[1] Twinning Mechanism via Synchronized Activation of Partial Dislocations in Face-Centered-Cubic Materials

B Q Li et al

In situ straining in a high-resolution transmission electron microscope and molecular dynamics simulations reveal a new deformation twinning mechanism in the face-centered-cubic structure. A twin forms via the simultaneous and cooperative activation of different Shockley partial dislocations on three (111) layers. The synchronized slip produces a zero net Burgers vector; such twining relieves local stress concentration in a shear confined to adjacent atomic layers, but induces no macroscopic shape change of the surrounding crystal.

[2] Importance of secondary and ternary twinning in compressed Ti

W Tirry et al

Twin formation during uniaxial compression of high-purity α-Ti at room temperature is investigated for both quasi-static and dynamic conditions using electron backscatter diffraction techniques. The initial texture is favorable for View the MathML source twinning, yet it is observed that secondary and ternary twins occur for both strain rates, showing a higher propensity in the dynamic case. While secondary twins may explain the difference in texture change and strain hardening between the two loading conditions, the ternary twins mainly contribute to grain fractioning.

[3] Limit of steady-state lamellar eutectic growth

Wang and Trivedi

Eutectic microstructure under rapid solidification conditions becomes unstable beyond a certain velocity that places a limit on the finest spacing and the largest interface undercooling that can be achieved for a eutectic microstructure. Expressions are developed to characterize the physics that lead to the limit of eutectic growth. Activation energy for diffusion and eutectic temperature are shown to play key roles at the limit of eutectic growth, and this limit modifies the high velocity branch of the coupled growth regime.

[1] In situ study of nucleation and growth of the irregular α-Al/β-Al5FeSi eutectic by 3-D synchrotron X-ray microtomography

S Terzi et al

In order to better understand the formation of β-Al5FeSi intermetallic plates during solidification of Al–Si casting alloys, an Al–8% Si–4% Cu–0.8% Fe alloy has been studied by in situ microtomography using high-energy X-rays in the synchrotron. After formation of the aluminium dendrites, the β phase forms as an irregular eutectic together with eutectic α-Al. Only four plates were nucleated in the sample, and all nucleated in the very early stage of the eutectic reaction and subsequently developed into complex connected three-dimensional plates. The plates display very rapid lateral growth and slow thickening, which, together with the observation of imprints of dendrites and ridges in the plates, suggest a very weakly coupled eutectic.

[2] Deformation of hierarchically twinned martensite

P Muellner and A H King

Shape-memory alloys deform via the reorganization of a hierarchically twinned microstructure. Twin boundaries themselves present obstacles for twin boundary motion. In spite of a high density of obstacles, twinning stresses of Ni–Mn–Ga Heusler alloys are very low. Neither atomistic nor dislocation-based models account for such low yield stresses. Twinning mechanisms are studied here on a mesoscopic length scale making use of the disclination theory. In a first approach, a strictly periodic twin pattern containing periodic disclination walls with optimally screened stress fields is considered. Strict periodicity implies that the twin microstructure reorganizes homogeneously. In a second approach, a discontinuity of the fraction of secondary twins is introduced and modeled as a disclination dipole. The stress required for nucleation of this discontinuity is larger than the stress required for homogeneous reorganization. However, once the dipole is formed, it can move under a much smaller stress in agreement with experimental findings.

[3] New Interpretation of the Haasen Plot for Solute-strengthened Alloys

W A Curtin

The Haasen plot (inverse activation area 1/Δa versus offset flow stress σ-σs) for solute-strengthened alloys is usually assumed additive, 1/Δa=1/Δas+1/Δaf, with 1/Δafnot, vert, similarβ(σ-σs) due to forest interactions. Experiments often show a slope < β. Here, a model for the dislocation activation enthalpy is proposed that predicts a slope 1/(Δasσs) determined only by solute parameters Δas and σs and not directly connected to forest hardening. This parameter-free prediction agrees well with a wide range of experiments on Al-X alloys at T=78K.

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.