A study on effects of TiB2 contents on reactive products and compressive strength of brazed CBN grains

The composite fillers were prepared by means of mixing Ag‐Cu‐Ti alloy and TiB₂ reinforcing particles. Experimental study was conducted on the chemical reaction and the reactive products at the interface between CBN grains and the composite filler. The compressive strength measurement of brazed CBN grains was performed. Results obtained show that the TiB₂particles disperse uniformly in the filler layer. Great influence of TiB₂particles is made on the quantity of the reactive products formed on the surface of brazed CBN grains. The destruction of the sharp edges of grains is restrained. Depending on the compact and fine reactive compounds, hard joining is realized for the CBN grain and the filler layer. When the content of TiB₂particles in the composite filler is 8 wt%, the thermal damage of the brazed CBN grains are controlled effectively. In this case the compressive strength of the brazed grains reaches 15.3 N, which is as high as that of the original ones. Copyright © 2009 John Wiley & Sons, Ltd.     

Columnar grain growth pattern with fluid flowing in molten pool

A coupled model was used to simulate columnar grain growth in TIG (tungsten inert‐gas) molten pool of nickel base alloy. The cellular automaton algorithm for dendritic growth is incorporated with solute transport model to take fluid flow into consideration. The results indicate that shear flow changes the solute distribution at the S/L (Solid/Liquid) interface, leading to asymmetrical growth of columnar grains. The dendrite arms on the upstream side grow fast, while the growth of dendrite arms on the downstream side is much delayed. However, dendrite arms on both sides are not as well‐developed as the grain growth without flow. With inlet flow velocity increasing, the phenomenon becomes more obvious. In addition, shear flow also results in more severe coring segregation. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Recycling process for 123-type bulk superconductors

Bulk (RE)-Ba-Cu-O (RE is a rare earth element or yttrium) high temperature superconductors have contained a lot of precious substances such as Ag and Pt, which led to a significant increase in production cost. However, the bulk superconductors have been thrown away once they were fractured during characterization or failed to grow in the form of single grain. The material recycling of these samples is, therefore, a great important and leads to a cost-effective production. Hence we have attempted a recycling process for Ag-doped Gd-Ba-Cu-O (GdBCO/Ag) bulk samples. The single grain GdBCO/Ag bulk sample, however, could not be grown by the cold-seeded melt-process albeit the recycled precursor powder is chemically identical to a mixture of commercial powder (reference powder). On the other hand, the hot-seeding technique enables us to fabricate readily the single grain GdBCO/Ag bulk samples even with employing the recycled precursor powder. The recycle-processed GdBCO/Ag bulk sample showed a 10% decrease in the trapped magnetic field compared with the reference bulk sample. However, most importantly this study demonstrates clearly, and for the first time, that the recycling of GdBCO/Ag bulk samples is possible, nevertheless a small degradation of superconducting properties is recognized.     

Phase-field simulation of the coupled evolutions of grain and twin boundaries in nanotwinned polycrystals

Nanotwinned polycrystals exhibit an excellent strength-ductility combination due to nanoscale twins and grains. However, nanotwin-assisted grain coarsening under mechanical loading reported in recent experiments may result in strength drop based on the Hall-Petch law. In this paper, a phase-field model is developed to investigate the effect of coupled evolutions of twin and grain boundaries on nanotwin-assisted grain growth. The simulation result demonstrates that there are three pathways for coupled motions of twin and grain boundaries in a bicrystal under the applied loading, dependent on the amplitude of applied loading and misorientation of the bicrystal. It reveals that a large misorientation angle and a large applied stress promote the twinning-driven grain boundary migration. The resultant twin-assisted grain coarsening is confirmed in the simulations for the microstructural evolutions in twinned and un-twinned polycrystals under a high applied stress.     

Dilatometric investigation of thermotropic linear chain copper (II) alkanoates

The molar volumes of three binuclear copper (II) alkanoates (cupric octadecanoate, docosanoate, and tetracosanoate) were measured over the temperature range 30°–200 °C, which encompasses their phase transition to a columnar mesophase. A sharp volume increase in a narrow temperature interval about the transition was observed, consistent with a first-order phase transition. Values obtained for the partial molar volume of a methylene group in the columnar mesophase (16.9±0.3 cm³/mol) are between those for crystalline paraffins and for other smectic or discotic mesogens, respectively, indicating a disordered state, but not a complete fusion, of the aliphatic chains of the copper soaps in their columnar mesophase. The stacking period of the binuclear copper complexes in a column was derived from a combination of dilatometric and X-ray data. Its value, 4.64±0.08 Å, is independent of the chain length of the complex and of the temperature.     

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their mixtures IX. Multi-mixtures of a monotrope and enantiotropes

Transition temperatures of the ternary and quaternary mixtures, prepared from the enantiotropic cyano and nitro derivatives of the two series, 4-hexadecyloxyphenyl 4-substituted benzoates (Ia,b) and 4-substituted phenyl-4'-hexadecyloxy benzoates (IIa,b) with the monotropic methoxy derivative (IIc), have been determined by DSC and identified with polarized light microscopy. The equation deduced to calculate the eutectic composition of a multicomponent system from values determined for their individual binary systems, was found to be fairly applicable to our non-ideal systems. Conversely, values of clearing temperatures (T_c), calculated from the polarizability anisotropy (Δα_X) of the individual C_Ar-X bonds, are often less than those measured, indicating complex formation between the methoxy and nitro derivatives.     

Electrical activity and structural configuration of transition metals (Cu, Ni, Fe) in silicon bicrystals

Deep-level transient spectroscopy and electron-beam-induced current measurements have been performed on =25, =13 and =9 silicon bicrystals contaminated with Cu, Ni or Fe. Among the studied grain boundaries, only the heat-treated =25 showed a barrier effect which seems to originate, for the uncontaminated samples, from the structural defects induced by the rapid disorder with temperature. An extrinsic origin would start to prevail, however, upon formation of impurity precipitates in the contaminated specimens. The evolution of the barrier height (BH) and the electronic interface states with the annealing temperature, T _a, allowed one to infer that the rectifying behavior of the buried silicides could be compared to that of the same silicide layer grown on a silicon surface. This was strongly suggested by the systematic lowering of the BH with T _a which is consistent with the increasingly greater contribution of type-A NiSi₂ silicides with their low BH. The opposite behavior of the Cu and Fe cases appeared due to the tendency towards the pinning of the Fermi level near the mid-gap. For EBIC, the different aspects of the contrast were assigned to the various microstructures of the interfaces. Finally, the formation of impurity particles were found to involve a net degradation of the minority-carrier diffusion length, in addition to a concomitant and consistent increase of both recombination velocity and density of interface states.     

Interaction of Solute Impurity with Grain Boundary: The Impurity Drag Effect

An equation of grain boundary motion in a binary polycrystal is derived. The derivation is based on minimization of free energy of the total systems. The equation takes into account an impurity segregation at the grain boundary, grain boundary curvature and energy.As an example, we apply this equation to the analysis of the impurity drag effect problem. It is shown, that the sign of the impurity effect on grain boundary velocity (delay or acceleration) does not depend on kinetic coefficients. The sign of the effect is determined by a thermodynamic function which combines the grain boundary segregation coefficient, the derivative of grain boundary energy with respect to absorbed impurity concentration, and the derivative of bulk free energy with respect to bulk impurity concentration.