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oys are widely used in the automotive and aerospace industries due to their high strength-to-weight ratio and castability. However, the plate-like coarse Si phase is usually accused of reducing mechanical properties of the alloy. To solve this problem, minor chemical modifications are used to modify the plate-like Si morphology into a fibrous or coral-like morphology so that mechanical properties, especially ductility, are improved. Various elements are reported to have a modification effect on Si, among which Sr and Na are the most commonly used.[1–3] Ba and Ca[4,5] are also reported to have a similar but reduced effect on Si. Of the commonly used modifiers, Sr remains longer in the melt than Na but is well known for its association with enhanced gas porosity.[6] McDonald et al.[7] investigated porosity in a Sr-treated alloy and found that, at any Si content, porosity was increased when Sr was added. Tiedje et al.[8] suggested that Sr modification would increase the amount of porosity by 5 to 10 times in an Al-Si alloy. The impurity-induced twinning (IIT) model[9] and twin plane re-entrant edge (TPRE) mechanism[10] are

QI CHEN and W.D. GRIFFITHS are with the School of Metallurgy and Materials, College of Engineering and Physical Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Contact e-mail: [email protected] Manuscript submitted April 23, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS A

accepted as explaining the modification behavior. Modifier atoms, with a ratio to Si of r (modifier):r (silicon) = 1.646), are selectively absorbed at re-entrant edges, deactivating the preferential Si growth directions. Another explanation would be that the modifier atoms are absorbed at the growth steps of Si crystals, interrupting the current growth of the Si phase. The Si atom was forced to fall into the next available vacancy and its stacking sequence was changed. The formation of twins of the Si phase was also increased. Li et al.[11] observed the absorption of Eu atoms (with r (modifier):r (Si) roughly equal to 1.646) at twin plane re-entrant edges using STEM. Timpel et al.[12] used atom probe tomography and TEM with nanometer resolution to investigate the behavior of Sr in Al-Si alloy and suggested that the segregation of Sr is responsible for the formation of multiple twins restricting the Si growth. Sr modification was successfully simulated by Eiken et al.[13] using 3D phase field simulation, based on the above two mechanisms. The topic of bifilm defects was raised by Campbell in the 1990s and since then has led to more awareness of liquid metal quality in the casting of light alloys.[14] Liquid aluminum will readily form a very thin layer of oxide on the surface of the melt. During the process of liquid metal transfer, the surface of the liquid metal may experience some turbulence. If the local velocity of the liquid metal exceeds about 0.5 ms1,[14] the liquid metal can fold over onto it trapping a pocket of gas. This ‘‘doubled-over’’ structure can then be entrained into the bulk melt and carried around by