Atomistic experimental and simulation investigation on the modification of Al-Si alloysFriday (10.11.2017) 10:55 - 11:15 Part of:
Modifying the eutectic Si from flake-like to fibrous is a key factor to improve the properties of Al-Si alloys. The impurity-induced twinning (IIT) mechanism and the twin plane re-entrant edge (TPRE) mechanism as well as the poisoning of the TPRE mechanism are generally accepted to be valid under certain conditions. However, IIT, TPRE or poisoning of TPRE mechanism cannot be used to interpret all the observations accompanying modification, indicating that other factors may be also valid. Therefore, a detailed atomistic experimental and simulation investigation on the modification of Al-Si alloys is still of great necessity to elucidate the possible modification mechanism. In this contribution, high resolution scanning transmission electron microscopy high angle annular dark field imaging and electron energy loss spectroscopy as well as atom probe tomography was used to elucidate the distribution of modifying elements within eutectic Si and at the interface between eutectic Al and eutectic Si. Density functional theory calculation was also performed to elucidate the bonding behaviour of modifying elements within Si twins and its effect on the Si twinning. Both experimental and simulation investigation reveals that modifying elements have four different roles: (i) the adsorption at the intersection of Si facets, inducing IIT growth mechanism, (ii) the adsorption at the twin plane re-entrant edge, inducing TPRE growth mechanism, (iii) the adsorption ahead of the growing Si twins, inducing a solute entrapment within eutectic Si, and (iv) the segregation at the interface between eutectic Si and eutectic Al, suppressing the growth of eutectic Si. This investigation not only demonstrates, for the first time, a direct experimental support to the well-accepted poisoning of the TPRE and IIT growth mechanisms, but also provides a full picture about the behaviour of modifying elements, including the solute entrapment within eutectic Si and the segregation ahead of eutectic Si growth front.