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

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 delta-Al4FeSi2 phase with tetragonal structure formed first from melt, followed by primary Si precipitation and then remaining liquid solidified finally into ternary eutectic of alpha-Al, Si and delta phases. However, in Al-30Si-5Fe alloy, primary Si formed first, followed by the delta phase precipitation and then eutectic solidification. During isothermal heat treatment of as-solidified alloys, phase transformation from the delta phase to equilibrium beta phase began at the interface between delta phase and alpha-Al matrix and progressed toward the inside of delta phase with co-precipitation of Si particles due to the difference in composition between delta-Al4FeSi2 and beta-Al5FeSi phases. (C) 1999 Kluwer Academic Publishers.