Growth of irregular eutectics and the Al [sbnd]Si system

Abstract

The directional solidification of the lamellar Al Si eutectic alloy is studied experimentally. It is shown that the average undercoolings and spacings lie on the theoretical ΔT−λ−V curve, but not at the extremum point. The operating factor, φ, describing this departure from the extremum conditions is experimentally shown to be independent of the growth velocity. The minimum spacings observed in the irregular microstructure formed are shown to be higher than the extremum value. The results are compared with those previously obtained in the Fe C and Fe Fe 3C alloys, the latter being shown to be an irregular eutectic despite the regularity of the microstructure formed. The theory of irregular eutectic growth is reexamined, based on the new model previously developed. A solution is obtained in the case of a non-isothermal s/l interface, which is identical to the isothermal solution if a weighted average undercooling is used. The effect of a kinetic undercooling term is studied, although it can usually be neglected even for irregular eutectics. It is shown that irregular eutectics grow with a nearly isothermal s/l interface, the different spacings observed in the microstructure being characterized by different growth rates. These rates oscillate locally around an average value, and significant departures from the steady-state conditions occur. The operating range is described by its average value (φ) and its extent (η). It is shown experimentally that the latter is always very close to the value of the average spacing, independent of the φ-value.