Evaluation of Silicon Twinning in Hypo-Eutectic Al-Si Alloys

Abstract

It is generally accepted that growth of eutectic silicon in aluminium-silicon alloys occurs by a twin plane re-entrant edge (TPRE) mechanism. It has been proposed that modification of eutectic silicon by trace additions occurs due to a massive increase in the twin density caused by atomic effects at the growth interface. In this study, eutectic microstructures and silicon twin densities in samples modified by elemental additions of barium (Ba), calcium (Ca), yttrium (Y) and ytterbium (Yb) (elements chosen due to a near-ideal atomic radii for twinning) in an A356.0 alloy have been determined by optical microscopy, thermal analysis, X-ray diffractometry (XRD) and transmission electron microscopy (TEM). Addition of barium or calcium caused the silicon structure to transform to a fine fibrous morphology, while the addition of yttrium or ytterbium resulted in a refined plate-like eutectic structure. Twin densities in all modified samples are higher than in unmodified alloys, and there are no significant differences between fine fibrous modification (by Ba and Ca) and refined plate-like modification (by Y and Yb). The twin density in all modified samples is less than expected based on the predictions by the impurity induced twining model. Based on these results it is difficult to explain the modification with Ba, Ca, Y and Yb by altered twin densities alone.

Eutectic Modification of Al-Si Alloys with Rare Earth Metals

“…Recent papers discussing the twin probability and TEM crystal morphology of eutectic silicon in alloys modified by Ba, Ca, Y, Yb 4) and misch metal 7) showed there was a low density of twinning, despite the structure being well modified. The aim of the present paper was therefore to investigate the individual effects of each rare earth element on the eutectic microstructure in an Al-10 mass%Si alloy.…”

“…Addition of other alkali, alkaline earth and rare earth metals have also been reported to cause modification, although very limited data is available in the literature. [1][2][3][4] It has been shown that the fibrous silicon phase in chemically modified alloys is heavily twinned. This impurity induced twinning has been explained by a model suggesting that atoms of the modifier are absorbed onto the growth steps of the silicon solid-liquid interface 2,5) creating a twin plane re-entrant edge (TPRE).…”

Effect of Co and Ni Addition on the Microstructure and Mechanical Properties at Room and Elevated Temperature of an Al–7%Si Alloy

“…10 The addition of 0.4 wt% Co did not produce any significant changes in the microstructure (see Figure 1b), and the SDAS values are similar (see Table 2), but some Co-rich intermetallics could be observed. Kaya et al 11 reported a reduction of eutectic silicon interflake spacing with addition of 2 wt% Co, but there is no clear effect in the present samples, perhaps caused by the lower addition level and higher growth rate in the present study.…”

Modeling of Eutectic Formation in Al-Si Alloy Using A Phase-Field Method

“…Many experimental studies are performed to investigate the nature of eutectic silicon growth, in Al-Si alloy during solidification and refining processes. The precipitation of silicon in Al-Si alloy has also been studied experimentally by many researches, including solidification refining from Al-Si melt [1], eutectic nucleation [2], irregular eutectic Al-Si [3] and hypo-eutectic Al-Si alloys [4]. The formation and chemical modification of the Al-Si eutectic is reviewed in Ref.…”