It is well-known fact that mechanical properties of Al-Si alloys are much dependent on micro-structural features such as eutectic silicon morphology, dendrite arm spacing (DAS), grain size, which play a vital role when the alloy is put into specific use. In order to improve mechanical properties, these alloys are generally subjected to suitable melt treatment processes. The most important among them is modification, which transforms the acicular silicon morphology to fibrous one resulting in a noticeable improvement in elongation and strength. Sodium and strontium are the common modifiers in use. There are several theories proposed for the mechanism of modification. They can be divided into two main classes, viz.. the restricted growth theory and restricted nucleation theory.
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