Materials Science Forum Vols. 638-642

Abstract: It is well known that multi-component alloys form bulk metallic glasses (BMGs) from the supercooled liquid state without rapid quenching. However, the mechanism of phase selection between crystal and glass states has not been fully clarified. To obtain an insight into the glass-forming processes, we carried out in-situ observation on the solidification of Zr-based BMG-forming alloys from its supercooled liquids by time-resolved X-ray diffraction combined with the conical nozzle levitation (CNL) technique to achieve a containerless melting. For Zr-based alloys, we succeeded in detecting the X-ray diffraction patterns during glass formation from the supercooled liquid state as well as the crystallization from the liquid state. Furthermore we performed the precise structure analysis of supercooled state of Zr-based binary liquids. Based on the liquid structure and in-situ observation results, we discussed about the phase selection mechanism between crystal and glass states.

Abstract: The growth of ultrananocrystalline diamond (UNCD) by pulsed laser deposition necessitates hydrogen atmospheres during the deposition. Optical emission spectroscopy was used to study the roles of the hydrogen atmosphere on the UNCD growth. Time-resolved images of a plume that expanded from a laser-irradiation spot toward a substrate were taken using a high-speed ICCD camera equipped with narrow-bandpass filters. While the plume disappeared at the longest within 1 s in vacuum, the emission from C+ lasted above the substrate surface for approximately 7 microseconds at a hydrogen pressure of 53.3 Pa. Since emission lifetimes of species are approximately 10 nanoseconds, this implies that C+ ions collided actively for such a long time. The hydrogen atmosphere has a role of forming a high number density of C+ ions. In addition, we believe that atomic hydrogen that might be generated by the collisions with carbon species contributes to the UNCD crystallite formation by their terminating the dangling bonds of carbon clusters as theoretically predicted in previous reports.

Abstract: In this study, Spark Plasma Sintering has been used to sinter pure iron with an initial crystallite size around 100 nm. The process parameters for sintering pure iron have been optimized in order to obtain fully dense materials and avoid excessive grain growth. Archimede's method has been used to calculate the relative density of the sintered samples. It appears that almost fully dense materials can be obtained (95%). X Ray diffraction applied to the sintered samples shows the presence of iron and of the wustite oxide FeO (around 6% wt) formed during the sintering process. Peak enlargement measurements show that the grain size after sintering is around 200nm. This is confirmed by TEM observations showing a dual distribution of grain size. Finally, mechanical characterization has been carried out. The sintered compact exhibits a very high hardness of about 400 Hv. Compression test reveals a very high maximal stress of about 1.2 GPa and that the ductility in compression is non negligible. Using the Hall and Petch law, the calculated grain size should be around 450 nm which is in accordance with direct observations.

Abstract: Magnetic nanocomposite of Sm/Co/C-system is prepared and characterized. It reveals chemical and thermal stability. SQUID magnetometry is applied to study hysteresis behaviour of the material. Modified Langevin formalism is used to distinguish between ferromagnetic and paramagnetic contribution to magnetization. It is shown that 80 % of low temperature remanence is stable at 350 K and coercive field at this temperature is 140 Oe.

Abstract: On the basis of generalization of research results obtained at the Institute for Metals Superplasticity Problems, principles of fabrication of bulk ultrafine-grained and nanostructured materials by multiple isothermal forging are formulated. Multiple isothermal forging is shown to be a universal high-performance deformation technique for the grain refinement in metals and alloys maximally exploiting the potential of dynamic recrystallization.

Abstract: FePt binary alloy nanopowder has been synthesized by a chemical vapor condensation process using a mixture of iron acetylacetonate and platinum acetylacetonate. Particle size of the synthesized powder was less than 10 nm and the powder had very narrow size distribution with relatively high dispersivity. FePt nanopowder possessing L10 ordered phase was synthesized at the condition of well controlled precursor mixing ratio and reaction temperature with some disordered cubic phase.

Abstract: Hot work tool steels generally consist of the tempered martensitic structure and they are high strength materials using all strengthening mechanisms, such as strain strengthening, grain refinement strengthening, solid solution strengthening and precipitation strengthening. It is necessary to use the grain refinement strengthening that can bring both higher strength at high temperature and toughness than those of conventional steels. In this study, hot work tool steels with ultra-fine grained structure were fabricated by the mechanical alloying treatment. The powder mixture of atomized AISI H13 steel powders and Y2O3 powders was mechanically alloyed by a planetary ball mill for 360ks. The mechanically alloyed powders were packed in a stainless steel tube in a vacuum and then consolidated by hot rolling for full densification. The consolidated material was austenitized at 1293K, which was general austenitizing temperature of H13 steel, and then oil-quenched. Through this process, an ultra-fine grained H13 steel with equiaxed grains of about 0.3 microns in diameter could be obtained. A quenched hardness of the developed steel was comparable to that of an AISI M2 high speed steel produced by melting and hot-working process. Furthermore, although tempered hardness of the M2 steel decreases with increasing tempering temperature of 848K or higher, that of the developed steel does not decrease so much. Therefore the hardness of the developed steel exceeds that of the M2 steel at tempering temperatures over 923K.

Abstract: The conductive metallic nanofibers were prepared by using a combined technology of electrospinning and metallization. The electrospun polyurethane and poly(vinyl alcohol) (PVA) nanofibers are metallized with different thicknesses of metal (Cu, Ni) layer via a metallization. The thickness of the metallic layer, which is ranging from 50 nm to 200 nm, was controlled. The resultant metallized nanofibers are characterized using field emission scanning electron microscopy (FE-SEM), wide angle X-ray diffraction (WAXD), and thermogravimetric analysis (TGA). FE-SEM micrographs demonstrate that the nano-scaled metallic layers are well deposited onto the nanofibers. TGA result indicates that thermal stability of the metallized nanofibers was enhanced due to the barrier effects of the metallic thin layer. WAXD data also confirm that the metallic layers are well deposited onto the nanofibers. Remarkably, the fibrous morphologies were satisfactorily conserved after removal of the nanofiber template by heat treatment at ca. 400 oC for 24 hr., suggesting the successful deposition of metal layer onto the nanofiber template, and thereby resulted in the formation of metallic nanofibers and nanotubes depending on the diameter of electrospun nanofibers and the thickness of the deposited metallic layers in the conductive metallic nanofibers. In addition, it was observed that the metallized nanofibers exhibit higher conductive properties depending on the thickness of the deposited metallic layers.

Abstract: Nickel- and Platinum-silicide nanodots with an areal density of the order of ~1011cm-2 were successfully formed on thermally-grown SiO2 through a process of ultrathin metal film formation on self-assembled Si quantum dots (QDs) on SiO2 and subsequent remote H2 plasma exposure. Chemical shifts in photoemission spectra of core lines and changes in valence band spectrum and work function value with the remote H2-plasma treatment show that silicidation of pre-grown Si-QDs is promoted by the remote H2-plasma treatment. Electrical separation among so-prepared nanodots was verified from the surface potential change after applying a dc bias between the AFM tip and the sample surface. From temporal decay in the surface potential after electron injection to the nanodots, we confirmed that silicide nanodots have superior charge retention to that of Si nanodots with almost the same size as expected in a deeper potential well for electrons in silicide dots than pure Si-QDs. In the application of silicide nanodots to a floating gate in MOS capacitors, distinct hysteresis characteristics caused by charging and discharging of several electrons per dot were verified by capacitance-voltage measurements.

Abstract: High temperature creep and superplastic flow in high-purity, polycrystalline oxide ceramics is very sensitive to a small amount of doping by various oxides. The doping effect is attributed to change in grain boundary diffusivity owing to grain boundary segregation of the doped cations. The doping effect on the grain boundary diffusivity is caused mainly by change of chemical bonding state in the vicinity of the grain boundary segregated with the doped cations. In other words, controlling of grain boundary nanostructure based on the doping process will be a useful way to develop new high-performance functional ceramics in the near future.