Partitionless solidification and anomalous triradiate crystal formation in drop-tube processed Al-3.9 wt%Fe alloys

Drop tube atomized Al-3.9 wt% Fe alloy has been investigated. Spherical samples were collected and sieved into 9 different size fractions ranging between 850 + µm to 38 µm, with corresponding estimated cooling rates ranging between 100 and 20,000 K s−1. X-ray diffraction analysis showed the presence of Al, Al13Fe4 and Al6Fe in all size fractions. Scanning electron microscopy and optical microscopy have been employed for microstructural evolution. Large proeutectic crystals of Al13Fe4 surrounded by α-Al, dendritic α-Al with interdendritic lamellar eutectic, lamellar eutectic and rod-like eutectic was observed in samples with d > 212 µm. In the smaller samples (d < 212 µm) primary Al13Fe4 disappeared and featureless Y-shaped structures with thin, triradiating arms start to emerge. These Y-shaped structures can be heavily fragmented in nature and appear to be the first phase to nucleate in the droplets, followed by divorced eutectic, microcellular α-Al, dendritic α-Al with lamellar interdendritic eutectic and lamellar and rod-like eutectics. Serial sectioning with a cumulative depth of 20.2 µm has revealed that the Y-shaped features have an internally connected sheet-like morphology. Transmission electron microscopy reveals that these Y-shaped features are composed of nano-sized needle-like and spherical precipitates. TEM diffraction from a Y-shaped region has revealed the presence of AlmFe in this region. EDX analysis shows that these Y-shaped features have the same bulk composition as the liquid from which they grew, suggesting they form via partitionless solidification of highly supersaturated α-Al, which subsequently undergoes solid-state decomposition. The formation of these feature also slightly increases the eutectic spacing from 0.35 µm in the 300–212 µm size fraction to 0.45 µm in the 212–150 µm size fraction. In order to understand the effect of nonequilibrium the solidification on the mechanical properties microhardness of the droplets was measured. The microhardness has risen from 50 HV0.01 to 83 HV0.01 for 850 + μm and 53 ≤ d ≤ 38 µm droplets respectively.