Microstructural development and mechanical properties of drop tube atomized binary Al-Fe and ternary Al-Fe-Si alloys

The effect of nonequilibrium solidification on the microstructural development and mechanical properties of Al-2.85 wt% Fe, Al-3.9 wt% Fe, and Al-4.1 wt% Fe-1.9 wt% Fe alloys was studied using a 6.5 m drop tube. Spherical particles with diameters ranging between 850 µm and 38 µm were obtained with the corresponding estimated cooling rates between 155 K s-1 and 20,000 K s-1, respectively. The spherical samples were examined using OM and SEM to understand the microstructural evolution, while XRD and TEM were employed for phase identification. Furthermore, microhardness testing was performed to observe the effect of rapid solidification on the mechanical properties of the alloys.
For drop tube atomized Al-2.85 wt% Fe alloys whose diameter were ranging between 850 µm and 53 µm, XRD analysis showed that while α-Al, Al6Fe, Al13Fe4 were formed in all samples, Al5Fe2 was observed in samples with a diameter smaller than 150 µm. SEM and OM results have revealed that samples with a diameter larger than 300 µm had three regions with distinct morphologies: microcellular, dendritic with lamellar interdendritic eutectic, and rod-like eutectic region, which disappeared with decreasing sample size. TEM result has shown that while the interdendritic lamellar eutectic is Al-Al13Fe4, rod-like eutectic is Al-Al6Fe eutectics. Using EDX, Fe content in α-Al has been found to be rising from 0.37 wt% Fe to 1.105 wt% Fe with decreasing sample size. As a result, the volume fraction of the eutectic measured to be decreasing from 49.7 vol.% to 26.7 vol.% with increasing cooling rate. Microhardness has increased from 55.3 HV0.01 to 66.5 HV0.01 for ≥850 µm and ≤75 µm droplets, respectively.
Drop tube atomized Al-3.9 wt% Fe alloy was sieved into 9 different sieve fractions ranging between 850+ µm and 38 µm. In large samples (d > 212 µm), large proeutectic Al13Fe4 surrounded by α-Al, dendritic α-Al with interdendritic lamellar eutectic, lamellar eutectic, and rod-like eutectic was observed. The proeutectic Al13Fe4 vanished with decreasing sample size. Featureless Y-shaped structures, which are the first phase to nucleate in the droplet, have emerged in samples with diameters smaller than 212 µm. The solidification in the droplet has proceeded with the formation of divorced eutectic, microcellular α-Al, dendritic α-Al with lamellar interdendritic eutectic and rod-like eutectic. SEM and OM showed that these Y-shaped structures are fragmented. Y-shaped was found to be an internally connected sheet-like morphology by employing serial sectioning. Y-shaped has been revealed to be composed of nano-sized needle-like and spherical precipitates by using TEM. AlmFe was formed in the Y-shaped region. The microhardness has increased 50 HV0.01 to 83 HV0.01 for 850+ µm and 53≤d≤38 µm droplets, respectively.
Drop tube atomized Al-4.1 wt% Fe-1.9 wt% Si samples with diameters ranging between 850-53 µm were analysed. XRD results have revealed that there are only two phases: α-Al and Al8Fe2Si regardless of sample size. Microstructural analysis has shown dendritic α-Al with interdendritic lamellar eutectic in large samples (d > 300 µm). However, with decreasing sample size, angular nucleation zone has started to emerge in the microstructure. The fraction of samples with such angular nucleation zone has increased with decreasing sample size. EDX analysis from this zone has depicted that while the Fe content is identical to that of the melt, Si content was found to be around 1 wt% Si regardless of sample size. In addition to the angular nucleation zone, propeller-like structures and Y-shaped structures have been observed in fine samples (d < 106 µm). The formation of propeller-like structures indicates that the growth mechanism of angular structure has changed from faceted growth to continuous growth. TEM analysis from the angular region has depicted the formation of clusters of faceted Al8Fe2Si formed due to solid-state decomposition. The microhardness of the samples has improved from 72 HV0.01 to 90 HV0.01 for between 850-150 µm samples, respectively. However, a further decrease in sample size has resulted in the microhardness from 90 HV0.01 to 80 HV0.01 for 150-53 µm.
drop tube