Nucleation and Growth of Eutectic Si in Al-Si Alloys with Na Addition
Al-5 wt pct Si-based alloys with Na additions (19 and 160 ppm) have been produced by controlled sand casting and melt spinning. Entrained droplet technique and differential scanning calorimetry were employed to investigate the nucleation behavior of eutectic Si. High-resolution transmission electron microscopy and atom probe tomography were used to investigate the distribution of Na atoms within eutectic Si and at the interfaces between eutectic Si and eutectic Al. It was found that (i) only 19 ppm Na addition results into a high undercooling (49 K (49 °C)) of the entrained eutectic droplet. However, further increasing Na addition up to 160 ppm exerts no positive effect on the nucleation of eutectic Si, instead a decreased undercooling (29 K (29 °C)) was observed. (ii) Na addition suppresses the growth of eutectic Si due to the Na segregation at the interface between eutectic Si and eutectic Al, and (iii) Na addition promotes significant multiple Si twins, which can be attributed to the proposed adsorption of Na atoms at the intersection of Si twins and along the 〈112〉Si growth direction of Si. The present investigation demonstrates, for the first time, a direct observation on the distribution of Na atoms within eutectic Si and thereby provides strong experimental supports to the well-accepted impurity-induced twinning growth mechanism and poisoning of the twin plane re-entrant edge growth mechanism.
KeywordsAtom Probe Tomography Glow Discharge Mass Spectrometer Atom Probe Tomography Measurement Na3P Phase Inductively Couple Plasma Atomic Emission Spectrum
J.H. Li gratefully acknowledges the access to TEM at the Erich Schmid Institute of Materials Science of the Austrian Academy of Sciences. The EU funding in the framework of the project AME-Lab (European Regional Development Fund C/4-EFRE-13/2009/Br) is gratefully acknowledged. The Atom Probe was financed by the DFG and the Federal State Government of Saarland (INST 256/298-1 FUGG). J. Barrirero acknowledges the Erasmus Mundus Programme of the European Commission within the Doctoral Programme DocMASE for financial support.
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