Effect of Co addition

February 28, 2010

Effect of Co addition on microstructure and magnetic properties of ferromagnetic CoFeSiB alloys

B S Chun et al

A correlation between composition, microstructure, and magnetic properties of sputter-deposited CoFeSiB alloy films has been studied. Various analytical tools and micromagnetic models were employed to understand the details of microstructural evolution and magnetic reversal processes, respectively. The CoFeSiB alloy film shows significantly different microstructure and magnetic properties depend on the Co concentration. When the Co concentration is below 75 at.%, the alloy has an amorphous phase exhibiting magnetic softness with negative remanence. Meanwhile beyond 75 at.%, the structure consists of nanocrystals precipitated in the amorphous matrix, which becomes magnetically hard with positive remanence as well as antiferromagnetic exchange coupling.

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Four Questions About Triple Lines

W Craig Carter et al

The identification of triple lines as one of a hierarchy of defects is presented. There are several distinct cases of triple junctions, and these are sorted into classes. Viewpoints about open questions and directions for future research are offered, including 1) the effect of induced order on the structure and energy of a triple line; 2) considerations of interfacial (complexion) transitions; 3) suggestions of methods for the direct measurement of triple-line energy; 4) observations of the possibility of triple-line anisotropy.

Incorporating Diffuse-interface Nuclei in Phase-field Simulations

T W Heo et al

We propose a computational framework for incorporating diffuse-interface critical nuclei in phase-field simulations. Using a structural transformation as an example, we first generate a table of diffuse-interface critical nuclei. We then incorporate them in phase-field simulations through the explicit nucleation algorithm. The temporal growth kinetics of the introduced nuclei is obtained by numerically solving the Allen-Cahn equation. The results are analyzed by comparing to the phase transformation kinetics using the classical nucleation and normal growth theory and the Kolmogorov-Johnson-Mehl-Avrami equation.

Modeling fluid flow in three-dimensional single crystal dendritic structures

J Madison et al

Convection during directional solidification can cause defects such as freckles and misoriented grains. To gain a better understanding of conditions associated with the onset of convective instabilities, flow was investigated using three-dimensional (3D) computational fluid dynamics simulations in an experimentally obtained dendritic network. A serial-sectioned, 3D data set of directionally solidified nickel-base superalloy measuring 2.3 × 2.3 × 1.5 mm was used to determine the permeability for flow parallel and normal to the solidification direction as a function of solid fraction (fS). Anisotropy of permeability varies significantly from 0.4 < fS < 0.6. High flow velocity channels exhibit spacings commensurate with primary dendrite arms at the base of the mushy zone but rapidly increase by a factor of three to four towards dendrite tips. Permeability is strongly dependent on interfacial surface area, which reaches a maximum at fS = 0.65. Results from the 3D simulation are also compared with empirical permeability models, and the microstructural origins of departures from these models are discussed.

Triple lines and triple lines

February 13, 2010

A couple of more triple lines papers:

[1] Triple lines in materials science and engineering

A H King

We assess the impact of triple lines in materials preparation and use by considering several examples of materials behavior in which they have identifiable effects. The microstructural roles of triple lines are also considered and some persistent scientific questions are raised.

[2] Grain boundary ridges and triple lines

B B Straumal et al

The classification of various grain boundary (GB) and surface triple lines is given. The observations of “degenerated” GB triple lines like first-order and second-order facet-to-facet, rough-to-facet and rough-to-rough ridges, as well as there influences on the GB mobility are discussed. Some interesting phenomena remain undisclosed in experiments and should be studied in the future.

Triple lines and nucleation

February 5, 2010

A letter from Greer in Scripta.

[1] Numerical solution and comparison to experiment of solute drag models for binary alloy solidification with a planar phase interface

Shu Li et al

A generalized solute drag model for binary alloy solidification with a planar phase interface was proposed as an extension of Hillert-Sundman model [M. Hillert, B. Sundman, Acta Metall. 24 (1976) 731]. Using a new thermodynamic parameter set of Si-As system, the present model can, at the expense of the introduction of three new adjustable parameters, fit the available experimental data of Si-9at.%As alloy. In addition, non-solute-drag models with the new set of thermodynamic parameters can not reproduce the experimental data.

[2] Model for coarsening of intergranular precipitates in multicomponent systems

J Svoboda and F D Fischer

Based on the thermodynamic extremal principle a model for coarsening of intergranular precipitates in multicomponent systems is developed. The model provides the evolution equations for individual precipitates within the mean-field approach and generalizes the recent models applicable only to a special binary system. The simulations based on the model are performed for precipitates with the starting LSW size distribution which changes after a certain time to a quasistationary one with a t{1/4} growth law being in agreement with recent models.