Positron annihilation study of the vacancy clusters in ODS Fe–14Cr alloys

Abstract

Oxide dispersion strengthened Fe14Cr and Fe14CrWTi alloys produced by mechanical alloying and hot isostatic pressing were subjected to isochronal annealing up to 1400 °C, and the evolution and thermal stability of the vacancy-type defects were investigated by positron annihilation spectroscopy (PAS). The results were compared to those from a non-oxide dispersion strengthened Fe14Cr alloy produced by following the same powder metallurgy route. The long lifetime component of the PAS revealed the existence of tridimensional vacancy clusters, or nanovoids, in all these alloys. Two recovery stages are found in the oxide dispersion strengthened alloys irrespective of the starting conditions of the samples. The first one starting at T > 750 °C is attributed to thermal shrinkage of large vacancy clusters, or voids. A strong increase in the intensity of the long lifetime after annealing at temperatures in the 800–1050 °C range indicates the development of new vacancy clusters. These defects appear to be unstable above 1050 °C, but some of them remain at temperatures as high as 1400 °C, at least for 90 min.     

Precise half-life measurements for 38Ca and 39Ca

The half-lives of ³⁸Ca and ³⁹Ca have been measured at ISOLDE of CERN. The REXTRAP facility was used to prepare ultra-clean samples of radioactive nuclei for precision decay spectroscopy. ³⁸Ca is one of the T _z = - 1 , 0⁺ $ \rightarrow$ 0⁺ $ \beta$ -emitting nuclides used to determine the vector coupling constant of the weak interaction and the V_ud quark-mixing matrix element. The result obtained, T _1/2 = 443.8(19) ms, is four times more precise than the average of previous measurements. For ³⁹Ca , a half-life of T _1/2 = 860.7(10) ms is obtained, a result in agreement with the average value from the literature.     

Kinetics and mechanisms of gas‐phase elimination of 2,2‐diethoxyethyl amine and 2,2‐diethoxy‐N,N‐diethylethanamine

The gas‐phase elimination kinetics of 2,2‐diethoxyethyl amine and 2,2‐diethoxy‐N,N‐diethylethanamine (320–380 °C; 40–150 Torr) in a seasoned reaction vessel are homogeneous, unimolecular and obey a first‐order rate law. These elimination processes involve two parallel reactions. The first gives ethanol and the corresponding 2‐ethoxyethenamine. The latter compound further decomposes to ethylene, CO and the corresponding amine. The second parallel reaction produce ethane and the corresponding ethyl ester of an α‐amino acid. The following Arrhenius expressions are given as: For 2,2‐diethoxyethyl amine For 2,2‐diethoxy‐N,N‐diethylethanamine Comparative kinetic and thermodynamic parameters of the overall, the parallel and the consecutive reactions lead to consider two types of mechanisms in terms of a concerted polar cyclic transition state structures. Copyright © 2008 John Wiley & Sons, Ltd.     

Variational Evaluation of the HSSH FIR Transitions: The Anomalous K-Doubling

The roto-torsional energy levels of HSSH and DSSD up to J = 20 are evaluated variationally with a Hamiltonian expressed in terms of internal coordinates. The kinetic and potential parameters are derived from ab initio calculations with full optimization of the geometry. The calculated levels are employed for the determination of the centrifugal distortion constants. HSSH is a near-prolate symmetric rotor. The most stable C(2) conformer, calculated with MP4(SDQ)/cc-pVQZ, exhibits a 90.55 degrees dihedral angle. For J = 0, the lowest energies of HSSH and DSSD are 413.4876 cm(-1) (n = 1), 798.0304 cm(-1) (n = 2) and 1151.5773 cm(-1) (n = 3), and 304.3185 cm(-1) (n = 1), 594.2919 cm(-1) (n = 2), and 869.3508 cm(-1) (n = 3), respectively. For J = 60, the ab initio calculations allow the reproduction of the anomalous type-K doubling predicted with perturbation theory. Copyright 2000 Academic Press.     

Comment on ‘Ab initio study: the potential energy curves and rovibrational spectrum of low-lying excited states of HCl+ cation’ (Molecular Physics, 114:19, 2817–2823)

In a recent article, Y. Liu, X. Cheng, H. Cheng, J. Cheng and X. Song [Mol. Phys., 114:19, 2817–2823] report on the calculation of potential energy curves and the derivation of spectroscopic constants and line intensities for the lowest energy electronic states of HCl⁺. There are several shortcomings in the article; the most notable being the incorrect ordering of the ²П_3/2 and ²П_1/2 state energies, erroneous selection rules, which lead to the absence of a Q-branch in their predicted spectra, incorrect Höln–London factors, and a missing appendix.     

Creep mechanism of gas-pressure-sintered silicon nitride polycrystals I. Macroscopic and microscopic experimental study

The creep behaviour and microstructure of two silicon nitride ceramics have been investigated. Compressive creep tests were performed at temperatures between 1450 and 1700°C at stresses between 6 and 90?MPa in an Ar atmosphere. The creep behaviour was characterized by a stress exponent lower than one for both materials, with an average value n?≈?0.6 over the whole range of stresses and temperatures, and with apparent activation energies between 470 and 530?kJ?mol^?1. The study of the microstructural evolution revealed the absence of dynamic grain growth and, in some cases, evidence of grain rearrangement. Partial coalescence of cavities was observed only at the highest stress, but this did not result in accelerated creep.     

Effective temperature in driven vortex lattices with random pinning

We study numerically correlation and response functions in nonequilibrium driven vortex lattices with random pinning. From a generalized fluctuation-dissipation relation, we calculate an effective transverse temperature in the fluid moving phase. We find that the effective temperature decreases with increasing driving force and becomes equal to the equilibrium melting temperature when the dynamic transverse freezing occurs. We also discuss how the effective temperature can be measured experimentally from a generalized Kubo formula.     

X‐ray absorption study of the local structure at the NiO/oxide interfaces

This work reports an X‐ray absorption near‐edge structure (XANES) spectroscopy study at the Ni K‐edge in the early stages of growth of NiO on non‐ordered SiO₂, Al₂O₃ and MgO thin films substrates. Two different coverages of NiO on the substrates have been studied. The analysis of the XANES region shows that for high coverages (80 Eq‐ML) the spectra are similar to that of bulk NiO, being identical for all substrates. In contrast, for low coverages (1 Eq‐ML) the spectra differ from that of large coverages indicating that the local order around Ni is limited to the first two coordination shells. In addition, the results also suggest the formation of cross‐linking bonds Ni—O—M (M = Si, Al, Mg) at the interface.     

Colloidal particles at fluid interfaces: Effective interactions, dynamics and a gravitation–like instability

Colloidal particles of micrometer size usually become irreversibly trapped at fluid interfaces if they are partially wetted by one phase. This opens the chance to create two–dimensional model systems where the effective interactions between the particles are possibly influenced by the presence of the interface to a great extent. We will review recent developments in the quantitive understanding of these effective interactions with a special emphasis on electrostatics and capillarity. Charged colloids of micrometer size at an interface form effective dipoles whose strength sensitively depends on the double layer structure. We discuss the success of modified Poisson–Boltzmann equations with regard to measured colloidal dipole moments. On the other hand, for somewhat larger particles capillary interactions arise which are long–ranged and analogous to two–dimensional screened Newtonian gravity with the capillary length λ as the screening length. For colloidal diameters of around 10 micrometer, the collective effect of these long–ranged capillary interactions will dominate thermal motion and residual, short–ranged repulsions, and results in an instability towards a collapsed state for a finite patch of particles. Such long–ranged interactions with the associated instability are also of interest in other branches of physics, such as self-gravitating fluids in cosmology, two–dimensional vortex flow in hydrodynamics, and bacterial chemotaxis in biology. Starting from the colloidal case we develop and discuss a dynamical “phase diagram” in the temperature and interaction range variables which appears to be of more general scope and applicable also to other systems.