Materials Science Forum Vols. 706-709

Abstract: For description of the mechanical performance of SiC/SiC composites and for safety design for practical use, it is needed to reveal the degradation mechanism especially of fiber under the oxygen atmosphere. In the present work, the fracture behavior and microstructure of the polycrystalline silicon carbide fiber exposed in air at 1173-1873 K for 20 and 3.6 ks were studied with monofilament tensile test, microstructure observation and fracture toughness determination test using newly developed FIB(focused-ion-beam)-method.

Abstract: Our co-worker, Hishinuma et. al. has established a new route Powder-In-Tube (PIT) process using a high Ga content Cu-Ga compound in order to improve the superconducting property of the V₃Ga compound wire. In this study, we investigated microstructure of this high Ga content Cu-Ga/V composite superconducting wire. The different contrasts of matrix, V-Ga phase and Cu-Ga core were observed by SEM observation in cross section of 19 multifilamentary wire. And V-Ga phase was confirmed by SEM mapping. The area fraction of V-Ga phase increased when Ga content increased from 30% to 50%. Thin film sample with V-Ga phase for TEM was fabricated by FIB and observed by TEM in detail. Selected area diffraction pattern was obtained for V matrix, V-Ga phase and Cu-Ga core. The ratio of V to Ga for V-Ga phase was probably V₃Ga according to the EDS result. There was a linear interface between V matrix and V-Ga phase, while the interface between Cu-Ga core and V-Ga phase was not linear. On the other hand, there were some granular grains observed in V-Ga phase wear Cu-Ga core.

Abstract: High-performance metal/polymer/metal hybrid sandwich composites are attractive materials for lightweight constructions in automotive, aerospace and naval engineering world-wide. Due to the excellent combination of mechanical, thermal and elastic properties and, as a result of high forming potential, they can be used in areas of high vibration, where high damping properties of the polymer are demanded and at the same time high strength and stiffness are given by the metal. Disadvantages can be given in case of mechanical or thermal joining of these polymer-based sandwiches because of the elastic behaviour as well as low melting temperature of the polymer. Local metal plate insertions in the soft core at the place of joining can be a solution for such kind of problems. But forming behaviour of sandwich materials with and without local inlays differs strongly. Sandwich composites of that type were produced by roll-bonding. Their quality and their position were controlled by Lockin thermography. The forming behaviour of sandwiches with different geometry, size, type and the position of the inlays was tested by deep drawing and bending and analysed with the help of digital photogrammetry and compared to experimentally obtained mechanical properties. As a result, the local inlays, as well as their geometry, size and type strongly influence the forming limit conditions. The differences in flow behaviour of non-reinforced and reinforced sandwich regions after deep drawing and bending will be presented, as well as the influence of the position of the inlays.

Abstract: Metal matrix composites reinforced with discontinuous reinforcement (short fiber, whisker or particle) are attractive for applications requiring higher stiffness and strength than traditional alloys. Unlike continuously reinforced composites, where the properties are mainly influenced by fibers, the properties of the discontinuously reinforced composites seem to be influenced more by matrix properties. Most of the discontinuously reinforced composites are based on age-hardenable light alloys, so that aging treatments can be applied to develop the optimum properties of the composites. The aging behavior of discontinuously reinforced composites has been a subject of great interest both from scientific and technological view points. Recently developed NZ30K (Mg-3wt.%Nd-0.5wt.%Zn-0.5wt.%Zr) alloys exhibit higher specific strength at both room and elevated temperatures, better strength and creep resistance than the existing commercial magnesium alloys. Accordingly, this alloy can be considered as a candidate material for potential automobile applications, such as engine blocks and pistons, which experience high service temperature. Its use could save considerable mass weight in powertrain systems. However, low elastic modulus and wear resistance of magnesium alloys limit their widespread applications. Metal matrix composites have been proposed as the feasible and economical solution. The aim of this study is to investigate the effect of alumina fibers on the aging hardening kinetics and age-hardening efficiency of squeeze cast NZ30K/Saffil/15p magnesium matrix composite. The aging behavior has been examined using Vickers, combined with microstructure observation developed during heat treatment by optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Abstract: Functionally graded materials (FGM) are characterized by a gradual change in the volume fractions of two or more components as a function of position along certain dimensions. FGM has been introduced as an alternative to laminated composites where a mismatch in properties across each layer interface is the origin of stress concentration and a source of delamination/failure. In addition, the use of natural wood fibres as reinforcement has the advantage of easy manufacturing, low cost, biodegradability, negligible health hazards and high specific properties. Using short fibres in a controlled manner to produce functionally graded composites can provide more balanced properties and lead to improved stiffness/strength properties across thickness. The aim of this paper is to evaluate the mechanical behavior of functionally graded natural fibre composites. To study the effect of composite property variation, the functionally graded composite is divided into a number of homogeneous layers in order to evaluate the mechanical behavior. In particular, the effect wood fibre content variation across thickness on the tensile properties of the composites is presented.

Abstract: As vapor grown carbon fiber (VGCF) possesses the good mechanical properties, high thermal conductivity, high electrical conductivity and low thermal expansion, VGCF/ Al composites are expected to be suitable materials for a high performance radiator. In this study, VGCF were dipped and treated with ultrasonic vibration in some kinds of solution at first. Then, the mixed powders including three kind of average particle size of Al (1, 3 and 30µm) was milled by wet process with three kinds of solution, which are acetone, ethanol and butanol. Ethanol is most suitable for mixing solution because of homogeneous distribution of VGCF and Al powders. The mixed powders were spark-sintered in order to obtain dense VGCF/Al composites. The densification mechanism of VGCF/Al composites was divided into the plastic deformation (2nd stage) and creep deformation (3rd stage) after the 1st stage of rectangular wave pulse discharge. The densification rate of VGCF/Al composite powder depended on Al powder for matrix, but independent on VGCF. VGCF are dispersed uniformly in the VGCF/1 µm Al composite. But the aggregations of VGCF exhibited a preferential orientation in VGCF/30 µm Al composite, which results from the deformation of Al powders under the uniaxial pressure during the hot pressing in sintering process.

Abstract: Aluminum syntactic foams were fabricated by pressure-infiltrating liquid pure aluminum into packed preforms of cenosphere fly ash. The morphology, true density and porosity of fly ash microballoons were characterized. The microstructure of the syntactic foams demonstrated uniform distribution of the microballoons in the aluminum matrix and seldom infiltration of cenosphere fly ash. These foams were subjected to quasi-static uniaxial compression tests and behaved like high strength aluminum foams under compressive deformation, exhibiting an extended plateau region in the stress–strain curves. With the decreasing of fly ash diameter, the plateau stress and absorbed energy of the syntactic foams increased. X-ray microcomputed tomography was used to examine the foam microstructures after interrupted compression and reveal the damage evolution. The current work provides a better understanding on the structure and mechanical properties of aluminum matrix syntactic foams.

Abstract: Function of functionally-graded (FG) foams as energy absorption material for impact was discussed on the basis of theoretical analysis, and fabrication process of the foams was proposed in the paper. The FG foams were found to be useful as impact absorber due to progressively local fracture or cushion in the theoretical analysis. Next the fabrication process of the FG foams was suggested. The graded dispersion of the micro-balloons was conducted before curing the matrix resin in the process. The density distributions in the FG foams were confirmed to be predicted by the numerical analysis on the basis of floating the micro-balloons. Finally, compression tests were carried out to evaluate mechanical properties.

Abstract: Mullite (3Al₂O₃•2SiO₂) undergoes a phase transition at 30 GPa with forming aligned nanocrystalline fragments in an amorphous phase. The direction of the crystal axes of mullite nanocrystals with the grain sizes less than 10 nm is that preserved from the starting specimen. To clarify the mechanism of the nanofragmentation in mullite, compositional and structural effects are investigated by comparative studies using several mullite-related aluminosilicates. Consequently, we proposed that the oxygen vacancies in the crystal structure in mullite play an important role to formation of the nanofragmentation textures. Also, we performed impact experiments using mullite as a bumper material, simulating a Whipple bumper shield for spacecrafts. Damage of impact could be considerably less with mullite bumper shield than with aluminum alloy bumper shield, suggesting that mullite could be an candidate for a Whipple bumper materials in the next generation.

Abstract: Quasi-static tensile tests and high strain rate split Hopkinson bar tensile experiments were used to mechanically characterize a commercially available Ti6Al4V alloy. The material was tested along different directions as to assess the influence of the initial crystallographic texture on the mechanical behaviour. In order to circumvent the problem of the limited thickness of the base material, special usage was made of the Linear Friction Welding technique. The results of the experiments were afterwards compared to the numerical results of a crystal plasticity code based on the viscoplastic self-consistent approach, taking into account twinning as an active deformation mode.