Advanced Materials Research Vols. 89-91

Abstract: Polymeric, biodegradable microspheres represent a good reliable system to investigate the release of bioactive substances in both in vitro and in vivo applications. Common biomaterials for the synthesis of these microspheres are aliphatic polyesters of the poly(α-hydroxy)acids, especially poly-L-lactides (PLA) and polyglycolides (PGA) or their copolymers poly-D,L-lactide-co-glycolides (PLGA). In our own previous studies we have developed PLGA microspheres with integrated PGE2 as model substance for a wide range of biomedical applications, especially in angiogenesis, fracture healing and cartilage repair. The synthesis is based on a binary solvent in water emulsion approach, where two different solvents are used to dissolve the active agent and the polymer, while being miscible in each other (CHCl3, ethyl acetate). Both, the degradation of the material and the release profiles were investigated using SEM and mass spectrometry coupled with gas- or high performance liquid chromatography. SEM and AFM measurements indicated a porous structure of the microspheres but could not resolve the true three dimensional structure of the microspheres. Therefore, synchrotron radiation-based µCT (SR-µCT) investigations were performed to link the release profile to the structural design of the microspheres. As a result, we were able to cross validate the experimental data from SEM and AFM with SR-µCT, demonstrating both micro-porosity and nano-porosity. The polymer itself appears to consist of 200 nm – 300 nm sized particles.

Abstract: Magnesium matrix composites are attractive for weight critical application, such as automotive and aerospace components, because of its high specific strength and stiffness. Extrusion process directly following vacuum infiltration (EVI) can eliminate the porosity and obtain the well-aligned and uniform fiber distribution during the fabrication of Csf/AZ91D composite. This process combines the advantages of gas pressure infiltration, squeeze casting, and semi-solid extrusion. The mechanical properties of the magnesium are improved greatly by introducing the carbon fibers into the magnesium matrix through the EVI process. In the present study, the carbon short fiber reinforced magnesium matrix composites Csf/AZ91D were fabricated by EVI process. The microstructure and tensile property of Csf/AZ91D composites were investigated. The results showed that the microstructure of the composite presented a uniform distribution of carbon short fibers in the matrix and good interfacial integrity. The yield strength and stiffness of the composites increased with increasing carbon short fiber content, but at the cost of ductility. Nonetheless, Csf/AZ91D can keep relatively high ductility during the improvement of strength compared with reported composites in the literatures. Increasing carbon fiber content in the composite was not always beneficial to the ultimate tensile strength at the same magnitude. When the fiber content exceeds 10%, the matrix was not strengthened as greatly as under 10% fiber content. The yield strength improvement was attributed to (i) load-bearing effects due to the presence of carbon short fiber reinforcements; (ii) grain size refinement due to the large extrusion deformation; (iii) generation of dislocations to accommodate CTE mismatch between the matrix and the particles.

Abstract: This paper presents an experimental methodology to determine a Friction Stir Welding (FSW) means of production based on the experimental study of the tool / material mechanical interactions generated during the plunging and welding stages. These two stages have been identified as being characteristic for the qualification of a FSW equipment. This paper presents the experimental results of the parametric study done on the plunging and welding phases. Ranges of forces and torques diagrams were established according to the processing parameters, in order to qualify a means of production and select the process parameters allowing the operation on the available FSW equipment.

Abstract: The stretch forming applied force during the forming process and exceed the yield stress of the raw material to make the part deforming plastically, and fitting the contour of a form block, then the purpose of forming can be reasonably achieved. The saddle-shaped skin located in the mating area between the vertical stabilizer and the empennage. This study investigates the stress distribution on the stretch forming of saddle-shaped skins. First, apply the FEM software-ABAQUS to analyses the stretch forming of saddle-shaped skin. Then use the statistical method to analysis the stress in different forming conditions. Finally, the stress of saddle-shaped skins on the critical location is analyzed and discussed. This study aims to modify the process parameters in advance to prevent the failure and defects of skin and provide valuable information for both production engineer and tool designer to improve their design of process parameters and form block.

Abstract: This study investigated stress corrosion cracking of two welded stainless steel alloys, austenitic 304L and duplex 2205, in an acidic chloride solution. Different heat inputs are selected for welding the alloys, using tungsten inert gas, with and without filler metal. The slow strain rate technique is utilized to estimate the susceptibility of each weldment to stress corrosion cracking. Different strain rates are used, and the experiments showed that the strain rate equal to 1.66x10-6/sec is a critical value that can be used for assessing the susceptibility of the alloys to corrosion cracking. A numerical index used in this study to evaluate this susceptibility, which is based on a ratio between elongation percent of each alloy in the solution to that in the air. The results showed that the austenitic alloy has higher ductility than duplex in air, while there was not a big difference between both alloys in the solution. Increasing the heat input in autogenous welding caused a brittleness, i.e. less elongation, for both alloys. The results showed that the austenitic alloy is exposed to stress corrosion cracking in the solution, before and after welding, with or without filler metals. On the other hand, the duplex alloy showed higher resistance to stress corrosion cracking than the austenitic alloy due to the high chromium content, and it is dual phase.

Abstract: Roll Diffusion Bonding (RDB) is a new process, developed at C.S.M., for producing Ti composites reinforced by long fibres. The prototypal “diffusion bonding” plant permits to co-roll at high temperature in superplastic rolling field (under temperature and strain rate control) foils of titanium alloy and fabrics made of SiC monofilaments. This study evidenced that the Ti6Al4V-SiCf composite produced by roll-bonding exhibits superior mechanical properties with respect the same material prepared by Hot Isostatic Pressing (HIP) owing to the smaller grain size and the higher dislocation density.

Abstract: In the present study, the effect of austenite deformation on the recrystallisation behaviour in terms of recrystallisation-stop and recrystallisation-limit temperatures (T5% and T95%) of an X70 niobium microalloyed pipeline steel have been investigated by interrupted plane strain compression tests. The extents of recrystallisation are calculated using a modified fractional softening parameter. And the 20% and 60% of fractional softening were correlated to T5% and T95%. Quantitative optical metallography indicates that this method provides for a convenient and reliable experimental measurement of the critical temperatures associated with the recrystallisation of austenite. The recrystallisation kinetics and the precipitation kinetics of Nb(CN) were calculated using two widely applied models. The experimental results from this study suggest that the current model of precipitation kinetics might overestimate the precipitation start time.

Abstract: We have explored a straightforward approach for achieving water-resistant properties of the electrospun PVA nanofibers. The electrospun PVA nanofibers are post-treated with a hydrophobic polyhedral oligosilsesquioxane (POSS) hybrid macromer via a direct urethane reaction between the hydroxyl group of PVA and the isocyanate group of POSS macromers. The POSS-modified PVA nanofibers are characterized by fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and water resistant property. The morphologies of the electrospun PVA nanofibers before and after POSS post-treatments are regular and a narrow distribution of diameters was observed, indicating a uniform post-treatment of POSS macromers onto the PVA nanofibers. Thermal decomposition behavior of the POSS-modified PVA nanofibers was altered compared to the pure PVA nanofibers, suggesting the suppression of thermal decomposition due to the incorporation of POSS macromers. In addition, the pure PVA nanofiber mats immersed in pure water exhibited no characteristic morphology, whereas the POSS-modified PVA nanofiber mats showed the texture morphology, indicating an enhanced water-resistant property.

Abstract: Two sintered magnets Nd15Dy1.2Fe77Al0.8B6 and Nd22Fe71B7 were modified by intergranular additions of Si3N4. The remanence as well as sintering density of the two magnets increased slightly with appropriate amount of Si3N4 additives. Meanwhile, there was an obvious increase in coercivity of the Nd-rich Nd22Fe71B7 magnet after 0.3 wt. % Si3N4 was added to magnets. Besides the effects on magnetic properties, an improved corrosion resistance was observed. Compared with the native magnets without any additions, corrosion potential of the magnets with Si3N4 additives is more positive and the current density in the anodic branch of the polarization curve is reduced. Microstructure observation reveals that Si3N4 additives have been incorporated into the intergranular phases in the magnets. Si is found to enrich in the Nd-rich intergranular phase with low oxygen content. With the introduction of Si3N4 additives, more intergranular phase with high oxygen content is formed, which may contribute to improved corrosion resistance. In addition, addition of Si3N4 refines the grain size of Nd22Fe71B7.

Abstract: The selection of new TBC materials is restricted by few basic requirements such as: high melting point, no phase transformation between room and the operation temperatures, low thermal conductivity, chemical inertness to the combustion gases and environment, thermal expansion match with the metallic substrate, good adherence to the metallic substrate and low sintering rate of the porous microstructure. Among these properties, one of the most important is thermal diffusivity. The number of material that can be used as TBCs is limited and so far only a few materials have been found to basically satisfy these requirements. Recent research has shown that certain rare-earth zirconates, such as Gd2Zr2O7, have even lower thermal conductivities than 7YSZ, and this has spurred an intensive research in discovering alternative TBC materials. The results of microstructure tests performed on the powders intended for thermally sprayed TBCs with APS method were presented in this article. The tests of phase and chemical composition of the analysed powder were performed. The carbon, sulphur and gas nitrogen contents were, among other things, determined during those tests. The x-ray powder diffraction phase identification in as received material was determined. The tested material showed the presence of Gd2Zr2O7 compound as the predominant one and Gd2O3 and ZrO2 oxides. The surface morphology analysis of the powder was carried out and its internal structure was characterized. The tested material shows porous structure typical for agglomerated powders. The second testing area applied to analysis of the powder thermal properties. The thermal diffusivity of the compressed samples with density similar to the solid material was determined with the laser flash (LF) method. The measurement results show that requirements for the materials used for new generation TBCs are met.