associated with pure metals and eutectic alloys, wherein solidification takes place by the advance of plane interface from the mold wall with the metal flow continuing until the channel is
finally choked (Fig. 10(a)). On the other hand, in alloys, constitutional under cooling and other phenomena produce independent crystallization in main liquid mass, leading to the presence of free crystals in the liquid, which can arrest the flow and hence reduce the fluidity (Fig. 10(b)). Further, the hindrance caused by crystals with irregular growth surfaces in long freezing range alloys is much greater than that of comparatively smooth crystallization interface front of pure metals and eutectic alloys. Thus,fluidity values of pure metals and eutectic alloys are greater than those of alloys solidifying over a temperature range. This is the reason for the wide practical preference for eutectic or near eutectic alloys for foundry purposes, particularly for casting with thin sections.