The structure of complex monoborides in γ-TiAl alloys with Ta and B additions

Fibrous primary borides grown from the melt in a Ti-48Al-9Ta-4.3B at.% alloy were characterized by conventional and high resolution transmission electron microscopy. The as-cast borides show predominantly the B27 crystal structure of TiB, but with twice as much Ta as Ti in the metal sublattice. The borides contained numerous planar faults parallel to the (100)_TiB planes, which multiplied and evolved into long platelike precitates with the structure of TaB (B_f) upon prolonged heat treatment. The initial faults could be described as rotational twins on the (100)_TiB plane, giving rise to monolayers of the B_f structure; migration of planar diffusion regions along the needle axis subsequently coarsens the TaB regions. Experimental and computer-generated diffraction patterns were consistent with the orientation relationship anticipated from the twinning model, (100)_TiB|(100)_TaB and [010]_TiB|[001]_TaB. High resolution images showed that the B_f precipitates are fully coherent with the parent B27 matrix and only a few atomic layers thick, but extend over many micrometers along the axis of the boride needles. Moreover, microchemical analysis revealed that the structural transformation occurs without apparent solute partitioning. The evolution of the boride structures is discussed in terms of phase stability, growth kinetics and lattice energy considerations.     

A rationalisation of fatigue thresholds in pearlitic steels using a theoretical model

From a detailed re-examination of results in the literature, the effects of microstructure sizes, namely interlamellar spacing, pearlitic colony size and the prior austentitic grain size on the thresholds for fatigue crack growth (ΔK_th) and crack closure (K_{cl, th}) have been illustrated. It is shown that while interlamellar spacing explicitly controls yield strength, a similar effect on ΔK_th cannot be expected. On the other hand, the pearlitic colony size is shown to strongly influence ΔK_th and K_{cl, th} through the deflection and retardation of cracks at colony boundaries. Consequently, an increase in ΔK_th and K_{cl, th} with colony size has been found. The development of a theoretical model to illustrate the effects of colony size, shear flow stress in the slip band and macroscopic yield strength on K_{cl, th} and ΔK_th is presented. the model assumes colony boundaries as potential sites for slip band pile-up formation and subsequent crack deflection finally leading to zig-zag crack growth. Using the concepts of roughness induced crack closure, the magnitude of K_{cl, th} is quantified as a function of colony size. In deriving the model, the flow stress in the slip band has been considered to represent the work hardened state in pearlite. Comparison of the theoretically predicted trend with the experimental data demonstrates very good agreement. Further, the intrinsic or closure free component of the fatigue threshold, ΔK_{eff, th} is found to be insensitive to colony size and interlamellar spacing. Using a criterion for intrinsic fatigue threshold which considers the attainment of a critical fracture stress over a characteristic distance corresponding to interlamellar spacing, ΔK_th values at high R values can be estimated with reasonable accuracy. The magnitude of ΔK_th as a function of colony size is then obtained by summing up the average value of experimentally obtained ΔK_{eff, th} values and the predicted K_{cl, th} values as a function of colony size. Again, very good agreement of the theoretically predicted ΔK_th values with those experimentally obtained has been demonstrated.     

Effect of partial substitution of La with Ce,Pr and Nd on the properties of LaNi5-based alloy electrodes

A comparison is made of the properties of LaB₅ (BNi_3.55Co_0.75Mn_0.4Al_0.3), La_0.7R_0.3B₅ (RCe, Pr, Nd) and MmB₅ (Mm is mischmetal in an atomic ratio of La:Ce:Pr:Nd = 0.7:0.2:0.05:0.05) alloy electrodes. X-ray diffraction results reveal that Ce, Pr, Nd substitute for La and decrease the unit cell volume. Pressure-composition isotherms of the electrode alloys are determined by an electrochemical method. The characteristics of the alloy electrodes, including initial activation, high-rate discharge, cycle life and self-discharge, are examined. It is found that partial replacement of La with Ce, Pr, Nd in the LaNi₅-based alloy improves greatly the activation, high-rate discharge and cycle life of the electrode, but increases the self-discharge due to a higher dissociation pressure of the metal hydride.     

Electrochemistry of phenyl-N-capped aniline oligomers. Evaluation of optical and electrochemical properties of ideal polyaniline

Potentials for the first reversible oxidation and optical bandgaps of phenyl-N-capped aniline oligomers (n = 1,2 and 4) are linearly related with 1/n. The comparison of the values extrapolated for the infinitely long polymer and those of electrochemically synthesized polymer samples indicates for the latter an average equivalent length of 6–10 monomeric units.     

Fast Electronic Digital Image Stabilization for Off-Road Navigation

Image stabilization can be used as front-end system for many tasks that require dynamic image analysis, such as navigation and tracking of independently moving objects from a moving platform. We present a fast and robust electronic digital image stabilization system that can handle large image displacements based on a two-dimensional feature-based multi-resolution motion estimation technique. The method tracks a small set of features and estimates the movement of the camera between consecutive frames. Stabilization is achieved by combining all motion from a reference frame and warping the current frame back to the reference. The system has been implemented on parallel pipeline image processing hardware (a Datacube MaxVideo 200) connected to a SUN SPARCstation 20/612 via a VME bus adaptor. Experimental results using video sequences taken from a camera mounted on a vehicle moving on rough terrain show the robustness of the system while running at approximately 20 frames/s.     

An explicit eighth-order method with minimal phase-lag for accurate computations of eigenvalues,resonances and phase shifts

A new explicit hybrid eighth algebraic order two-step method with phase-lag of order ten is developed for computing eigenvalues, resonances and phase shifts of the one-dimensional Schrödinger equation and coupled differential equations arising from the Schrödinger equation. Based on this new method and on the method developed recently by Simos we obtain a new variable-step procedure for the numerical integration of the Schrödinger equation. Numerical results obtained for the integration of the resonance problem for the well known case of the Woods-Saxon potential, for the integration of the eigenvalue problem for the well known case of the Morse potential and for the integration of coupled differential equations show that this new method is better than other variable-step methods.     

Molecular materials for electronic and optical applications

In the past two years molecular rectification and single electron transistor effects in molecular devices have been demonstrated. Because of the ambiguity of assigning these effects to particular molecules, research has turned to carbon nanotubes which are larger than more classical molecules. For optical applications bright and multicoloured organic light emitting diodes are available.