Abstract
The heterogeneous nucleation of primary Si and eutectic Si can be attributed to the presence of AlP. Although P, in the form of AlP particles, is usually observed in the centre of primary Si, there is still a lack of detailed investigations on the distribution of P within primary Si and eutectic Si in hypereutectic Al-Si alloys at the atomic scale. Here, we report an atomic-scale experimental investigation on the distribution of P in hypereutectic Al-Si alloys. P, in the form of AlP particles, was observed in the centre of primary Si. However, no significant amount of P was detected within primary Si, eutectic Si and the Al matrix. Instead, P was observed at the interface between the Al matrix and eutectic Si, strongly indicating that P, in the form of AlP particles (or AlP ‘patch’ dependent on the P concentration), may have nucleated on the surface of the Al matrix and thereby enhanced the heterogeneous nucleation of eutectic Si. The present investigation reveals some novel insights into heterogeneous nucleation of primary Si and eutectic Si by AlP in hypereutectic Al-Si alloys and can be used to further develop heterogeneous nucleation mechanisms based on adsorption.
Highlights
Conventional focused ion beam (FIB) ‘lift out’ techniques were used to attempt the extraction of one such AlP particle within the primary Si region imaged in Fig. 1d, marked with a white arrow, for further transmission electron microscopy (TEM) sample preparation and characterisation
No further AlP particle was observed within the primary Si after final thinning, indicating that AlP particles located in the centre of primary Si are relatively small in size
Considering the very low solubility of P in are very close (Al), the random P distribution may only be possible when the P concentration is at a trace level (e.g. 0.44 ppm in high purity Al-Si alloy6). (ii) P combines with Al in the form of small AlP clusters or AlP patches within the Al matrix when the P concentration is in the range of 0.44 ppm to 3 ppm[6]
Summary
HAADF STEM imaging and electron energy loss spectroscopy (EELS) were used to elucidate the distribution of P in hypereutectic Al-Si-P alloys. The aim of this investigation is to elucidate the heterogeneous nucleation of primary Si and eutectic Si by AlP in hypereutectic Al-Si alloys and to further develop the heterogeneous nucleation mechanisms based on adsorption. It should be noted here that the investigation in hypereutectic Al-Si based alloys can be used to interpret the heterogeneous nucleation of eutectic Si in hypoeutectic Al-Si based alloys, the P concentration in hypoeutectic Al-Si based alloys is usually much lower than that in hypereutectic Al-Si based alloys, guiding the choice of the hypereutectic case for this study
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