An effect of antiphase boundaries on the kinetics of short-range ordering by a vacancy mechanism

In Monte Carlo simulations of chemical short-range ordering on a square lattice, the number of single-atom domains was found to depend on the presence of antiphase boundaries and on the mechanism by which ordering occurred. When antiphase boundaries were present and the ordering occurred by a vacancy mechanism, the number of single-atom domains was found to increase with decreasing temperature, in contrast to thermodynamic predictions. This is understood as a consequence of highly correlated vacancy motions in those regions of the lattice away from antiphase boundaries

Two-phase coexistence in Fe–Ni alloys synthesized by ball milling

We used mechanical alloying with a Spex 8000 mixer/mill to synthesize a series of Fe100–xNix alloys from x=0 to x=49. The Spex mill was modified so that it could also operate at a reduced milling intensity, and we compared the alloys synthesized after long times with the normal and reduced milling intensities. X-ray diffractometry and Mössbauer spectrometry were used to measure the volume fractions of the bcc and fcc phases in the alloys, and to determine the chemical compositions of the individual phases. We found that the composition ranges of the bcc and fcc single phase regions were extended well beyond their equilibrium ranges. At the higher milling intensity, we found that the bcc phase was destabilized with respect to the fcc phase, and the two-phase region shifted to lower Ni concentrations. For those alloys with coexisting bcc and fcc phases, we present evidence that the chemical compositions of the two phases are nearly the same. We explain the destabilization of the bcc with milling intensity as originating with a higher defect density in the bcc alloys than in the fcc alloys. We argue that this defect density is not homogeneous throughout the alloy, however, and the distribution of defect enthalpies can explain the two-phase coexistence in the as-milled alloys

Charge Redistribution and Phonon Entropy of Vanadium Alloys

The effects of alloying on the lattice dynamics of vanadium were investigated using inelastic neutron scattering. Phonon densities of states were obtained for bcc solid solutions of V with 3d, 4d, and 5d transition metal solutes, from which vibrational entropies of alloying were obtained. A good correlation is found between the vibrational entropy of alloying and the electronegativity of transition metal solutes across the 3d row and down columns of the periodic table. First-principles calculations on supercells matching the experimental compositions predicted a systematic charge redistribution in the nearest-neighbor shell around the solute atoms, also following the Pauling and Watson electronegativity scales. The systematic stiffening of the phonons is interpreted in terms of the modified screening properties of the electron density around the solutes

First-principles study of phonon linewidths in noble metals

Phonon lifetimes in Cu, Ag, and Au at low and high temperatures were calculated along high symmetry directions using density functional theory combined with second-order perturbation theory. Both harmonic and third-order anharmonic force constants were computed using a supercell small displacement method, and the two-phonon densities of states were calculated for all three-phonon processes consistent with the kinematics of energy and momentum conservation. A nonrigorous Grüneisen model with no q-dependence of the anharmonic coupling constants offers a simple separation of the potential and the kinematics, and proved semiquantitative for Cu, Ag, and Au. A rule is reported for finding the most anharmonic phonon mode in fcc metals

Chemical Environment Selectivity in Mössbauer Diffraction from 57Fe3Al

Mössbauer diffraction was used to measure different autocorrelation functions for 57Fe atoms in different chemical environments. The sample was polycrystalline 57Fe3Al with the ordered DO3 structure. Diffraction peaks from a fcc structure with a doubled unit cell were detected when the incident radiation was tuned to the Mössbauer resonance of the Wyckoff 4(b) Fe site, but not for tuning to the 8(c) site, thereby distinguishing the spatial arrangements of these two Fe sites

Magnetomechanical damping by polycrystalline TbDy

Vibration damping in polycrystalline TbDy alloys was studied at cryogenic temperatures. Mechanical hysteretic losses were measured at various strains, frequencies, and loading configurations at 77 K. Some textured TbDy materials demonstrated 22.6% energy dissipation in mechanical measurements at low frequency (0.01 Hz) and a mean logarithmic decrement of 0.23 at a higher frequency (25 kHz). Ultrasonic velocities of longitudinal and shear elastic waves were measured on single and polycrystalline TbDy; little variation in ultrasonic velocities was found even for samples with large variation in crystallographic texture and magnetomechanical properties

The Character of Dislocations in LiCoO2

Dislocations in LiCoO2 were observed by transmission electron microscopy, and their Burgers vectors were determined by analysis of diffraction contrast in tilting experiments. The configuration of all dislocations indicates that they are glissile, and dislocation configurations were found that are indicative of active slip planes. Perfect dislocations of a/3 type Burgers vectors were observed on {0001} habit planes. These perfect dislocations sometimes dissociate into Shockley partial dislocations with a/3 type Burgers vectors. Glide of these partial dislocations can account for the sequence of crystal structures O3, H1-3, O1 that occur with the delithiation of LiCoO2. The presence of glissile dislocations also suggests possible damage mechanisms during cycling

Vibrational entropy of L12 Cu3Au measured by inelastic neutron scattering

The phonon density of states of elemental Au, Cu, and Cu3Au with L12 chemical order were measured by inelastic neutron scattering and used to calculate the vibrational entropy of formation of the ordered compound from the elemental metals. A vibrational entropy of formation of (0.06±0.03) kB/atom at 300 K was obtained, with the vibrational entropy of the ordered alloy being larger than that of the elemental metals. The phonon DOS of the disordered Cu3Au was simulated by adding the phonon DOS curves of fcc Cu, L12 Cu3Au, and fcc Au to match the numbers of first-nearest-neighbor pairs in a disordered alloy. The vibrational entropy obtained with this simulated DOS disagrees with calorimetric data and theoretical estimates, indicating that the phonon DOS of disordered Cu3Au depends on chemical order at spatial lengths larger than is set by first-nearest-neighbor pairs

Negative Entropy of Mixing for Vanadium-Platinum Solutions

The phonon densities of states for pure vanadium and the solid solutions V-6.25% Ni, Pd, Pt were determined from inelastic neutron scattering measurements. The solute atoms caused a large stiffening of the phonons, resulting in large, negative vibrational entropies of mixing. For V-6.25%Pt, the negative vibrational entropy of mixing exceeds the conventional positive chemical entropy of mixing. This negative total entropy of mixing should extend to lower concentrations of Pt, and the effect on the bcc solvus line is discussed. The experimental data were inverted to obtain interatomic force constants by using a Born–von Kármán model with an iterative optimization algorithm. The stiffening of bonds responsible for the decrease of entropy was found to occur mainly in first-nearest-neighbor solute-host bonds, and correlates in part with the solute metallic radius