Materials Science Forum Vols. 636-637

Abstract: In this work, SiOx films were deposited on crystalline silicon substrates and their microstructure and photoluminescent properties are reported. The films were deposited by the Hot Filament Chemical Vapor Deposition (HFCVD) technique using molecular hydrogen (H2) and silica glass (SiO2) as reactants. The H2 becomes atomic hydrogen when is flowed through a tungsten wire heated at 2000 °C. According to the chemical reaction, the atomic hydrogen reacts with the solid source (SiO2) and a SiOx film on a substrate is obtained. From FTIR and room temperature photoluminescence measurements can be concluded that, regions with different average size of silicon nano-clusters in the oxide are formed and they probably are the responsible for the light emission in the visible range.

Abstract: Titanium dioxide (TiO2) thin films were prepared by RF magnetron sputtering technique at an argon flow rate of 4.9 sccm and room temperature during 4 h. These films were deposited on pyrex substrates with an RF power of 300 W and annealed at different temperatures (500, 600, 700 and 800 °C). The optical and structural properties were studied by spectrophotometry and X ray Diffractometry respectively. The obtained results show an amorphous structure for the unheated TiO2 films and an apparition of an anatase phase after the annealing process. The transmittance is increased with an annealing of 500°C from 45 to 80 % in the visible and near-infrared regions. The direct band gap, refractive index, extinction coefficient and grain size were investigated. The reflection in the visible range for a silicon (Si(p)) substrate covered by TiO2 thin films is decreased to be about 20%. The results of this work suggest that the variation of the annealing temperature allow the control of the physical properties of TiO2 thin films as antireflective coating for silicon solar cells.

Abstract: Although new structural and advanced materials have been used in the automotive and aircraft industries, especially lightweight alloys and advanced high strength steels, the successful introduction of such materials depends on the availability of proven joining technologies that can provide high quality and performance joints. Solid-state joining techniques such as Friction Stir Welding (FSW) are a natural choice since their welds are produced at low temperatures, so the low heat input provides limited, slight distortion, microstructural and mechanical degradation. Great effort has currently been devoted to the joining of Al-Cu-Mg and the Al-Mg-Si alloys because of their high strength, improved formability, and application in airframe structures. FSW is a continuous, hot shear, autogenous process involving a non-consumable and rotating tool plunged between two abutting workpieces. The backing bar plays an important role in heat transfer from stir zone (SZ), which can influence the weld microstructure as well as the consolidation of material in the root of the join. This study aims at investigating issues concerning heat generation, within the SZ of friction stir welded aircraft aluminium alloys.

Abstract: Rod-like Al3Ni intermetallic structures have been widely studied by Bridgman techniques of solidification. However, there is a lack of experiments conducted under unsteady-state solidification conditions. Such conditions are very close to the industrial reality since the thermal solidification variables (tip cooling rate, tip growth rate and thermal gradient) are freely changing as solidification progresses. In this research, Al3Ni structures found in hypoeutectic Al-Ni alloys were characterized under transient solidification conditions. Two Al-Ni alloys (1.0 and 5.0 wt%Ni) were directionally solidified. SEM (Scanning Electron Microscope) micrographs were obtained along the casting length (P). It was possible to observe with adequate magnifications the distribution of rod-like Al3Ni particles along the interdendritic regions. In order to emphasize the examination of morphology and distribution of such particles, the aluminum-rich matrix was dissolved by immersion of the sample in a fluoride acid solution (0.5%HF + 99.5% H2O). The effects of nickel content, dendritic arrangement and Al3Ni distribution on mechanical properties were investigated by tensile tests.

Abstract: In the present work the age hardening parameters of a 14Ni (200) maraging steel are studied in order to optimize mechanical properties of the steel. The initial characterization of the as received solution annealed steel has been carried out by optical and scanning electron microscopy (LOM and SEM), and hardness measurements. To identify the structural changes during ageing, differential scanning calorimetry (DSC) tests were performed. Different time-temperature combinations were considered for the precipitation hardening treatment of as-quenched material samples. After hardness measurements, three of these treatments were selected for an in depth study. The obtained microstructure at the maximum hardness peaks then analysed (LOM, SEM and TEM) and mechanical behaviour (strength, toughness and wear resistance) was studied for the final selection of the age hardening conditions.

Abstract: The intermetallic phase precipitation, mainly sigma () and chi () phases, was studied in a comparative manner in two stainless duplex steels: a duplex type UNS S31803 and a superduplex type UNS S32520. The -phase precipitated at ferrite/ferrite grain boundaries prior to the -phase precipitation, which occurred preferentially at ferrite/austenite interfaces and at ferrite/ferrite grain boundaries. The -phase precipitation is a eutectoid type reaction of ferrite, leading to -phase and austenite (γ). The -phase precipitated at lower temperatures and in smaller amounts than sigma. The -phase is metastable in the studied steels and was consumed during the -phase precipitation.

Abstract: Thermogravimetry was used to study the oxidation behaviour of a lamellar Ti46Al8Nb alloy during holding at 700°C in synthetic air. A parabolic plot of the oxidation kinetics shows three different regimes over the total duration (50 h) of the tests corresponding to decreasing values of the parabolic rate constant. The oxide scale was characterized by glancing-angle X-Ray diffraction and transmission electron microscopy. The scale was found to be bi-layered with an outer part that consists of amorphous aluminium rich oxide whilst the inner layer is made of very small cristalites of titania distributed in the same amorphous oxide.

Abstract: Corrosion behaviour of a Mg-Y alloy (commercial WE54) has been studied. This alloy presents excellent retention of mechanical properties and corrosion resistance at elevated temperatures, a combination of properties that can be of interest in many technology applications. To evaluate the effect of heat treatment on the corrosion resistance, WE54 samples in extruded state and after T6 heat treatment were studied. Corrosion behaviour was evaluated by electrochemical and immersion tests in 3.5 wt.% NaCl solution at room temperature and neutral pH. Surface examination was carried out by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD). It was found that corrosion resistance of the magnesium alloy WE54 in chloride medium was improved by applying the heat treatment to the material, a fact that was correlated with the developed microstructure in T6 condition.

Abstract: Studies performed earlier show that the thermal shock loading has influence, together with the normal service loadings, in the fatigue life of some critical welded joints present in the exhaust systems of naval gas turbines [1-2]. In order to prevent the fatigue crack propagation that was verified in service [3], a recently developed ultrahigh-strength austenitic stainless steel was selected (Cr-Mn steel - number 1.4376) and its mechanical properties and the fatigue resistance are under study. This new material could replace, locally, the current material used in the main structure of the exhaust system (AISI 316L). Experimental data are shown. The temperature measured at the critical locations was about 350°C and the pressure applied in the system was calculated through a Computational Fluid Dynamics simulation (CFD), whose results are presented in the paper. The stresses induced by the loadings will be important to estimate the lifetime from the fatigue resistance tests (S-N curves) that will be performed, at 350°C, in butt and T-welded joints of AISI 316L stainless steel with Cr-Mn austenitic stainless steel.

Abstract: The microstructure evolution of a two-phase austenitic-ferritic steel after hot rolling and cold rolling (reduction amounts: 20, 40, 60 and 80%) is investigated with the help of the FEG-SEM / EBSD technique. Special phases orientation relationships (Bain, K-S, N-W and G-T) evolution between neighbouring grains of  and  phases have been characterised as a function of the cold rolling reduction. As this later increases fraction of these particular phase orientation relationships decrease. The same evolution is noticed for the 3 grain boundaries inside the  phase. These results are compared to those observed by TEM.