Volume 105, Issue 11

The Ta–W, W–Zr and Ta–W–Zr systems are critically reviewed and modeled using the CALPHAD technique. The enthalpy of formation of the stoichiometric compound W 2 Zr in the W–Zr system is predicted from first-principles calculations. The solution phases (liquid, bcc and hcp) are modeled by the substitutional solution model. The compound W 2 Zr is treated with the formula (Ta,W) 2 Zr in the Ta–W–Zr system because of a significant solid solubility of Ta in W 2 Zr. All experimental data, including the Gibbs energy of formation, enthalpy of formation, activity of Ta and W of bcc phase at 1 200 K, Ta–W and W–Zr phase diagrams, and three isothermal sections of the Ta–W–Zr system at 1 073, 1 098, and 1 873 K, are reproduced in the present work. A set of self-consistent thermodynamic parameters of the Ta–W–Zr system is obtained.

In order to investigate the influence of crystal-lattice defects (dislocations) and of a relatively large Al content on the development of different modifications of aluminium nitride in ferrite and on the associated uptake of excess nitrogen, recrystallised and cold rolled specimens of Fe-4.75 at.% Al alloy were subjected to gaseous nitriding treatments at 550 °C employing a nitriding potential of 0.104 atm −1/2 . In contrast with earlier results for an alloy of relatively low Al content, in both the nitrided cold rolled and the nitrided recrystallised specimens nanosized AlN precipitates developed, both of the metastable, cubic, NaCl type modification and of the stable, hexagonal, wurtzite type modification, which exhibit with the ferrite matrix a Baker–Nutting orientation relationship and a Pitsch–Schrader orientation relationship, respectively. A high-temperature hydrogen reduction treatment confirmed the stoichiometry of the precipitated nitrides as AlN. The fraction of cubic, NaCl type, AlN is significantly higher in the nitrided cold-rolled specimens because of its preferential development along dislocations. As compared to the nitrided recrystallised specimens, the higher dislocation density and the higher amount of cubic AlN in the nitrided cold-rolled specimens leads to a larger amount of excess nitrogen in the nitrided cold-rolled specimen.

The application of physical fields is currently a common method to improve the solidification structure of Al alloys. In this study, rotating magnetic field (RMF) and power ultrasonic field (USF) were applied during the solidification process of Al-8 wt.%Si alloy. The experimental results have been compared to those of conventional casting and have verified that the alloys processed under individual RMF or USF exhibit obvious refinement in their solidification structure as well as an enhancement in the mechanical properties. However, the refinement under RMF is not fully effective, while the ultrasonically treated region is inadequate. For the case of the compound fields, it could be furthermore observed that these improvements become more pronounced. Owing to the advantages of both RMF and USF, RMF enlarges the ultrasonic treated region, and USF improves the refinement effect of RMF as well.

The effects of holding temperature, time and cooling rate on the microstructure of Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1Si (at.%) alloys are studied. Three kinds of segregations are found in the as-cast material. In back scattered electron images these segregations are observed as dark regions formed by the solidification process, bright regions with irregular shaped blocks and imaged regions of lighter contrast formed by the cooling process from β phase to α phase and from α phase to (β + γ) two phase or (α + β + γ) three phase, respectively. Addition of small amounts of Cr, Ni and Si to the Ti-45Al-7Nb alloy shifts the (β + γ) two phase state and (α + γ + β) three phase state to a lower Nb concentration range. While cooling from the α single phase state to the (β + γ) two phase or (α + β + γ) three phase states, sequential type phase transformation occurs. The amounts of Cr, Ni and Si are too small to induce the pearlitic mode of transformation. Therefore, the sequential mode of the ternary alloy containing Nb occurs. The microstructures change depending on the cooling rate from α single phase region. Massive transformation occurs in the range of 300 K s −1 to 50 K s −1 . However, the α phase is partially retained at the cooling rate of 300 K s −1 . A fully lamellar structure appears at cooling rates lower than 10 K s −1 .

In order to determine the most suitable chemical composition for the basis of zinc-based ternary and quaternary alloys, five binary Zn-(5–25) Al alloys were prepared by permanent mould casting. Their microstructures and mechanical properties were investigated using metallography and hardness, tensile and compression tests. The microstructure of Zn-5Al alloy consisted of eutectoid α + η and eutectic η phases. However, the microstructures of relatively high aluminium-containing alloys (Zn-10Al and Zn-15Al) consisted of proeutectic β dendrites surrounded by η phase and small rounded α particles. The near eutectoid alloys (Zn-20Al and Zn-25Al) exhibited similar microstructures to those of Zn-10Al and Zn-15Al alloys, but with α-cored β dendrites surrounded by α + η particles. As the aluminium content increased the hardness and strength of the alloys increased, but their impact energy and density decreased. The total percentage elongation of the alloys also decreased with aluminium content, but after showing an initial increase. These observations are discussed in terms of the microstructural features of the alloys.

The hot ductility of rolled IMI834 titanium alloy has been studied by conducting tensile tests with a strain rate of 0.1 s −1 and temperature range of 750–1 100 °C to obtain the optimum hot working conditions. The alloy showed minimum hot ductility in the lower alpha–beta region in the temperature range 750–950 °C. Further microstructural characterizations showed improvement in hot ductility by increasing temperature, which was attributed to reduction of volume fraction of high strength alpha phase. The best hot ductility was observed at 1 000 °C, i. e. in the upper alpha–beta region. The better hot ductility at higher temperature could be related to the increase in the volume fraction of beta phase and the occurrence of dynamic restoration phenomena. The second decline in hot ductility appeared at higher temperatures in the beta region and was attributed to the high stacking fault energy and self-diffusion of beta phase leading to limitation of dynamic recrystallization.

Densification, grain growth, magnetic and dielectric properties of sintered Li–Zn ferrite (Li 0.3 Zn 0.4 Fe 2.3 O 4 ) doped with nano silicon dioxide (10 nm) were investigated. The density and microstructural results revealed that increasing the nano-SiO 2 content resulted in lower density and higher porosity at sintering temperatures below 1 000 °C, whereas at higher temperatures, this led to higher density and grain growth. In addition, nano-SiO 2 content greater than 2 wt.% resulted in more reduction in porosity and had no effect on grain growth. Moreover, the highest density and the maximum induction ( B s = 380 mT) were obtained for the sample containing 2 wt.% nano-SiO 2 and sintered at 1 100 °C for 1.5 h, in comparison with undoped samples. Furthermore, the variation of magneto-dielectric properties as a function of frequency were explained according to Koop's theory and electron hopping between the Fe + 2 and Fe + 3 ions.

Mixtures of 5, 10, 20 and 30 wt.% of tungsten carbide powder and 7075 aluminum alloy powder were ball milled for 20 hrs to investigate the effect of percentages of the reinforcement on microstructural and mechanical properties of the produced composite. The milled powders were sintered in a hot press and analyzed using X-ray diffraction. The microstructure, physical and mechanical properties of the specimens were studied. Results showed that the milling process synthesized Al–WC composite powders in which crystallite sizes of the produced phases were in the nano-scale regime. Although the higher amounts of tungsten carbide increased bulk density of the sintered samples, the apparent density decreased. The increase in the amount of the tungsten carbide phase increased effectively the hardness of the sintered specimens, however, the ultimate tensile strengths showed an maximum value at 20 wt.% of the reinforcement phase.

This paper investigates the microstructure and texture evolution of as-rolled Mg-5Li-3Al-2Zn-1.2Y-0.8Nd. During rolling (both warm rolling and hot rolling), many twins appear in the alloy, some of which are cross-twins. Dynamic recrystallization occurs, resulting in grain refinement. With the increase of rolling reduction percentage, basal texture appears, and then finally non-basal texture appears. The non-basal texture during warm rolling begins to appear under a smaller rolling reduction percentage than that during hot rolling. When the rolling reduction is 28 %, the angles between the majority of c -axes and rolling direction are mainly about 25°. When the rolling reduction percentage is 52 %, the textures are strengthened.

Ti 2 AlC/TiAl composites were successfully in-situ synthesized by reaction hot pressing using Ti, Al, TiC and CNTs as starting materials. The results indicate that the as-sintered composites mainly consist of Ti 2 AlC and TiAl phases. The mechanical properties of the 23 wt.% Ti 2 AlC/TiAl composite exhibit the highest values, with a flexural strength and fracture toughness of 652.5 ± 76.8 MPa and 6.6 ± 0.5 MPa m 1/2 , respectively. The complex structure coupled with dispersed Ti 2 AlC micro-particulates is responsible for the enhancement in the strength. Toughening of the composite is mainly attributed to crack deflection, crack bridging, crack branching and pull-out of the Ti 2 AlC particles, as well as transgranular cracking.

The thermal fatigue behavior of superalloy Inconel 713LC was investigated at peak temperatures of 900, 1 000 and 1 100 °C. Thermal fatigue tests were performed on self-constraint single-edge V-type notch specimens. The effect of the maximum temperature, T max in the thermal cycles on the crack characteristics was studied. Under identical test conditions a comparable crack propagation rate was observed for all three peak temperatures. The results showed that cycle numbers for crack initiation decreased while crack propagation rate increased with the increase in peak temperature. Therefore, it was concluded that in the current experimental conditions, thermal fatigue resistance reduced dramatically with the rise of upper temperature. Through optical microscopy and scanning electron microscopy observation, it was found that the primary cracks generally propagated along a preferential direction. Microstructure changes after thermal fatigue are also discussed on the basis of scanning electron microscopy observation.

We have established the effect of copper dopant concentration (Cu/Sn = 0–30 at.%) on the structural, morphological and optical properties of SnO 2 : S films grown on glass substrate using a spray pyrolysis technique. According to the experimental evidence and data analysis, we found that the polycrystalline layers in undoped condition mainly consist of mixed phases such as SnS 2 and SnO 2 , whilst in doped ones, Cu 2 SnS 3 and Cu 4 SnS 4 phases are dominant. With increasing Cu concentration, a decrease in the grain size with a change in the shape of the grains is observed. Optical measurement analysis showed that these films have a direct band gap energy of about 2.98–3.59 eV.

Polycrystalline zirconium nitride films (560 nm) were deposited on stainless steel (SS-316) substrates using the multi-target cathodic arc sputtering technique. Deposition was carried out under N 2 reactive atmosphere (4 × 10 −3 mbar) at two different temperatures, 150 and 200 °C. X-ray diffraction studies show that reflections from planes changed from (111) to (200) for deposition temperatures of 150 °C and 200 °C, respectively. Films deposited at 150 °C and 200 °C bear a critical load of 6.4 N and 6.8 N respectively, showing better adherence at higher temperature. This may be the result of film–substrate diffusion at the interface.

A melt-quench method is used to synthesize a series of tellurite glasses containing fixed concentration of Er 2 O 3 , with and without silver nanoparticles. The existence of metallic nanoparticles (average diameter ∼24 – 36 nm) inside the glass is confirmed by transmission electron microscope imaging. From UV – Vis absorption spectroscopy, a surface plasmon resonance band is detected in the visible region (∼484 nm). Furthermore, the intensity of both fluorescence and Raman scattering is found to be enhanced by embedding silver nanoparticles inside the glass matrix. These enhancements are attributed to the surface plasmon resonance effect in which direct plasmon excitation enhances the local field at the surface of the nanostructures. Enhanced fluorescence influenced by silver nanoparticles may contribute towards the development of optical displays, laser and optical memory devices whereas amplification of the Raman signal is promising for Raman amplifiers.

Porous composite insulating scaffolds were prepared from mixtures of forsterite, salt (Na 2 CO 3 ), and phenolic resin by sintering at 900 to 1 100 °C for 10 h, and the sintering behaviour and mechanical properties were examined. The results show that the changes in temperature did not significantly affect the phases present in the samples. However, changing the Na 2 CO 3 content of a sample will change its phase composition. When the Na 2 CO 3 content was 40 wt.% and sintering undertaken at 1 000 °C, the apparent porosity of the sample was 57 %, and its strength was 3.1 MPa. The large number of pores in the samples had diameters of approximately one micron, while the mesopores had diameters concentrated over the interval of 3 to 5 nm.