Key Engineering Materials Vols. 510-511

Abstract: The paper presents the role of equiaxed α׳ in the bimodal microstructure to attain an optimal combination of ductility and strength. The study revealed that the production of bimodal microstructure and volume fraction of equiaxed α׳ were reliant on the forging temperature and subsequent heat treatment. The Ti-6Al-4V alloy was forged in the two phase region and different heat treatment cycles were employed to get the desired bimodal microstructure and thus the combination of strength and ductility. The mechanical properties of fully lamellar microstructure were compared with bimodal microstructure containing equiaxed α׳. The experimental results showed that the amount of equiaxed α׳ in the bimodal microstructure was critical for achieving a well-balanced profile of mechanical properties.

Abstract: The direct energy band gap in the range of ~1.5eV and the high absorption coefficient (~105cm-1) makes Cadmium Telluride (CdTe) a suitable material for fabrication of thin film solar cells. Thin film solar cells based on CdTe (~ 1cm area) achieved efficiency of 15.6% on a laboratory scale. CdTe thin films were deposited by thermal evaporation technique under vacuum 2×10-5mbar on glass and stainless steel (SS) substrates. During deposition substrates temperature was kept same at 200C for all samples. The structural properties were determined by the X-ray Diffraction (XRD) patterns. All samples exhibit polycrystalline nature. Dependence of different structural parameters such as lattice parameter, micro strain, and grain size and dislocation density on thickness was studied. Also the influence of the different substrates on these parameters was investigated. The analysis showed that the preferential orientation of films was dependent on the substrate type.

Abstract: Superplasticity is a feature of a material or alloy which allows the material to deform plastically to an extremely large strain at low values of stress under certain loading conditions of strain rate and temperature. Eutectic tin-lead alloy is a practical material for research investigations as it possesses a superplastic behavior at room temperature and low strain rate which makes it a useful tool in simulating the ordinary engineering materials at high strain rate and temperature. This alloy has been extensively used as a model material to simulate behavior of engineering materials at high strain rates and temperatures. In this paper, superplastic tin-lead alloy was used at room temperature to simulate the closure of cavities in steels at high temperatures in the hot region under dynamic loading (high strain rate) under the effect of compressive loads using flat platens (open dies). Hollow specimens having different values of bore diameter (D_b) to outer diameter (D_out), of the same height and volume were investigated under different values of height reduction percentages ranging from 20% to 80% , and the percentage of cavity closure at each reduction percentage was determined. It was found that the cavity closure percentage increases or decreases at slow rate for reduction percentage in height less than 40% and increases more rapidly for reduction percentages in height above this value. Furthermore, specimens having smaller values of ratio (D_b/D_out) resulted in higher percentage of cavity closure than specimens having higher ratios at the same value of reduction in height percentage. Complete cavity closure has occurred in specimens having the ratios of 0.1 and 0.2 at 75% reduction in height.

Abstract: The surface of Kevlar fibers is chemically modified by treatment with Phthalic anhydride (PA) and the effect is examined by SEM for the laser cut, three point bending and interlaminar shear delaminated surfaces. The surface modification improved the adhesion to epoxy resin that clearly leads to cohesive fracture as opposed to interfacial failure in the untreated specimen. SEM reveals marginal surface roughening of fibers without compromising their strength. The interface modification technique described in this paper is attractive thermodynamically as it does not compromise surface free energy of the polymer matrix or that of the fiber itself to enhance wetability.

Abstract: BaFe_2-xPb_xO₉ (x=0.0 to 1.0) samples were prepared by co-precipitation method. All samples were sintered at 960±5°C for three hours. Structural properties were investigated by X-ray diffraction (XRD) and Scanning electron microscopy (SEM). Variation in structural parameters was calculated from XRD obtained data. SEM micrographs were used to study the morphology and grain growth. No regular increase or decrease in structural parameters and morphology is observed with increase in Pb content. For dc electrical characterizations, dc resistivity was measured as a function of temperature up to 750K. Data obtained was further analyzed and different dc parameters like activation energy and mobility were measured. For dielectric analysis, capacitance C was measured in the frequency range from 100 kHz to 3MHz. High dielectric constant for this material makes this material suitable for applications like multilayer capacitors (MLC), microwave devices and dynamic random access memory (DRAM).

Abstract: Phase pure Strontium doped PZT (Pb_0.96Sr_0.04Zr_0.53Ti_0.47O₃) was synthesized by sol gel process. Zirconium and Titanium alkoxides were used as starting materials and Lead (II) Acetate trihydrate was used as Pb source. The alkoxides mixture was stabilized by acetylacetonate. The vacuum dried gel was calcined at different temperatures to study the calcination temperature and the resulting samples were analyzed by DSC, TG and XRD. The powder remained amorphous until a temperature of 550°C. The morphology and composition of the powder was studied using EDX, stereomicroscope and high resolution SEM.

Abstract: Eddy current as a non-destructive technique has been used for evaluation of thermal barrier coatings (TBCs) system. In this study the TBCs system which consists of substrate material of stainless steel 316, Ni-20Cr as a bond coat and top coat of CaZrO₃ was produced by air plasma spraying technique. Isothermal treatment of TBCs system was performed and its effect on the microstructures and oxide formation was investigated. The chromium oxide formation as a function of temperature and time was observed. The observed changes in microstructure and formation of chromium oxide were then correlated with variation in eddy current values.

Abstract: One of the potential applications of TiO₂ is its use in gas sensor technology. The aim of this work was to study the gas sensing properties of TiO₂ thin films in combination with the effect of post-deposition annealing treatment. Titanium dioxide thin films with thickness 100 nm were prepared by the reactive dc magnetron sputtering. The thin films were deposited on sapphire substrate from a titanium target in an oxygen atmosphere. The samples were then post-annealed in air in the temperature range 600 °C 1000 °C. Crystal structure, surface topography and absorption edge of the thin films have been studied by X-ray Diffraction technique, Atomic Force Microscopy and UV-VIS Spectroscopy. It was found that the phase gradually changed from anatase to rutile, the grain size and roughness tended to increase with increasing post-annealing temperature. The effect of these factors on gas sensing properties was discussed. For electrical measurements comb-like Pt electrodes were prepared by standard photolithography and the films were exposed to different concentrations of H₂ gas up to 10000 ppm in synthetic air at various operating temperatures from 200 °C to 350 °C.

Abstract: Ultrasonic testing has a strong application in defect detection. An efficient tool for characterizing and life assessment of material structure and components by nondestructive ultrasonic velocity was developed about thirty years ago. Cold rolling results in increase in strength and hardness. The work discussed here is to study quantitative ultrasonic longitudinal velocity for characterizing a change in microstructure due to cold rolling in austenitic stainless steel samples. Samples were cold rolled upto 80 percent in 10 percent step. It was found that the use of velocity measurements is a useful quantitative and non-destructive tool for characterizing amount of cold rolled austenitic stainless steel.

Abstract: The paper presents the effect of vanadium addition to aluminum and aluminum grain refined by titanium on the micron level, in the range from 0.005 wt % to 0.236 wt %, on their corrosion resistance in acidic solution, HCl, at three different temperatures namely: 25 °C, 40 °C and 60 °C. It was found that the corrosion rate was slightly increased by the addition of any percentage of vanadium at 25 °C. Furthermore, it was found that the corrosion rate increased with the increase of temperature at any percentage of vanadium addition in the case of both aluminum and aluminum grain refined by titanium. However it was found that addition of vanadium at any percentage to either aluminum or aluminum+ titanium, resulted in decrease of the corrosion rate i.e. improvement in their corrosion resistance in acidic solution at 40 °C and 60 °C. The maximum achieved reduction in corrosion rate was 77 % at 40 °C and 0.148 wt % vanadium addition.