Solidification behavior of Al-Si-Fe alloys and phase transformation of metastable intermetallic compound by heat treatment

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Solidification behavior of Al-20 wt% Si-8 wt% Fe and Al-30 wt% Si-5 wt% Fe alloys during cooling with a cooling rate of 10 K/min has been studied using optical microscopy, X-ray diffractometry, differential thermal analysis, scanning electron microscopy and energy dispersive X-ray spectroscopy. In Al-20Si-8Fe alloy, metastable δ-Al42 phase with tetragonal structure formed first from melt, followed by primary Si precipitation and then remaining liquid solidified finally into ternary eutectic of α-Al, Si and δ phases. However, in Al-30Si-5Fe alloy, primary Si formed first, followed by the δ phase precipitation and then eutectic solidification. During isothermal heat treatment of as-solidified alloys, phase transformation from the δ phase to equilibrium β phase began at the interface between δ phase and α-Al matrix and progressed toward the inside of δ phase with co-precipitation of Si particles due to the difference in composition between δ-Al4FeSi2 and β-Al5FeSi phases.

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