Volume 104, Issue 6

Continuous cooling transformation diagrams were plotted for a microalloyed steel in dilatometric tests at different cooling rates, and maximum and minimum cooling rates for bainite formation were determined. Austenite grain and bainite packet sizes were measured in three bainitic microstructures obtained by continuous cooling from austenitisation temperatures of 950, 1 050 and 1 150 °C, respectively. The results show that bainitic packet growth cannot exceed the austenite grain boundaries where it has nucleated. V(C, N) particle size was measured using field emission gun scanning electron microscopy and transmission electron microscopy, respectively. Both techniques yielded very similar values, but transmission electron microscopy is preferable due to its higher resolution and the possibility to carry out electron diffraction and dispersive X-ray analyses. The presence of precipitates has contributed to raising the yield strength, which can be predicted by Orowan's expression.

Notches lead to a decrease in the strength of machine parts in comparison to unnotched structures. The strength reduction is caused by stress concentration at the notch base. In static or dynamic calculations this effect is considered by the notch stress concentration factor. In this paper, simulations with a titanium single-crystal were carried out to analyze the dependence of the notch-stress concentration factor on the orientation of the anisotropic single-crystal material. Furthermore, numerical simulations and experiments with notched commercially pure titanium specimens have been performed to compare the stress–strain relationships until the ultimate tensile stress. The strain rate and grain size have been varied to identify dependencies on the microstructural situation which may influence the strength during the tests. After the tests, interrupted at ultimate tensile stress, metallographic sections were taken to reveal possible changes in microstructure. In the present investigation it is shown that for the single-crystal the notch-stress concentration factor depends strongly on the crystal orientation. Evidence is found that the grain size affects the strength of a notched specimen negatively, when the grain size is of the same size as the notch radius.

In this study, we used scanning probe microscopy to observe the abrasion damage of annealed Si–Ge superlattices. Specimens were more amenable to plastic deformation after annealing, thereby providing higher friction coefficients as a result of decreased strain energy. Restricted dislocation movement appeared not only in the combined Si–Ge superlattices but also at the interfaces, suggesting that thermal treatment could decrease the shear resistance. The microstructures contained a relatively large number of defects after annealing; these defects presumably led to the change in the oscillation trends under the ramped force of the sliding cycles. The component damage and oscillation events of the specimen that had been treated only at room temperature were more serious than those of the diffusion annealing specimens.

A dental Ag–Au–Cu–Pd alloy with small amounts of Zn and Ir was examined to determine the association of the age-hardenability with the phase transformation process and microstructural changes by means of hardness testing, X-ray diffraction study, field emission scanning electron microscopic observation and energy dispersive spectrometry. The fast and apparent age-hardenability of the present alloy was possibly attributed to phase separation by spinodal decomposition and the formation of a fine and coherent cuboidal structure composed of the Ag–Au-rich α 1 phase and the metastable Cu–Au-rich α 2 ′ phase. The lamellar-forming grain boundary reaction was induced by internal strain before the maximum hardness value was obtained. Moreover, growth of the grain boundary lamellar structure toward the grain interior and its coarsening acted as a strain-releasing softening mechanism, which caused a decrease in age-hardenability. A change in Ag and Pd content might alter the start point of lamellar-formation and the age-hardenability of Ag–Au–Cu–Pd alloys.

The results of calorimetric study and phase equilibria investigation of the Au–Ga system, in the Ga-rich region, are presented. Calorimetric study was done using Oelsen methodology and partial and integral molar thermodynamic quantities were determined at a temperature of 873 K. Phase equilibria investigations were done experimentally – using differential thermal analysis, light optical microscopy and scanning electron microscopy with energy-dispersive X-ray spectroscopy, and theoretically – using the CALPHAD method. Results agree well with available literature data.

Various silver nanostructures were fabricated by means of ethylene glycol reduction of silver nitrate together with poly (vinyl pyrrolidone) through a polyol-thermal method. Variable valency metal ions (iron and copper) affected the morphologies of these nanostructures. Because of enhanced reduction ability compared with Cu + ions, Fe 2+ ions resulted in Ag + being reduced to Ag 0 efficiently. Regular silver nanowires were prepared using copper salt, while quasi-flag types of silver nanostructures were obtained in the case of iron salt. Quasi-flag types of silver nanostructures were also created successfully using both CuCl 2 and FeCl 2 in the reaction system. Furthermore, the diameter and morphologies of silver nanostructures were adjusted by changing the amount of poly (vinyl pyrrolidone) and CuCl 2 . A mechanism is proposed to understand the morphology evolution of silver nanostructures.

Nanowires, nanobelts, and nanotubes of vanadium oxides have been fabricated using the hydrothermal method with surfactants in one reaction system. The nanowires, nanobelts and nanotubes of vanadium oxides can be synthesized selectively by adjusting and controlling the reaction conditions such as surfactants selected, reaction temperature and time, etc. The as-prepared one-dimentional nanomaterials were characterized by means of X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, electron diffraction, and high-resolution transmission electronmicroscopy. The reaction mechanism is also explained according to the experimental data.

In the present study, ultrafine TiO 2 nanowires (∼80 nm in diameter) exhibiting large aspect ratio in the order of 10 3 were synthesized hydrothermally. Phase and morphological analysis of the nanowires was carried out using X-ray diffractometry, X-ray photoelectron spectroscopy and scanning electron microscopy. High resolution transmission electron microscopy revealed the wire exhibiting growth in (101). A Tauc plot derived from UV analysis showed the average band gap values for nanowires to be less than for nanoparticles of similar diameter. It was observed that nanowires exhibited a high degree of photoactivity in an eosin-based dye system which was found to be 20 – 30 % more than that of nanoparticles. This high photoactivity in nanowires was attributed to the longer charge retention which was observed during lifetime measurements, resulting in easy radical formation and dye degradation. Lifetime measurements on the nanowires showed the recombination time to be 54 ns as compared to 43 ns for nanoparticles.

The kinetics of thermal dehydration of mixed hydroxide hydrogels in MgO–ZrO 2 system was studied as a function of composition by following the isothermal heat treatment route. Kinetic parameters were calculated through the application of the Guggenheim equation. The expulsion of both loosely bound water and constitutional OH groups were not continuous processes but proceeded in steps. The applicability of 1 st order reaction kinetics for the major portion of the reaction for all compositions suggests that during the dehydroxylation process the dehydration is essentially controlled by the orientation of H 2 O molecules and the mutual interaction of hydroxyl groups. Due to a decrease in the concentration of the reacting species the activation energy was always higher at the final stage of dehydration.

The reaction process of carbothermal nitridation synthesis of β-Si 3 N 4 from serpentine is investigated in this paper. Samples with different molar ratios of carbon black to serpentine were calcined under nitrogen atmosphere at 1300 °C, 1400 °C and 1500 °C for 3 hrs. The phase composition and microstructure of the synthesized powders were determined using X-ray diffraction and scanning electron microscopy. The reaction process was analyzed by means of differential thermal analysis. The results show that the main reaction was that forsterite from serpentine reacted with carbon black to form SiO, Mg vapor and CO, then SiO and N 2 generated β-Si 3 N 4 . Fine β-Si 3 N 4 powders were produced under nitrogen atmosphere at 1500 °C when the carbon black/serpentine molar ratio was 6.0. The synthesized β-Si 3 N 4 powders had uniform grain size and high β-Si 3 N 4 content.

Silicon carbide has been used in refractories, ceramics, and numerous high-performance applications with very good mechanical properties and high thermal conductivity. Silicon carbide composites have excellent potential as a low-activation structural material for fusion and industrial applications. Thermal properties of silicon carbide ceramics are important for the design and safe operation in these situations. To obtain the thermal properties of silicon carbide ceramics, based on the nonequilibrium molecular dynamics simulation method, the size effect on thermal conductivity in different directions is described. 6H – SiC thermal conductivities in the normal and tangential directions are predicted in a nanoscale cuboid system. It is found that there is an obvious size effect and anisotropy of thermal conductivity in different directions because of the impact of the boundary scattering. The normal and tangential thermal conductivity equations have been obtained as good power function expressions. The results show that boundary scattering is strong in phonon transport in different directions within thin film silicon carbide.

While “like attracts like” is common sense for large surfaces, here we show that in a nanopore the effective solid–liquid interfacial tension can be quite independent of the liquid polartiy. Moreover, as the liquid molecules and ions are confined, their behavior can be either reversible or irreversible, depending on the liquid composition. These unique phenomena can be attributed to the confinement effects and the absence of bulk liquid phase in the nanoenvironment.

This paper evaluates the compression deformation behaviour and energy absorbing properties of honeycomb paperboards with different thicknesses under homogeneous vertical static compression loading. Each piece of the static compression stress and strain curves of the honeycomb paperboards is similar and comprises three parts of compression deformation (elastic region, plateau region, and densification region), which reflects the mechanical properties such as approximately linear elastic deformation, large plastic deformation with local collapse and densification. The unit volume deformation energy and strain curves and two kinds of static compression cushioning curves show that within the elastic region and plateau region of the stress and strain curves, the honeycomb paperboards hold better energy absorbing properties and the change of the cushioning factor with an increase in the thickness of honeycomb paperboards is very small.

V 2 AlC was synthesized via self-propagation high-temperature sintering using V/Al/C powders as raw materials. Results showed that the main phase of the product was V 2 AlC in the sample from 2V/Al/C powders. A low amount of VC 0.75 was also observed in the product. The weight contents of V 2 AlC and VC 0.75 in this sample were 94.5 % and 5.5 %, respectively. Excess Al additives clearly improved the synthesis of V 2 AlC. For the raw material 2V/1.1Al/C, single-phase V 2 AlC material can be obtained.

Densification behavior, microwave and mechanical properties of the low temperature co-fired CaO–B 2 O 3 –SiO 2 glass-ceramic with additions of La 2 O 3 –B 2 O 3 and SiO 2 were investigated. The results showed that the La 2 O 3 –B 2 O 3 addition promotes the crystallization of the CaSiO 3 and clearly enhances the flexural strength of the CaO–B 2 O 3 –SiO 2 glass-ceramics. Si was added to adjust the shrinkage rate of the glass-ceramics to meet the actual use and the dielectric and mechanical properties were not significantly affected. The CaO–B 2 O 3 –SiO 2 -6 wt.%La 2 O 3 –B 2 O 3 -7 wt.%Si sample showed a flexural strength of 160 MPa, ∊ r of 6.13 and loss of 12.34 × 10 –4 (10 GHz), shrinkage rate of 14.9 %.