinforced polyester (GFRP). Indication of the strain rate and temperature dependence of the glass fiber bundles; Result of tensile tests on unidirectional GFRP at different temperatures; Establishment of a bi-modal Weibull statistical constitutive equation to describe the material.

This article focuses on relaxor ferroelectrics which have attracted considerable attention both in fundamental understanding of the structural and physical properties and in practical applications. The obvious feature of relaxor ferroelectric is the broad peak that appears in the dielectric constant as a function of temperature and frequency. The positions of the maxima in the dielectric constant are shifted toward higher temperatures as the frequency is increased. Analogs to spin glass in magnetism, the dipolar glass behavior of relaxor ferroelectrics was proposed. It is usually believed that the dielectric behavior of polar species, due to composition and field in homogeneities, is responsible for the glassy behavior, although the mechanism of the relaxor ferroelectric behavior has not been fully understood.

The article focuses on effect of annealing temperature on tensile behavior of 5052 Al alloy deformed at cryogenic temperature. Recently, deformation at cryogenic temperature has attracted much attention as one of the effective processes to produce ultrafine grained materials. The suppression of dynamic recovery during deformation at extremely low temperatures is expected to preserve a high density of defects generated by deformation, which can act as the potent recrystallization sites. Accordingly, the cryogenic deformation would require less plastic deformation for achieving ultrafine grains, compared to the severe plastic deformation processes at ambient or elevated temperatures.

This article presents information on fabrication of Ti&sub2;AlC-Ti&sub3;AlC&sub2;Ti&sub3;SiC&sub2; composite by spark plasma sintering method from elemental powders. Recently, a new remarkable group of materials known as ternary-layered compounds or machinable ceramics has attracted interest of materials scientists. These ternary-layered ceramics are thermally and electrically conductive, relatively soft and readily machinable, they are relatively tough and not susceptible to thermal shock, and behave plastically at elevated temperatures.