[1] High saturation magnetization and microstructure in melt-spun Fe-P ribbons

R Gopalan et al

A high saturation magnetization, Bs of 1.9 T and coercivity, Hc of 0.5 Oe were obtained in melt-spun ribbons of Fe-0.63 at%P after magnetic annealing at 400°C. Transmission electron microscopy studies revealed the presence of coarse Fe3P precipitates in as-spun ribbons, while these precipitates were found to be finer in annealed samples. The coercivity variation in the samples processed at various conditions was explained due to the formation of the Fe3P precipitates.

[2] Molybdenum carbide precipitation in an Fe-C-Mo alloy under a high magnetic field

Z N Zhou and K M Wu

The molybdenum carbide precipitation during isothermal reaction in an Fe-C-Mo allloy was influenced by applying a 12-T magnetic field. (Fe,Mo)6C was precipitated at earlier transformation stages when the magnetic field was applied; but the carbides Fe3C, (Fe,Mo)2C and (Fe,Mo)3C were precipitated at earlier transformation stages when no magnetic field was applied. The observed results indicate that the precipitation of (Fe,Mo)6C is promoted whereas the precipitation of Fe3C, (Fe,Mo)2C and (Fe,Mo)3C are greatly depressed when a 12-T magnetic field is applied.

[3] Effects of Ti addition on the microstructure and magnetic properties of magnetostrictive Tb-Dy-Fe alloys

J A Chelvane et al

Alloys of Tb0.3Dy0.7Fe1.95-xTix were vacuum induction melted and investigated for microstructural features and magnetic properties. Ti addition aids in diminishing the formation of pro-peritectic (Dy,Tb)Fe3 phase at the expense of TiFe2 phase formation. The magnetostriction was found to improve with the addition of Ti. Ti addition was found to influence the saturation magnetization, spin re-orientation, Fe magnetic moment and Curie temperature of (Tb,Dy)Fe2 phase.

[1] The transformation of narrow dislocation dipoles in selected fcc metals and in γ-TiAl

H Wang et al

Transformations of vacancy dipoles of dissociated edge dislocations are analyzed in Cu, Ni and γ-TiAl by molecular dynamics. Dipole heights up to 20 {1 1 1} interplanar distances are investigated at temperatures ranging from 0 K to near the melting points of Cu and Ni and slightly below the upper boundary of the single phase γ-TiAl domain. Three model configurations, hollows, vertical compact and inclined dipoles, are considered and their relative stabilities compared. Except for dipoles one interplanar distance high, hollows are either metastable or unstable and they are never formed by mutually approaching dipolar dislocations. The three configurations transform into a variety of height- and temperature-dependent layouts including cores containing ordered free volumes, zigzagged faulted dipoles and agglomerated stacking-fault tetrahedra (SFT). At the highest temperatures, small individual SFTs are formed by short-range pipe-diffusion along the dipole cores. There is no critical height below which small-height dipoles or their debris would just simply disappear.

[2] Thermodynamics of grain boundary premelting in alloys. II. Atomistic simulation

P L Williams and Y Mishin

We apply the semi-grand-canonical Monte Carlo method with an embedded-atom potential to study grain boundary (GB) premelting in Cu-rich Cu–Ag alloys. The Σ5 GB chosen for this study becomes increasingly disordered near the solidus line while its local chemical composition approaches the liquidus composition at the same temperature. This behavior indicates the formation of a thin layer of the liquid phase in the GB when the grain composition approaches the solidus. The thickness of the liquid layer remains finite and the GB can be overheated/oversaturated to metastable states slightly above the solidus. The premelting behavior found by the simulations is qualitatively consistent with the phase-field model of the same binary system presented in Part I of this work [Mishin Y, Boettinger WJ, Warren JA, McFadden GB. Acta Mater, in press]. Although this agreement is encouraging, we discuss several problems arising when atomistic simulations are compared with phase-field modeling.

Anomalous triple junction surface pits in nanocrystalline zirconia thin films and their relationship to triple junction energy

H Kim et al

Triple junctions (TJs) are the lines where three grains or grain boundaries meet and become increasingly important in nanocrystalline materials where they have a high areal number density and occupy a significant fraction of the total volume of the material. Surface pits are associated with TJs, just as surface grooves are associated with grain boundaries, and these pits may have particularly deleterious effects on the behaviors of thin films. We evaluate the surface topography associated with TJs in nanocrystalline ZrO2 thin films using thickness mapping images produced by energy-filtered transmission electron microscopy (EFTEM), and compare our results with theoretical predictions. While many of the pits conform to the standard theoretical treatment, some of them exhibit considerably increased depth, possibly indicating that the junctions have line energy. No pits were observed with less than the theoretically predicted depth.

Onset of sidewise instability and cell–dendrite transition in directional solidification

J Teng, S Liu and R Trivedi

The transition from cellular to dendritic microstructure in directional solidification is investigated in succinonitrile (SCN)–camphor alloys. This transition is found not to be sharp, but occurs locally over a range of velocities or thermal gradients, and the diffuseness of the transition is related to the existence of a range of primary spacing. Within the transition zone, critical cell spacing λcd is present where a cell just develops sidewise perturbations. An expression for the critical spacing for the onset of sidewise perturbation is obtained, and it is used to establish the conditions for the start and end of the transition. The results of the present study are then synthesized with those in the SCN–acetone system to incorporate the effect of system parameters on the onset of sidewise instability.

An MD study from Srolovitz, Jim Warren and co-workers on the glassy nature of gbs in the latest PNAS:

Polycrystalline materials are composites of crystalline particles or “grains” separated by thin “amorphous” grain boundaries (GBs). Although GBs have been exhaustively investigated at low temperatures, at which these regions are relatively ordered, much less is known about them at higher temperatures, where they exhibit significant mobility and structural disorder and characterization methods are limited. The time and spatial scales accessible to molecular dynamics (MD) simulation are appropriate for investigating the dynamical and structural properties of GBs at elevated temperatures, and we exploit MD to explore basic aspects of GB dynamics as a function of temperature. It has long been hypothesized that GBs have features in common with glass-forming liquids based on the processing characteristics of polycrystalline materials. We find remarkable support for this suggestion, as evidenced by string-like collective atomic motion and transient caging of atomic motion, and a non-Arrhenius GB mobility describing the average rate of large-scale GB displacement.

[1] On the frequency of occurrence of tilt and twist grain boundaries

A. Morawiec

Homophase grain boundaries are frequently classified into twist, tilt and mixed-type boundaries. With small deviations from pure twist and tilt allowed, there are finite probabilities of occurrence of these particular boundary types in a set of random boundaries. These probabilities are determined for the case of cubic crystal symmetry. If the limit on the deviations is 3°, then 3.9% of random boundaries have near-twist character, and as many as 84.0% of random boundaries are near-tilt boundaries.

[2] A study of low-strain and medium-strain grain boundary engineering

V Randle and M Coleman

Grain boundary engineering (GBE) processing schedules, involving low-strain (5% deformation) iterative treatments, have been carried out on copper. Misorientation and grain boundary plane statistics have been derived, plus tensile and hardness measurements. The Σ3 length fraction and Σ9/Σ3 number ratio decreased during the first two processing iterations, whereas maximum GBE misorientation statistics were achieved after three processing iterations. Analysis of mechanical properties data revealed an accumulation of strain energy throughout the first three processing iterations, sufficient to provide enough driving force for extensive Σ3n interactions. The density of Σ3 boundaries had a larger effect on the rate of hardening than did the density of grain boundaries. This finding indicates the effectiveness of Σ3 interfaces as barriers to plastic flow, which plays an important role in the early stages of GBE processing. Data from samples that had undergone the low-strain iterations were also compared to medium-strain (25% deformation) processing iterations.

[3] Phase-field model study of the crystal morphological evolution of hcp metals

R S Qin and H K D H Bhadeshia

An expression for anisotropic interfacial energy of hexagonal close-packed metals has been formulated which is able to reproduce published data obtained using the modified embedded-atom method, covering the variation in interface energy as a function of orientation for a number of metals. The coefficients associated with the expression can be determined fully by measured or calculated interfacial energies of just three independent crystal planes. Three-dimensional phase-field model simulations using this representation of interfacial energy have been found to yield convincing crystal morphologies. The apparent rate of crystal growth as a function of orientation in the phase-field simulation agrees with predictions made by surface energy theory.

[4] Metallographic evidence of carbon diffusion in the growth of bainite

A Borgenstam, M Hillert and J Agren

There are two paradigms regarding the formation of bainite. One is based on the first stage being rapid, diffusionless growth of acicular ferrite and the subsequent formation of carbide occurring by precipitation from the supersaturated ferrite. An assumption that the first stage occurs as a series of subsequent rapid steps resulting in sub-units plays an important role as an explanation of the not so rapid growth observed macroscopically. The other paradigm is based on the first stage being the formation of acicular ferrite under carbon diffusion and on the subsequent growth of carbide and ferrite side by side. Metallographic observations are presented that support the second paradigm. It is difficult to see how they can be accounted for by the first paradigm, in particular the observation of the shapes of sub-units.

[5] Temperature and structure dependency of solid–liquid interfacial energy

K Mondal et al

A new model has been proposed for the prediction of solid–liquid interfacial energy for pure elements. It is assumed that the interface between crystalline solid embryo and bulk liquid consists of a monolayer of atoms having a similar atomic packing factor as that of the crystalline solids. It has been observed that the solid–liquid interfacial energy is a strong function of temperature and structure of the solid and planar density of the interface. The solid–liquid interfacial energy has a lower value close to melting temperature and it reaches a maximum at some intermediate temperature. This model tries to correlate the classical nucleation phenomena and structure model of interfaces.