Intermetallic phase selection in dilute Al-Fe-Si alloys
The DC casting of 1000 series AI-Fe-Si alloys often results in the formation of a defect known as the Fir Tree Zone (FI'Z) caused by variations in the type of binary AI-Fe eutectic intennetallic phase. These include AI3Fe, which is the equilibrium phase, and ~Fe, AImFe and AlxFe which are all common metastable phases found in DC cast ingots. Variation in the solidification conditions across a DC cast ingot.in particular, the cooling rate and local solidification velocity, bring about transitions from one intennetallic phase to another. The composition of the alloy, especially the presence of minor trace elements is also known to affect the transitions, but its effect has been less well studied. Equipment was constructed which reliably simulated DC casting conditions and enabled the FfZ to be reproduced. Experiments were conducted to examine the effect of the concentration of Fe, Si (and Fe:Si ratio) and of low levels «0.04 wt %) of Mg, Ti, Ti-B, V and Mn on the occurrence of phases causing the defect. Intennetallic phases extracted from the matrix by dissolution in hot butanol were identified by XRD. It was found that presence of Ti-B grain refmer had a large effect on the intennetallic phase selection, whilst the other trace element additions had lesser effects. In order to understand the way in which each element was affecting phase selection, experiments were conducted using DSC and a Bridgman furnace to isolate the variables of cooling rate and growth velocity respectively. A curve fitting technique was used to obtain values of eutectic nucleation onset temperature from DSC data where the primary aluminium and eutectic peaks overlapped. A technique was developed to extract intennetallic particles from DSC samples for examination in the TEM. It was found that the addition Ti-B grain refmer affected the phase selection by altering the nucleation temperature. The addition of Mg was found to stabilise the equilibrium phase by its affect on both growth temperature an nucleation temperature The effect of Ti was less clear in that it stabilised the equilibrium phase in the DSC experiments but tended to stabilise the metastable phase in the Bridgman experiments.