An investigation on primary Si refinement by Sr and Sb additions in a hypereutectic Al-Si alloy
Reza Haghayeghi , Giulio Timelli
Hypereutectic Al-Si alloys have got interests in many industrial applications such as automotive and wear-resistance environments. However, the hypereutectic Al-Si alloys have a few drawbacks. The acicular shape of eutectic Si acts as a stress concentration point for fracture and makes the alloys brittle. Moreover, large primary Si blocks decrease the mechanical properties. To address above, many researches have been performed. All have focused on two approaches, namely physical and chemical. In the former, by applying ultrasonic , electromagnetic or their combination , have refined both primary and eutectic Si particles. Regarding the chemical route, the addition of alloying elements such as Sr , and Sb can change the size and morphology of Si. Both recommend that segregation or impurity induced twinning (IIT) is responsible for the refinement of primary Si and the modification of eutectic Si. However, the theory could not explain some phenomena. The impurity twining theory cannot describe the over-modification by modifiers such as sodium. It would not provide any justification on why sodium, whose atomic radius (r) ratio with that of Si (rSi) is 1.59, is more effective than ytterbium (rYb/rsi = 1.66); even though its ratio for eutectic modification is farther than the ‘‘ideal” value of 1.65 . Finally, no significant changes in twin density were observed between the modified and unmodified alloys [8]. In addition, most of above studies have focused on eutectic modification and no consensus has been achieved on primary Si refinement yet . This study investigates the theory and provides a mechanism that could answer the above concerns on primary Si evolution in a hypereutectic A390 alloy.
