CRYSTALLIZATION BEHAVIOR OF IRON-CONTAINING INTERMETALLIC COMPOUNDS IN 319 ALUMINUM-ALLOY

The crystallization behavior of iron-containing intermetallic compounds in industrial grade 319 aluminum alloy has been investigated by means of thermal analysis and metallography. In the absence of manganese, the iron compound crystallizes in the beta phase, at all cooling rates ranging from 0.1-degrees-C/s to 20-degrees-C/s under normal casting temperatures (750-degrees-C). However, when the melt is superheated to a high temperature (about 200 to 300 degrees above the liquidus temperature), the iron compound crystallizes in the a phase at high cooling rates. This is due to the fact that gamma alumina, which forms at low melt temperatures (less-than-or-equal-to 750-degrees-C), acts as a nucleus for crystallization of beta phase. When the melt is superheated to high temperature (greater-than-or-equal-to 850-degrees-C), the gamma alumina transforms to alpha alumina. This is a poor nucleus for the beta-phase crystallization, and as a result, alpha phase forms. The importance of nucleation and growth undercooling for the crystallization of iron compounds is highlighted. In the presence of manganese, the iron compound crystallizes in alpha phase at low cooling rates and in both the alpha and beta phases at high cooling rates. This reverse crystallization behavior is explained in terms of phase diagram relationships.