Volume 102, Issue 11

The effect of a “non-contacting” electric field (Method A) applied during the solution heat treatment of the Al – Mg – Si – Cu alloy AA6111 on subsequent natural aging measured by resistivity was determined and compared with that without field and with a field in which the specimen is connected to the positive terminal of the power supply (Method B(+)). Method A gave an increase in the level of the aging curve compared to that without field, but the increase was less than that with Method B(+). Neither field method had an effect on the general form of the aging curve, which was in accord with the Avrami equation with n = 0.49 and k = 0.02. The maximum resistivity for natural aging increased linearly with the as-quenched resistivity for the range in solutionizing temperatures (475 – 550 °C) without and with an electric field (0.2 and 2.5 kV cm −1 ) considered. This relationship gave: (a) the magnitude of the contribution of the insoluble constituents to the resistivity and (b) the ratio of the resistivity due to clusters of solutes formed during natural aging to that due to the as-quenched solute distribution. An electric charge on vacancy – solute atom clusters appears to be an important factor in the influence of an applied electric field on solubility.

Thermal diffusivity measurements of uniaxially cold pressed NiO and NiWO 4 were carried out in a dynamic mode in order to monitor the kinetics of hydrogen reduction of the above-mentioned materials using a laser flash unit. The calculated activation energy was found to be higher than that for chemically-controlled reaction obtained earlier by thermogravimetry. The difference has been attributed to physical changes occurring along with the chemical reaction. The activation energy of sintering of the products was evaluated to be 33 and 36 kJ · mol −1 for NiO and NiWO 4 , respectively. Thermal conductivities were calculated taking into consideration the change in heat capacity considering the compositional and the structural changes with the progress of the reaction. The potentiality of the laser-flash method as a complementary technique to thermogravimetry in understanding the mechanism of gas – solid reactions is discussed.

Different fragments of martensitic Cu-14.86 Zn-5.81 Al-0.5 Fe (mass.%) shape memory alloy were subjected to heating, up to 453 K, with different rates ranging from 1.66 ×10 −2 K s −1 to 54.6 × 1.66 × 10 −2 K s −1 , performed by means of a differential scanning calorimeter. In all cases, during heating, an endothermic peak was observed which was associated with the martensite reversion to parent phase. By means of the differential scanning calorimeter charts the critical transformation temperatures of martensite reversion were determined using the tangent method. The effects of heating rate were evaluated from the point of view: (i) of variation tendencies of critical transformation temperatures; (ii) of deviations of experimental values from linear fit and extrapolation to zero heating rate; and (iii) of corroborating morphological changes of martensite (sub)plates with heat flow variation particularities. The results prove that there is an obvious tendency of critical transformation temperatures, of reverse martensitic transformation, to linearly increase with heating rate. The effectiveness of the linear relationships was checked for two heating rate values located inside and outside the above mentioned range, respectively and the difference between the experimental and calculated values of critical transformation temperatures fell within the range (−3 … +4) ‰.

Mg-10Al- x Ce ( x = 0 – 1.0 wt.%) alloys were prepared using metal mould casting, and the effects of Ce content on the microstructures of semisolid Mg-10Al alloys produced via isothermal heat treatment were investigated. The results show that the solid particles in the semisolid Mg-10Al alloy without Ce addition are smaller and more spherical than those in the alloys with Ce addition. Furthermore, among the semisolid Mg-10Al-0.25Ce, Mg-10Al-0.5Ce and Mg-10Al-1.0Ce alloys, the best rheological parameters are obtained for the 0.5 % Ce containing alloy. The optimum isothermal temperature and time for Mg-10Al-0.5Ce alloy are 560 °C and 20 min, respectively. The main phases in the as-cast alloys are α-Mg, β-Mg 17 Al 12 and Al 4 Ce. The formation of Al 4 Ce phases decreases the content of β-Mg 17 Al 12 , and these Al-Ce phases at the grain boundaries restrict grain growth. Furthermore, the addition of Ce decreases the undercooling in front of the α-Mg tip which makes the α-Mg dendrites more developed. These microstructure changes with Ce addition in the as-cast state greatly influence the transformation of semisolid microstructure during isothermal heat treatment.

Low loss and low-temperature sintered Ba(Fe 0.5 Nb 0.5 )O 3 (BFN) ceramics were prepared via a conventional solidstate method with BaCu(B 2 O 5 ) (BCB) addition. The effect of the BCB content on the dielectric properties of the BFN ceramics was investigated. Low amounts of BCB can significantly reduce the dielectric loss of BFN ceramic from 2 to 0.2, as well as the sintering temperature from 1400°C to 1100°C. The results show that the as-sintered BFN ceramic is a promising candidate material for use in capacitors.

The recovery stress evolution during thermal cycling (heating, isothermal holding, cooling) to temperatures significantly above the austenite finish temperature in Ni – Ti – Cu thin wires was evaluated in a dynamic mechanical analyser DMA TQ800. Ni – Ti – Cu wires were annealed at four different temperatures (350 °C, 450 °C, 550 °C and 650 °C). It was observed that the maximum recovery stress significantly decreases with increasing annealing temperature. It was also observed that the recovery stress during four isothermal holdings shows similar behaviour for all annealing temperatures, i. e. stress relaxation occurs after initial stress increase during the first isothermal holding and no stress relaxation is observed during isothermal holdings in subsequent thermal cycles. The maximum recovery stress after each thermal cycle decreases and shows a tendency to saturate with increasing number of thermal cycles. It is found that even when the recoverable strain is significantly decreased during thermal cycling, Ni – Ti – Cu wires are still able to generate very high recovery stresses.

In this paper the influence of the Ni binder metal and Al as an additional alloying element on the microstructure and mechanical properties of WC-based cemented carbides processed by conventional powder metallurgy was studied. Microstructural examinations of the cemented carbides with 3 and 5 wt.% of Al in the binder metal indicated the presence of a very low and evenly distributed porosity as well as the presence of islands of metal binder in the microstructure. With the cemented carbide with 7 wt.% of Al in the metal binder, the presence of brittle needle-like regions was observed. The WC particles inside these regions were rounded and had a larger mean free path. Vickers hardness and flexural strength tests indicated that the cemented carbide WC-Ni-Al with addition of 5 wt.% of Al in the binder metal presented bulk hardness similar to the conventional WC-Co cemented carbides as well as superior flexure strength and fracture toughness.

The microstructures of two metastable high-alloyed CrMnNi cast TRIP steels and a stable AISI 316L austenitic stainless steel were studied in detail after tensile and cyclic deformation. Electron backscattered diffraction was employed to localize the martensitic phase transformation and electron channelling contrast imaging to describe the typical dislocation arrangements. These were complemented by transmission electron microscopy and by scanning transmission electron microscopy performed in a scanning electron microscope. The TRIP steel with the lowest austenite stability shows a more pronounced martensitic phase transformation realized from the austenite via the intermediate formation of ∊-martensite. Martensitic phase transformation also occurred in the stable 316L austenitic stainless steel with a small volume fraction of α′-martensite, but only with cyclic deformation at low temperatures and/or at very high plastic strain amplitudes.

The high temperature forming behaviour of a modified P92, type 9 % Cr, steel is studied by means of torsion tests. The data obtained from these tests allowed correlation of the number of turns to failure, a measure of ductility, as a function of strain rate and temperature. In addition, the data were correlated by the Garofalo equation with a stress exponent of 4.78 and an activation energy of 390 kJ mol – 1 . This equation was used to predict the formability behaviour for the rolling process and also to determine the maximum forming efficiency and stability of the steel. A temperature of 1 140 °C is found to give the optimum forming temperature.

Mineralized collagen fibrils are the basis for various connective tissues such as bone and cartilage. Injectable biomaterials incorporating mineralized collagen fibrils are applicable to a wide variety of implant types for bone regeneration. A mineralized collagen fibrils/chitosan thermo-sensitive and injectable scaffold for bone regeneration was prepared successfully. The thermo-sensitive properties of the mineralized collagen fibrils/chitosan system depended on the concentrations of mineralized collagen fibrils. 0.02 g mL −1 of mineralized collagen fibrils filler was appropriate for application since it allowed lower gelation temperature and more rapid gelation following injection, due to the increased hydrogen bonds between the collagen in mineralized collagen fibrils and chitosan.

CuGeO 3 nanoflowers have been synthesized by solvothermal treatment of GeO 2 and Cu(CH 3 COO) 2 · H 2 O in n-heptane solvent without using any surfactants. The influence of temperature and reaction time on the formation of the CuGeO 3 nanoflowers has been analyzed. The room temperature photoluminescence spectrum of the CuGeO 3 nanoflowers has also been investigated. The synthesized nanoflowers are composed of orthorhombic phase, based on X-ray diffraction and scanning electron microscopy characterizations. The temperature and reaction time have important roles in the formation and size of the CuGeO 3 nanoflowers. The room temperature photoluminescence spectrum of the CuGeO 3 nanoflowers demonstrates strong blue light emission centered at 421 nm and 488 nm and a poor green light emission centered at 530 nm and 543 nm, suggesting that the CuGeO 3 nanoflowers have good optical properties.

A simple “green” method was applied for preparation of Zn 1 – x Mn x O nanoparticles ( x = 0, 0.025, 0.05, 0.075, 0.15 and 0.20) in water by refluxing for 2 h at about 90 °C. This method is mild and highly efficient involving no templates, surfactants or additives; therefore, it is very promising for low-cost and large-scale production. Powder X-ray diffraction patterns demonstrate that the nanoparticles have the same crystal structure, and loading of Mn 2+ ions does not change the structure of ZnO. Scanning electron microscopy images show that the nanoparticles are mainly spherical. UV-Vis diffuse reflectance spectra of the samples demonstrate increasing visible light absorption with mole fraction of Mn 2+ ions. Photocatalytic degradation of methylene blue on the nanoparticles was evaluated under visible light irradiation. Among the prepared nanoparticles, Zn 0.95 Mn 0.05 O has the highest photocatalytic activity. Moreover, effects of refluxing time and calcination temperature on photocatalytic activity were investigated.

Dimethyltelluride has been used as a dopant source for GaSb epilayers grown via atmospheric pressure metalorganic chemical vapour deposition (MOCVD). It has been observed that the electron concentration ( n ) is proportional to the Te partial pressure in the vapour phase, until n saturates at high Te partial pressure. Electron concentrations as high as 1.36 × 10 18 cm – 3 have been measured with imperfect morphology, and as high as 1.22 × 10 18 cm – 3 with excellent, mirror like, morphology. These appear to be the highest electron concentrations reported to date for any MOCVD-growth epitaxial n-type GaSb doped with DMTe and grown at 540 °C with a V/III ratio of 1.4. The absorption spectra of GaSb doped with DMTe show that the heavily doped samples have a less abrupt edge. The absorption coefficient (α) strongly depends on the free carrier concentration. PL spectra of the epilayers are also reported.

High surface area porous silicon carbide was synthesized via a modified sol-gel method. In the sol-gel method, furfuryl alcohol and tetraethoxysilane were used respectively as carbon and silicon precursors, polymethylhydrosiloxane (PMHS) was employed as a pore-adjusting agent for preparing a carbonaceous silica xerogel. SiC was obtained by the carbothermal reduction of the carbonaceous silica xerogel at 1 300 °C in argon flow and then purified by removing excess silica, carbon and other impurities. Depending on the amount of PMHS added in the sol-gel process, the SiC products are found to have high specific surface area ranging from 120 to 167 m 2 g −1 and pore volume ranging from 0.49 to 1.01 cm 3 g −1 . It is therefore suggested that PMHS enhances the production of mesoporous pores in the SiC products.

A mixed hydroxide hydrogel of the MgO – ZrO 2 system has been synthesized by wet interaction in the aqueous phase. Both the mole ratio of MgO: ZrO 2 and the preparatory conditions strongly influence the particle morphology. This was also reflected in the dehydration – rehydration characteristics studied under equilibrium conditions. Both dehydration and rehydration followed a discontinuous path thereby indicating heterogeneity in the bonding of water to the gel structure.