Few papers of interest from Acta and Scripta:

[1] Phase-field simulation of void migration in a temperature gradient

S Y Hu and C H Henegar Jr

A phase-field model simulating vacancy diffusion in a solid with a strong vacancy mobility inhomogeneity is presented. The model is used to study void migration via bulk and surface diffusion in a temperature gradient. The simulations demonstrate that voids migrate up the temperature gradient, and the migration velocity varies inversely with the void size, in agreement with theory. It is also shown that the current model has the capability to investigate the effects of surface diffusion, temperature gradient and vacancy concentration on the void migration velocity. An interesting potential application of the model is to study the kinetics of void migration and the formation of a central hole in nuclear fuels.

[2] Analysis by high-resolution electron microscopy of elastic strain in thick InAs layers embedded in Ga0.47In0.53As buffers on InP(0 0 1) substrate

C Gatel et al

Elastic strain has been investigated by transmission electron microscopy in nanometric InAs layers grown on Ga0.47In0.53As/InP(0 0 1) by molecular beam epitaxy using a residual Sb flux. Deposits of 10 and 15 monolayers of InAs (3 and 4.5 nm) remain elastically stressed with a two-dimensional growth mode. The out-of-plane strain in the layers is analyzed by cross-sectional high-resolution electron microscopy. A distortion of the substrate below and on top of the InAs layers is detected and is attributed to a significant surface relaxation effect due to thinning. Surface relaxation is modeled by three-dimensional finite element modeling. An additional relaxation effect is obtained when the sample is not infinite along the direction perpendicular to the thinning. This effect enhances the buffer distortion of the buffers below and on top of the strained layers. Taking into account thin foil effects, the experimental out-of-plane strain is in excellent agreement with the theoretical value calculated for a pure InAs layer (i.e. 0.035), demonstrating the high level of strain and stress in the layers.

[3] The influence of solid-liquid interfacial energy anisotropy on equilibrium shapes, nucleation, triple lines and growth morphologies

M Rappaz et al

The anisotropy of the solid-liquid interfacial energy plays a key role during the formation of as-solidified microstructures. Using the ξ-vector formalism of Cahn and Hoffman, this contribution presents the effect that anisotropy has on the equilibrium shapes of crystals and on surface tension equilibrium at triple lines. Consequences on heterogeneous nucleation of anisotropic crystals and on dendritic growth morphologies are detailed with specific examples related to Al-Zn and Zn-Al alloys.

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.

A new resolution standard for spherical and chromatic aberration corrected electron microscopy has apparently been set; here is the commentary.

Electron channeling contrast imaging of twins and dislocations in twinning induced plasticity steels under controlled diffraction conditions in scanning electron microscopy

I Gutierrez-Urrutia, S Zaefferer, and D Raabe

Dislocation cells and mechanical twins have been imaged by electron channeling contrast imaging (ECCI) in a scanning electron microscope under controlled diffraction conditions in a deformed Fe-22Mn-0.6C twinning induced plasticity steel using a novel set-up. The approach uses electron backscattered diffraction (EBSD) for orientation-optimized ECCI with enhanced dislocation and interface contrast. The observations provide new insights into the strain hardening mechanisms of TWIP steels.

A paper in the recent Nature Materials (via iMechanica):

In situ observation of dislocation nucleation and escape in a submicrometre aluminium single crystal

S H Oh et al

‘Smaller is stronger’ does not hold true only for nanocrystalline materials but also for single crystals. It is argued that this effect is caused by geometrical constraints on the nucleation and motion of dislocations in submicrometre-sized crystals. Here, we report the first in situ transmission electron microscopy tensile tests of a submicrometre aluminium single crystal that are capable of providing direct insight into source-controlled dislocation plasticity in a submicrometre crystal. Single-ended sources emit dislocations that escape the crystal before being able to multiply. As dislocation nucleation and loss rates are counterbalanced at about 0.2 events per second, the dislocation density remains statistically constant throughout the deformation at strain rates of about 10-4 s-1. However, a sudden increase in strain rate to 10-3 s-1 causes a noticeable surge in dislocation density as the nucleation rate outweighs the loss rate. This observation indicates that the deformation of submicrometre crystals is strain-rate sensitive.

In Nature this week

May 11, 2007

A monolayer of manganese on a tungsten substrate, spin sensitive scanning tunneling microscopy and first principle electronic structure calculations result in the unveiling of chiral magentic structurs, which, apparently mix electronic, optical, magnetic and structural properties. Here is the paper; here is the commentary.

The perspective article by Volker Schmidt and Ulrich Goesle summarises the results rather neatly:

As expected, above the eutectic temperature, nanowire growth involves a liquid droplet on top of the germanium nanowires (…). However, when the authors reduced the temperature to below the eutectic temperature while keeping the supply of germanium constant, they observed two distinctly different phenomena (…). Some gold nanodroplets remained liquid even though the temperature was, in one case, more than 100°C below the T_E of 361°C. The authors observed this VLS-type growth mostly for nanowires with relatively large diameters.

In contrast, for nanowires with relatively small diameters, the gold droplet became solid as the temperature fell below T_E. The nanowires continued to grow, but did so much more slowly than in the case of VLS growth (…). Further cooling experiments showed that the transformation of the gold caps from liquid to solid at temperature below T_E could be delayed for tens of minutes. Kodambaka et al. show that this delay depends on various parameters, such as the vapor pressure, the temperature, and the diameter of the nanowires.

The bibliographic details of the paper referred to above are as follows:

Title: Germanium nanowire growth below the eutectic temperature

Authors: S Kodambaka, J Tersoff, M C Reuter, and F M Ross

Source: Science May 4 2007. Vol. 316, No. 5825, pp. 729-732.

Abstract: Nanowires are conventionally assumed to grow via the vapor-liquid-solid process, in which material from the vapor is incorporated into the growing nanowire via a liquid catalyst, commonly a low–melting point eutectic alloy. However, nanowires have been observed to grow below the eutectic temperature, and the state of the catalyst remains controversial. Using in situ microscopy, we showed that, for the classic Ge/Au system, nanowire growth can occur below the eutectic temperature with either liquid or solid catalysts at the same temperature. We found, unexpectedly, that the catalyst state depends on the growth pressure and thermal history. We suggest that these phenomena may be due to kinetic enrichment of the eutectic alloy composition and expect these results to be relevant for other nanowire systems.