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

“…This occurs when a large group of atoms uniformly shift position across a twin plane in the crystal structure. A self-perpetuating groove of 141 is produced at 2,3) have confirmed that modified silicon fibres contain more twins than unmodified silicon, and have a rough microfaceted surface.…”

“…It is notable that the twin density is not very high in these samples which is contrary to results in the literature for modified alloys. 2,3,6) A high density of twins is a basis for explaining the modification effect in Al-Si alloys. From these results, it is difficult to explain the modification of Ba, Ca, Y and Yb using only an impurity induced TPRE model.…”

“…Recent papers discussing the twin probability and TEM crystal morphology of eutectic silicon in alloys modified by misch metal (mixture of Lanthanum, Cerium, Praseodymium and Neodymium) 2,8) concluded that misch metal additions resulted in only a limited increase in the silicon twin density. This is despite reports of misch metal being an effective modifier.…”

“…Addition of other alkali, alkaline earth and rare earth metals have also been reported to cause modification, although very limited data are available in the literature. [2][3][4] An accepted theory to explain the mechanism of twinning of silicon in modified alloys suggests that atoms of the modifier are absorbed onto the growth steps of the silicon solid-liquid interface. Consistent with the impurity induced twinning model, a growth twin is created at the interface when the atomic radius of the element relative to silicon (r=r Si ) exceeds 1.65.…”

“…2,3,5) Even though the impurity induced twining theory has been well accepted, there are still some questions or contradictions that remain, i.e. ;…”