Self-organized Sr leads to solid state twinning in nano-scaled eutectic Si phase

A new mechanism for twin nucleation in the eutectic Al-Si alloy with trace Sr impurities is proposed. Observations made by sub-angstrom resolution scanning transmission electron microscopy and X-ray probing proved the presence of <110> Sr columns located preferentially at twin boundaries. Density functional theory simulations indicate that Sr atoms bind in the Si lattice only along the <110> direction, with preferential positions at first and second nearest neighbors for interstitial and substitutional Sr, respectively. Density functional theory total energy calculations confirm that twin nucleation at Sr columns is energetically favorable. Hence, twins may nucleate in Si precipitates after solidification, which provides a different perspective to the currently accepted mechanism which suggests twin formation during precipitate growth.

2 resulting in a drastic reduction of the Sr-columns intensity in the HAADF high resolution STEM images. Therefore, only a very quick acquisition (~30 s at 300 kV) preserved their initial contrast in the HAADF image.
In order to preserve the number of Sr atoms in the region investigated by X-ray Spectrum Imaging, low dose conditions measurements were needed: a voltage of 60 kV with a low beam current of ~40 pA and cumulative measurements (seven subsequent spectrum images, 0.01 s pro spectrum, total acquisition time of 15s pro SI).
Electron energy loss spectrometry (EELS) could not be used in this investigation since Sr presents a major edge at very high energy losses: L 3 at 1940 eV and L 2 at 2007 eV. In this region the signal to noise ratio is very low making the detection of trace elements in low dose conditions (very short acquisition times) unreliable. Figure S1 shows two further examples of EDX investigations of bright spots at the twin end representing typical findings from such areas. The spectra present the Al, Si, Cu, and Sr signals.

Experimental results
Relative quantification indicates concentrations of Al -14.9 at%, Si -82.6 at% and Sr-2.5 at%. We note that the amount of Al is the same in all regions over all investigated particles within an error of 0.5 at%, regardless of twins or interstitial Sr columns.
Cu K Cu K Sr K Sr K Figure S 1. Typical EDX spectra acquired from interstitial Sr columns (Spectrum images at 60 kV) as marked with yellow arrows in Fig. 1(a). Cu-signal comes from the TEM holder.

Image simulation
An examination of the HAADF intensities around the interstitial Sr columns reveals a clearly visible The general rule for identification of trace signals above image-or spectra-noise in an experiment is that the signal to noise ratio should be equal or higher than 3. However, the simulated images do not contain experimental noise; therefore the threshold contrast was calculated relative  The energy calculation data are in qualitative agreement with the results given by Yue (2013) which indicate that Sr binds preferentially with Si. However, some degree of screening should occur in the disordered, melt phase and in presence of Al since the pair distribution function of Sr does not show a peak at distances corresponding to the first or second neighbor positions of the crystal. This difference can also be due to the lack of statistics in the liquid containing very few Sr atoms.
For the interaction of the Sr column with the twin we considered the twins to be two lattice spacing thick as observed experimentally in Figure 1. An important requirement, considering that they are the thinnest possible twins, is that the energetic interaction between twin boundaries is zero. The interstitial Sr columns are placed at the two twin boundaries and are composed from Sr atoms located in the nearest neighbor position, which corresponds to the interstitial Sr column configuration discussed above (Figure 2 (a)). The distance between columns in the {110} plane is sufficiently large to prohibit significant direct interaction between them. Periodic boundary 7 conditions are used in all directions and the model size is large enough to preclude significant interactions of Sr columns and their images or of twin boundaries.