Materials Science Forum Vol. 985

Abstract: The influence of post weld heat treatments (PWHT) at 400°C, 600°C, 900°C on microstructures in heat affected zone (HAZ) of dissimilar welds between carbon steel and austenitic stainless steel was studied. As-welded condition, the fully Martensitic layer along the fusion line, Widmanstatten Ferrite, Bainite, Pearlite phases in the HAZ of carbon side and the fully austenitic zone in the weld metal can be observed. After PWHT, the microstructures of these zones were dramatically modified as a result of carbon diffusion from the carbon steel toward the weld metal. Decarburization of the base metal led to the formation of a zone with large Ferrite grains. Bainite or fine Pearlite were formed by carbon diffused to both the interfacial Martensite and the purely Austenite zone. The lowest hardness value in the decarburization zone was 92HV on average after PWHT at 900°C and the peak hardness value that was documented in the carburize zone with 366HV at 600°C. Carbides precipitation (M₂₃C₆, M₇C₃) were found in both the HAZ of carbon steel and austenitic stainless steel.

Abstract: Manganese dioxide (MnO₂) is a promising electrode material for electrochemical supercapacitor applications due to its low cost, eco-friendly and high theoretical specific capacitance in a wide potential window. In this study, MnO₂ and Ag-doped MnO₂ are prepared by cathodic electrodeposition on graphite substrate from electrolyte with the main compound of potassium permanganate using pulse potentiostatic technique. The effect of Ag doping on the morphology, structure and electrochemical properties of MnO₂ materials are investigated. Scanning electron microscopy (FESEM), Energy Dispersive X-ray Spectroscopy (EDX), cyclic voltammetry (CV), galvanostatic charge-discharge measurement and electrochemical impedance spectroscopy (EIS) are used for characterization of the prepared materials. The results show that doping Ag into the MnO₂ structure has improved electrochemical characteristics of materials. The specific capacitances are calculated for pure MnO₂ and Ag-doped MnO₂ to be 272.84 and 277.48 F/g, respectively. The prepared materials exhibit the high charge-discharge stability, maintaining at about 92 % for MnO₂ and 95 % for Ag-doped MnO₂ after 500 cycles of the charge-discharge operation.

Abstract: Hydroxyapatite (HAp) and octacalcium phosphate (OCP) coatings were formed on a Mg-3Al-Zn (AZ31) alloy with a chemical solution deposition method using a Ca-EDTA solution at various pH levels. The adhesion strength of the coatings was examined using the pull-off method. The microstructures of HAp and OCP coatings were measured X-ray diffraction (XRD). The morphology and composition of the surface and cross section of the samples before and after the adhesion test were characterized using scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), and a 3D profilometer. The results showed that plate-like OCP crystals grew from a continuous OCP layer on the surface of the AZ31 substrate in the case of a pH 6.3 coating solution. At pH values of 7.5 and 8.6, the HAp coating showed a two-layer structure with a HAp rod-like outer layer and a HAp continuous inner layer. Regardless of the pH of the coating solutions, a very thin Mg (OH)₂ intermediate layer was formed between OCP or HAp coating and substrate. The highest adhesion strength of the coatings was 6.7±0.5 MPa, achieved at a coating solution pH value of 7.5. A part of Mg (OH)₂ layer remained on the substrate, indicating that the delamination occurred in the Mg (OH)₂ intermediate layer. The primary particles in the inner layer formed at pH 7.5 was smaller than those at pH 8.6. This result indicates that the initial corrosion of substrate AZ31 at pH 7.5 was more rapidly than that at pH 8.6, presumably leading to the formation of mixed layer of Mg (OH)₂ and calcium phosphate. Further investigation is necessary to understand the better adhesion strength at pH 7.5 than that at pH 8.6. This good adhesion could be due to the flawless and rod-like uniform crystal, which had the densest and finest structure on the surface.

Abstract: Silicon carbide (SiC) was synthesized from Vietnamese rice husk and diatomite by electric arc discharge furnace with the arc current of 100, 150, and 200 A. The synthesized products were characterized by Raman spectroscopy, XRD, FE-SEM, and EDS. The result showed that α-SiC material with the purity of up to 85.12 % had been successfully synthesized from Vietnamese diatomite and rice husk charcoal. Also, there is remained amount of carbon and SiO₂ which can be removed by burning and leaching processes.

Abstract: In this work, commercial Viscose (cellulosic based precursor) rayon-based carbon fibers were oxidized to make activated carbon fibers (ACFs). Carbon fibers were made from Viscose fibers in carbonization process at 1200°C. The ultimate carbon fibers possessed carbon content above 94 mass% and fiber dimension about 8 mm. These fibers were activated to make ACFs by oxidizing gas such as steam of carbon dioxide. The experiments were conducted at temperature ranged from 800°C to 900°C with carbonic steam’s flow of 3 l.min^-1. The vaporous benzene adsorbability of activated carbon fibers was then measured by Mark Bell method. The adsorbability (a) and specific surface area (A_BET) of ACFs were determined. The properties of the produced ACFs were investigated and analyzed by SEM and TEM imaging. The results showed that obtained fibers have maximum benzene adsorbability of 4.58 mmol.g^-1 and BET surface area reached 1357 m².g^-1. These activated carbon fibers were able to use for toxic chemical prevention equipment.

Abstract: This paper reports a tunable luminescence of europium (Eu ³⁺) doped zirconia (ZrO₂) nanoparticles as a function of hydrothermal temperature, europium concentrations, time and pH value. The nano-sized Eu doped ZrO₂ (Eu:ZrO₂) particle was synthesized by hydrothermal method at the temperature of 200 °C and time up to 48 h. The nano-sized Eu:ZrO₂ particles have a diameter of about 10 nm. The luminescent properties of nano-sized Eu:ZrO₂ particle was enhanced in the sample when high temperature, high Eu ³⁺ concentration and prolonging hydrothermal time were used at pH 7. These results suggest the use of Eu:ZrO₂ nanoparticles followed by thermal annealing in tuning the luminescence of Eu:ZrO₂ nanoparticles which have potential applications as phosphors in solid state lighting.

Abstract: Steel grade 11 is widely used to fabricate superheater pipe in the thermal power plant. Since the steam has a high temperature (above 300oC) and pressure (max. 20 MPa), lifetime of the tube/pipe decreases or the failure occurs during the operation. Problems of the steel include micro-crack, creep degradation, pit corrosion, mechanical failure, etc.; and increasing working time raises the failure potential and decreases the economy if the thermal plant has to stop operating. This research investigated the change in the mechanical properties of steel grade 11 (ASTM A335) loaded constantly at room temperature. The steel specimens were cut from a new pipe P11, hanged under various stresses (6.45 – 9.68 – 12.9 MPa) for 2160 and 4320 hours. After the loads were removed, the steel specimens were tensile tested and observed by optical microscopy and SEM. The results shown that strengths of the steel increased with increased the stress for 2160 hours, but slightly decreased for 4320 hours. There was no significant change of the microstructure including 87 vol.% ferrite and 13 vol.% pearlite, and the grain sizes were estimated as about 18 μm for all conditions. It was considered that a deformation hardening occurred in the beginning stage of loading, but the creep degradation started as the loading time was lengthened. The fact that a significant change in the microstructure was not observed in the present steels revealed that high temperature played an important role in degradation of the practical steels.

Abstract: Normally, the microstructure of eutectic and hypereutectic Al-Si alloys consists of Al-Si eutectic or Al-Si eutectic + Si_I, that is why they have good wear-resistance properties, but their ductility decreases somewhat, which can be improved by the appearance of non-equilibrium α-aluminum grains. In present work, a recrystallization and partial melting (RAP) technique was applied in the preliminary research to investigate the solidification behavior of the eutectics at high cooling rate. The results show that numerous α-Al dendrites appeared instead of the eutectic structure. In the followed researches, the combination of some technological procedures, based on the competitive growth of dendrites and eutectics, such as heterogeneous nucleation and high cooling rate, were applied: A413.0 and A390.0 alloys were melted in resistant furnace and poured into a mold, made of copper at one side and of steel at another, which is connected to the temperature measuring device via four K-class thermocouples. The various pouring regimes: gravity, via 45⁰ tilt cooling slope and injection with Argon gas for 2 min. before pouring, were applied at different temperatures of 680, 650, 630⁰ C. The results show that the hetero-structure of eutectic and hyper-eutectic Al-Si alloys, consists of non-equilibrium α-Al grains, primary silicon and eutectics was obtained. Grain size varies with cooling rate, with minimum value of 10 μm when the specimen thickness is 5 mm. Non-dendritic grains were achieved with semi-solid treatment (pouring via cooling slope). The elongation of alloy is expected to be enhanced due to the appearance of α-Al grains.

Abstract: Dense [boron carbide (B₄C)]/[carbon nanomaterials] composites were synthesized and sintered simultaneously using pulsed electric-current pressure sintering (PECPS) at 2173 K for 6.0×10² s (10 min) under 50 MPa in a vacuum. The starting powders were amorphous B and C nanopowders and nanocarbons. The latter were acid-treated carbon nanofiber CNF and carbon nanotube CNT. The sintered composites were evaluated from the viewpoints of mechanical properties at high temperatures up to ~ 2023 K in inert atmosphere. Thus fabricated composites with 10vol%CNF maintained high bending strength σ_b around 750 MPa even at 1973 K; this temperature is 100 K higher than that of conventional B₄C/CNF composites, and furthermore 600 MPa at 2023 K. These high σ_b at elevated temperatures might be explained by both the low content of catalytic Fe particles and the rough surface of CNF after the acid-treatment. On the other hand, B₄C/CNT composites displayed 770 MPa at 1723 K. The stress-strain curves demonstrate that B₄C/CNF composite deformed elastically until 1273 K and plastically up to 1973 K, however, the B₄C/CNT composites displayed elastic deformation up to around 1873 K.

Abstract: This work concerns microstructure and mechanical properties investigations of aluminum based composite strengthened with the TiC particles being in nanometer size. The composites were fabricated by the casting method combined with in-situ formation of TiC particles. Applying a suitable composition of components and moderators of SHS reaction which occur during casting, it was possible to cast the samples with TiC particles of size of 150 nm and faceted shape. Microstructure investigations using scanning and transmission electron microscopy (SEM and TEM) allowed to identified the distribution of TiC particles and their preferred location in the microstructure of composites. Also the additional precipitates with different size and shape were identified in investigated samples. Significant increase of strength was observed in in-situ cast composites in comparison to Al-1000 alloy mainly due mainly due to coefficient of thermal expansion and elastic modulus mismatch between the reinforcements and the metal matrix, Hall-Pecht relation and also in minority the Orowan effect.