Chemical inhomogeneity of intermetallic phases precipitates formed during solidification of Al-Si alloys

In this work, the results of an examination of the solidification process of the AlSi6 and AlSi12 alloys as influenced by the content of the transition metals, Fe, Mn and Cr, are presented. The concentration of these elements in alloys prepared from pure components has been changed in the following range: Fe 0.5-3 wt.%, Mn 0-2 wt.% and Cr −0.3 wt.%. The thermal effects during solidification of alloys at a cooling rate of 3.3 K/min (between liquidus and solidus) connected with forming of structural components of alloy were determined. The temperature of the phase transformations was evaluated and correlated with chemical composition of alloys. It was stated that, on an average, an increase in Fe and Mn content of about 1 wt.% increases the temperature of primary proeutectic iron-bearing phase precipitation by about 40 K (in the examined range of the alloys compositions). By means of microscopic observations of the alloy microstructure, the morphology of the primary phase precipitates frozen at a chosen temperature was estimated. On the basis of the results of X-ray microanalysis (EDS), the chemical composition of the primary iron-bearing phases was analysed, and a relation among the components (i.e., Al, Si, Fe, Mn and Cr) was established. The sequence of microstructural component formation during solidification of the alloys examined was presented.