Effect of transition elements on the microstructure and tensile properties of Al–12Si alloy cast under ultrasonic melt treatment

This paper shows that the addition of transition elements significantly influences the microstructure and tensile properties of Al–12Si alloy cast under ultrasonic melt treatment (UST). The alloys contained primary Si, eutectic Si and a variety of intermetallic compounds (IMCs); the addition of Mn and Ni changed the solidification sequence by forming primary α-Al₁₅(Fe,Mn)₄Si₂ and ε-Al₃Ni prior to primary Si formation. The primary Si and primary IMCs competitively consumed the nucleation sites introduced by UST, thereby influencing their refining efficiencies; the efficiency of primary Si refinement by UST increased with increasing the fraction of primary Si formed prior to IMC formation, whereas it deteriorated with increasing primary IMC fraction. The refining efficiency of eutectic Si and eutectic IMCs was not affected by the type and fraction of the primary phase. Using a Ti sonotrode for UST caused Ti contamination, resulting in grain refinement by forming Ti(Al,Si)₃ particles and increasing the amount of Ti solutes. Regardless of transition elements content, the application of UST improved the tensile strength at 25 °C and 350 °C. The refinement of IMCs caused by UST allowed the alloy to contain more IMCs without deterioration of room-temperature tensile strength. This enhanced the increment of high-temperature tensile strength by IMCs. Ductility of heavily alloyed systems was also improved by UST.