Solubility of iron into pure magnesium and Mg–Al alloy melts

Abstract

We conducted this study to determine the solubility of iron in pure magnesium and Mg–Al molten alloys. Alloy ingots were prepared using high-purity magnesium (< 99.99%) and high-purity aluminum. These alloys were melted in a closed vessel with introducing a mixed gas of SF₆ and CO₂ to prevent oxidation or burning. Iron was added by a pure iron plate, which was immersed in the molten alloys (pure magnesium) at temperatures of 933 K (963 K) , 998 K and 1073 K. Molten alloys were held at the each temperature for 86.4 ks at maximum. A 5 ml specimen subjected to chemical analysis was collected from the top of melt by sucking through pre-heated titanium tube at given time intervals and then determined iron solubility. Following an equation was led as the experimental formula of iron solubility in pure magnesium (963~1073 K),
log N_Fe^solu. = -7981/T + 3.931 (1)
where N_Fe^solu. is the solubility of iron in the liquid phase (molar fraction), and T is the absolute temperature. This equation shows that iron solubility indicated by this experiment is smaller than those reported by any other researchers for temperatures up to 1073 K. We also led following equations, (2), (3) and (4), about Mg–3.2Al, Mg–6.5Al and Mg–9.0Al alloys (933~1073 K), respectively.
log N_Fe^solu. = -4911/T + 0.723 (2)
log N_Fe^solu. = -5115/T + 1.007 (3)
log N_Fe^solu. = -4298/T + 0.263 (4)
The solubility of iron slightly increases with increasing aluminum concentration of alloy. We conjectured that, in this case, the solubility increases because the increase in Al concentration causes the activity coefficient of Al in the molten alloy to decrease.