Materials Science Forum Vols. 514-516

Abstract: Several high temperature fatigue and possibly creep-fatigue cracks have nucleated and propagated through the 3.7 mm wall thickness of a gas turbine exhaust system of a navy combat ship made of a grade type AISI 316L annealed stainless steel. The main cracks propagated near some welded joints, where the measured working temperature was approximately equal to 350°C (Fig.1). The paper presents tensile, fatigue and creep data obtained from experimental tests that were performed in several test specimens obtained from steel plates used in-service. Results of optical microscopy for the microstructure of the material and analysis of the fracture surfaces carried out with the SEM have identified the failure mechanisms at test temperatures. The paper also presents microhardness and grain size measurements carried out together with microstructural observations in the SEM. A research work to investigate carbide precipitation in virgin thin sheet specimens, as used in these exhaust tubes, was also performed and it is presented. The influence of stages time (100, 200, 100+100 and 4x50 hours) and of thermal exposure temperatures (500 and 550°C) was assessed to compare the metallurgical properties of the material. Finally, the paper shortly analyses other materials that could replace the used one.

Abstract: The present work shows the microstructural and mechanical results obtained in joints WC-Co / steel 90MnCrV8 (high strength steel). The joints were carried out using diffusion welding processes and soft interlayers Cu-Ni. The binary Cu-Ni alloys were obtained by electrochemical deposition techniques. The microstructural and mechanical changes provoked in the steel, due to heat treatments, were studied in order to know plastic deformation during the joints. The diffusion welds were obtained in an Edwards vacuum furnace (10-3 - 10-4 Pa), three welding temperatures were used, (825 °C, 850 °C and 875 °C) and 30, 45 and 60 minutes, as welding times. The constant pressure was 5 MPa. The welding interface microstructure was studied by scanning electron microscopy (SEM-EDX). In order to determine the mechanical behaviour of all joints were carried out shear tests. The optimum results were obtained at 850 °C / 45 min and 850 °C / 60 min. Nevertheless, the diffusion welding conditions allowed attaining high quality welding interfaces in all joints of WC-Co / 90MnCrV8

Abstract: In this work, two different consolidation techniques were used to obtain AlN samples: slip casting and dry pressing of granules obtained from aqueous suspensions by freeze granulation. The green samples were sintered at a temperature (1750°C) lower than those usually used (>1800°C), using CaF2 and YF3 as sintering aids. The microstructural features were analysed by scanning electron microscopy (SEM) and the crystalline phases formed were identified by X-ray (XRD). Full dense AlN-based ceramics were obtained from both consolidation techniques; however the thermal conductivity values were quite different. The presence of carbon in the samples derived from the freeze granulated powder containing organic processing additives de-waxed in an inert atmosphere revealed to be helpful in the cleaning of oxygen present at the grain boundaries and consequently, for enhancing the thermal conductivity.

Abstract: Aluminosilicates represent the major group of solid inorganic matter of the terrestrial crust. In specific conditions of temperature and pressure, the aluminosilicates may, in strong alkaline environment, react among themselves, giving place to the formation of stable materials of high mechanical strength, able to support high levels of temperature and resistant to acid action. They also have the ability for the immobilisation of heavy metals and radionuclides. When subject to a previous thermal treatment, certain aluminosilicates like kaolin, can be directly activated at room temperature and normal pressure. In this work, the conditions of preparation of metakaolin (a meta-phase of kaolin) are optimised. Temperatures of kaolin dehydration and exposition time to temperature are studied by the use of XRD, FTIR and NMR-MAS, and so far by the application of Murat-Bachiorrini Criterion. The results obtained show that energy savings are considerable.

Abstract: PXRD (powder x-ray diffraction), FT-IR (Fourier Transform infrared), N2 adsorption at - 196 °C and TG/DTA (thermogravimetric and differential thermal analyses) techniques have been used for characterisation of MgAl- and ZnAl-CrO4 LDHs, which had been prepared by the ion exchange method from the corresponding chloride LDH (layered double hydroxides) precursors. The results indicates that the oxometalate intercalation in both systems produces interlayer microporosity and a basal spacing of 8.7 Å; This gallery height decreases when the samples are calcined in the temperature range 100-300 °C, due to a grafting process. A larger thermal stability is detected for MgAl-CrO4 sample than for the zinc-containing one.

Abstract: This work aims at characterising an alumina powder obtained by combustion synthesis, and at evaluating the technical properties of sintered bodies. Particle size distribution and XRD were used to characterise the alumina powder; electrophoresis and rheological measurements were used to select the most suitable processing conditions. Sintered density, flexural strength, Vickers hardness and SEM analysis were used to characterise the bodies sintered at 1600°C. The experimental results showed that a density of 3.60 g/cm3 could be obtained in absence of sintering aids and that the sintered bodies exhibited a flexural strength of 220 MPa and a Vickers hardness of 1100.

Abstract: In the present work, glass fibre polypropylene (GF/PP) woven fabrics were manufactured from cost-effective flexible thermoplastic towpregs produced by a developed dry coating process. The paper describes the modifications made on the developed coating line to allow producing thermoplastic towpregs able to be woven in textile equipments. The produced fabrics were observed under optical microscopy and submitted to tests in order to evaluate their textile-like properties, glass fibre content and flexibility. Composites processed from those fabrics by compression moulding were also submitted to mechanical testing in order to assess their performance. The obtained experimental results have shown that the woven fabrics produced are cost-effective and present properties good enough to be applied in large-scale commercial markets (e.g. automotive). Future research efforts will be carried to try decreasing the towpreg frictional properties and the amount of polymer lost during the textile processing and improving the feeding technology to warp yarns directly from a creel.

Abstract: This paper describes the work that is being done at the University of Minho concerning the development of braided rods for concrete reinforcement. A preliminary research study has been conducted to understand the mechanical behaviour of braided fabrics. Various samples have been produced varying the type of fiber (glass, polyester and aramid), the type of braided fabric (simple, hybrid and core reinforced) and in the latter case, the number of core reinforcing yarns. The tensile properties of these samples have been evaluated and the results presented. The influence of each factor on the tensile properties of braided fabrics has also been analysed and discussed. In order to produce braided reinforced composite rods to use as a concrete reinforcement, a special technique has been developed using a standard vertical braiding machine. The braided reinforced composite materials have been produced in rib structure to improve adhesion between them and the concrete. Special samples have been prepared and tested to evaluate the adherence between both materials involved. The tensile and bending properties of braided reinforced composite rods have been evaluated and the results obtained presented and discussed.

Abstract: The theoretical treatment of a cavity resonator consists of solving the Maxwell equations in that cavity, respecting the boundary conditions. The resonance frequencies appear as conditions in the solutions of the differential equation involved and are not significantly affected by the fact that the cavity walls have a finite conductivity. Solutions for rectangular cavities and for the lowest resonant mode, where the probability of mistaking one mode from another is slight, are readily obtained. The measurement of the complex permittivity, ε* = ε´-iε´´, can be made using the small perturbation theory. In this method, the resonance peak frequency and the quality factor of the cavity, with and without a sample, can be used to obtain the complex dielectric permittivity of the material. We measure the shift in the resonant frequency of the cavity, f, caused by the insertion of the sample, which can be related to the real part of the complex permitivitty, ε´, while the change in the inverse of the quality factor of the cavity, (1/Q), gives the imaginary part, ε´´. In this work we report the construction details, the performance tests of the cavity to confirm the possibility of the use of the small perturbation theory, and the application of the technique to measure the complex permittivity of a reinforced plastic.

Abstract: In this paper the influence of tribophysical activation on non-isothermal sintering of barium titanate has been investigated. BaTiO3 powders were tribophysically activated in a planetary ball mill for 0, 60 and 120 min., pressed and non-isothermally sintered up to 1380oC. Dilatometric analysis was performed in air in the temperature range from room temperature to 1380oC with heating rates of 10, 20 and 30oC/min. The samples were analyzed by the X-ray powder diffraction method. Investigation of the morphology of microstructure constituents was performed using the scanning electron microsocopy method. With the purpose of optimizing technological parameters the results obtained by microstructure analysis were correlated with the results of quantitative dilatometric analysis.