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

M Albu,R C Picu,C Gspan,F Hofer,G Kothleitner,A Pal

doi:10.1038/srep31635

Abstract

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.

Highlights

A new mechanism for twin nucleation in the eutectic Al-Si alloy with trace Sr impurities is proposed
Several theories have been proposed to explain the mechanisms leading to refinement and twinning with the most broadly accepted ones being “impurity-induced twinning” (IIT)[6] and the “twin plane re-entrant growth mechanism” (TPRE)[7,8,10]
This work targets the twinning mechanism in the presence of Sr using a combination of sub-angstrom resolution analytical scanning transmission electron microscopy (HR Scanning Transmission Electron Microscopy (STEM)) and ab-initio Density functional theory (DFT) calculations

Summary

Introduction

A new mechanism for twin nucleation in the eutectic Al-Si alloy with trace Sr impurities is proposed. Yue et al.[29] used ab-initio molecular dynamics to study the structure of liquid Al-Si-Sr at the eutectic concentration of Al and Si. Yue et al.[29] used ab-initio molecular dynamics to study the structure of liquid Al-Si-Sr at the eutectic concentration of Al and Si They indicate that the presence of Sr reduces the diffusion coefficient of both Al and Si in the melt and observed that Sr binds preferentially with Si. This work targets the twinning mechanism in the presence of Sr using a combination of sub-angstrom resolution analytical scanning transmission electron microscopy (HR STEM) and ab-initio DFT calculations. Analytical evidence for the presence of Sr atomic columns, arranged only along certain crystallographic directions and at twin boundaries, is backed up by DFT total energy calculations, aiming to answer whether a binding with the twin boundaries exists, and to shed some light on the energetic driving force for twin nucleation from such sites

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Conclusion