In alloys of eutectic type, abnormal structure or eutectic halo, which is a structure where primary crystals exist in eutectic matrix surrounded by pro-eutectic secondary crystals, appears either in hypo- or in hyper-eutectic alloys. Concerning this abnormal structure various kinds of binary alloys of eutectic type were investigated microscopically to determine abnormal structure appears, in alloy of hypo- or hyper-eutectic composition and it is recognized that it appears in hyper-eutectic alloys if eutectic point S is nearer to P than to K (i.e. PS<SK) on eutectic line PSK and in hypo-eutectic alloys if S is nearer to K than to P (i.e. PS>SK) and that this relations are not affected by the nature of constituents.
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