Papers by Author: Mario Lima de Alencastro Graça

Abstract: SiCf/SiC composite has a high potential for applications such as thermal protection systems due to its high oxidation resistance, high heat emissivity and low through-the-thickness (transverse) thermal conductivity, mainly if hollow fibers are used as reinforcement phase of the composite. PAN polymer is a precursor widely used and studied for developing high performance carbon fibers. Most commercial carbon fibers are basically derived from PAN polymer. In this work, the influence of the fiber specific mass on the morphology of SiC fiber obtained by conversion reactions from PAN oxidized fiber precursor was investigated. These oxidized fibers were obtained by controlling the time and temperature during the process of stabilization/oxidation.

Abstract: Gamma-TiAl alloys are potential replacements for nickel alloys and conventional titanium alloys in the cooler sections of turbine engines, as well as in orbital platform vehicles. The combination of high specific stiffness and good oxidation resistance at intermediate temperatures can provide significant weight savings. However, they have a limited plasticity at room temperature and the tendency to brittle fracture. Powder metallurgy is a near net shape process that allows the parts production with complex geometry at low costs. An improved plasticity of the Ti-Al alloys is received by adding alloying elements and by microstructure modification. An alloy of two-phase structure Ti–48Al–2Cr–2Nb (at.%) was investigated using the blended elemental technique. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering at 1500 °C, in vacuum. It was shown that the samples presented a two-phase structure consisting of lamellar colonies of alternating layers of gamma and α2 phase.

Abstract: This paper is related to the understanding of the relationship between the apparent interlaminar shear strength and the temperature-moisture content by the analysis of the dominant failure mode. It assesses the fracture mode of laminates with carbon fiber fabrics in (0, 90, 45)S ply orientation reinforced Polyphenylenesulphide (PPS) polymer matrix, tested for Interlaminar Shear Strength by the Short Beam method. Tests were conduced at different temperatures: (+20°C), (+0°C), and (-55°C), under various moisture-absorbed contents, namely Dry, 25%, 50%, and 75% of the saturated condition. Fractographic analyses were made and reveal, by the analysis of the fracture surface, the dominant mode of failure in every temperature-moisture content pair. As expected it is verified that temperature is a degradation factor. It is also found that humidity is not a degradation factor and, above the water freezing temperature, the moisture content even becomes a strengthening factor on this thermoplastic composite

Abstract: Powder compaction characteristics is a very important parameter to control in order to obtain products with best mechanical properties made by P/M techniques. This work presents a study on the densification of titanium powders trying to optimize the particle size distribution for the best packing and the maximum densification by pressure compaction. The powders used were made from titanium sponge obtained by the Kroll process. The powders were embrittled by mean of the Hydride-Dehydride process (HDH) and milled in a rotative ball-mill under vacuum. Powders with different particles sizes distributions were mixed in several proportions according to Alfred's and Andreasen’s Theory. The samples were compacted by uniaxial and isostatic pressing and sintered under vacuum. The evaluation of the densification was made following the Standard method of test for density of glass by Buoyancy (ASTM – C693-74) and by scanning electron microscopy (SEM). The samples made with powder milled during 36 hours and 12 hours presented better densification than the ones milled during shorter time and the ones with distributions combinations.