Crystallization behavior of iron-containing intermetallic compounds in 319 aluminum alloy

Abstract

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 theβ phase, at all cooling rates ranging from 0.1 °C/s to 20 °C/s under normal casting temperatures (750 °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 α phase at high cooling rates. This is due to the fact that γ alumina, which forms at low melt temperatures (≤750 °C), acts as a nucleus for crystallization ofβ phase. When the melt is superheated to high temperature (≥85O °C), the γ alumina transforms to a alumina. This is a poor nucleus for the β-phase crystallization, and as a result, a 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 a phase at low cooling rates and in both the α andβ phases at high cooling rates. This reverse crystallization behavior is explained in terms of phase diagram relationships.