Materials Science Forum Vols. 539-543

Abstract: This study examined the possibility of synthesis of biphasic calcium phosphate by sintering a mixture of hydroxyapatite and calcium phosphate glass. The effect of the concentration of calcium phosphate glass in a mixture on the proliferation and differentiation of MG-63 preosteoblast-like cells in a hydtoxyapatite scaffold was investigated. The addition of 5 wt% of calcium phosphate glass significantly improved the level of attachment, proliferation and differentiation of MG-63 cells onto the hydroxyapatite scaffolds, particularly when the surface was modified with 2% bovine serum albumin (p<0.05). Under these conditions, type I collagen was expressed and the extracellular matrix was formed by 1 week, and the ALP gene was expressed at 4 weeks.

Abstract: Influence of applied tensile and bending strains on the local and overall transport critical current Ic and n-value at 77 K of multifilamentary Bi2223-composite superconductor was studied, where the n-value refers to the sharpness of the transition from super- to normal conducting state. Under both tensile and bending strains, the damage such as transverse and longitudinal cracking of the Bi2223 filaments and interfacial debonding between the filament and silver progressed. The extent of damage and accordingly the critical current was different among the local portions. The relation of the local current and n-value to overall ones was analyzed with a voltage summation model, with which the experimental result was described well. Further analysis revealed that the distribution of local critical current could be described by the Weibull distribution function and n-value could be expressed as a function of critical current. Based on these results, a Monte Carlo simulation was carried out to predict the overall critical current from the distribution of local critical current, with which the experimental results could be described.

Abstract: One of the authors proposed a non-equilibrium powder metallurgy process, which enables the fabrication of a near net-shape product using TiC and TiN/Silicide ceramic composites. The PM process in combination with mechanical alloying (MA) and Spark Plasma Sintering (SPS) are applied to produce nano-grain composite, TiC/Ti5Si3. Powders of elements Ti and SiC whose composition is Ti-20 mass%SiC are blended for MA. After the alloying, the MA powder whose average particle size is 20~30 μm, has amorphous-like structures, and then the MA powder is compacted by SPS. Results of compression-tests indicate the occurrence of unusual hightemperature deformation behaviors such as low flow stress at the lower deformation temperature or at the high initial strain rates were observed in the SPS compacts. TEM observations of the deformed compacts after the compression-tests indicate the microstructure has no-strain equiaxial - grains and clear boundaries. This serves as proof of a superplasticity deformation. In addtion, the results of the XRD analysis of the compressed-compacts show that new phases are formed during the compression-test. Therefore, the above deformation is attributed to a "pseudo" superplasticity in which the phase transition of metastable microstructure occurs during the deformation.

Abstract: This investigation analyses the influence of dispersed alumina particles on the wear behaviour of the Al/albite composites in a corrosive environment. The composites were prepared by modified pressure die-casting technique. The corrosiveerosive wear experiments were carried out on a proprietary corrosion-erosion wear tester to study the wear characteristics of the composites. The slurry was made up of water and alumina(size: 90-150μm, proportion: 0 to 30 wt %) , while H2SO4 (0.01N, 0.1 N and 1 N) was added to create the corrosive conditions. Experiments were arranged to test the relationships among the corrosive-erosive wear rate, concentrations of H2SO4 and alumina in the slurry, weight percent of albite in the composite, erosion speed and distance. Wear rate varies marginally at low speeds but sharply increases at higher speeds. The corrosive wear rate logarithmically increased with the increasing concentration of the corrosive medium. The effect of abrasive particles and corrosion medium on the wear behaviour of the composite is explained experimentally.

Abstract: Fully dense composite materials of M3/2 high speed steel reinforced with 5, 10 and 15vol. % of high purity niobium carbide were developed using powder metallurgy processing. This work describes and discusses the mechanical behavior of the various composite systems. Mechanical properties were characterized by Rockwell hardness, compression and three point bending tests. It was found that the addition of ceramic particles causes a very small increase in the hardness and 0.2% yield strength, but a decrease in the transverse rupture strength is observed. In order to eliminate the influence of martensite on the mechanical properties, measurements were also conducted after tempering. After this treatment, the reinforced materials showed a moderate yield stress increase at room temperature respect to the unreinforced M3/2. However, the bend strength values were not affected significantly by this treatment. At high temperatures, the addition of reinforcement particles causes a slight increase in the strength. Strain rate-change tests in compression were performed at strain rates ranging from 3 x 10-6 to 2 x 10-3 s-1 in the temperature range from 650 to 750°C. The deformation behaviour was characterized by a stress exponent ranging from 5 to 7 and an activation energy for plastic deformation similar to that found in high alloyed ferritic steels.

Abstract: A new cost effective process to produce discontinuously reinforced (TiB) TMCs has been developed. The article presents general features of the composites, microstructural characteristics and mechanical properties. The production and characterization of two potential commercial applications are also discussed.

Abstract: In order to fabricate the metal matrix composites by low-pressure infiltration by gravity casting machine, the preform made from FeCrSi fiber with 40μm and matrix of A336.0 aluminum alloy was used. The volume fraction of fiber in preform was about 20%. The temperatures of die, preform and molten alloy were 200 oC, 400 oC and 750 oC, respectively. By controlling the infiltration of molten aluminum alloy to one direction by using barrier plate, the quantity of pores caused by curling of air degrades dramatically. Molten aluminum alloy was able to infiltrate at low pressure of 0.2MPa. As increasing the pressure, porosity in composites decreased. The composite with no pores was obtained by barrier plate and 0.8 MPa of molten alloy pressure. This composite had high strength at high temperature of 200-400oC.

Abstract: Composites made of niobium filaments within an oversaturated α-CuSn bronze matrix were processed by restack bundle drawing. The differences in mechanical behaviour of the various components entail an heterogeneity of deformation. Such a feature is mostly unfavourable not only for the phase transformations that govern the microstructure and the electromagnetic properties but also for the composite mechanical integrity. The obtained results finally demonstrate the chief importance of the selection of both the design of the composite and the processing steps.

Abstract: In the present talk, MgAl2O4 in the Al2O3/Al-1.0mass%Mg2Si alloy composite was also observed by a scanning electron microscope equipped with the low energy electron (SLEEM) adaptation aiming at examination of its morphology and orientation relationships to the Al2O3 particles. Owing to its much smaller interaction volume of signal exciting electrons in the target and hence more localized information, together with a favorable combination of secondary (SE) and backscattered (BSE) electron signals, the SLEEM method provided much better readable and detailed images of all particles, their shapes and mutual orientations, in comparison with conventional SE and BSE images at the electron energies usually used in the SEM. MgAl2O4 (spinels) were formed on facets of Al2O3 as small particles, and their shape well corresponded to an octahedron consisting of 8 equiaxial triangles.