Microstructural development and mechanical properties of drop tube atomized Al-2.85 wt% Fe

Abstract

• Fe solubility in α-Al increases with increasing cooling rate. • Micro-hardness increases with increasing cooling rate. • Al 13 Fe 4 forms at growth velocities in excess of 20 mm s −1 • α-Al was undercooled by 189 K Al-2.85 wt% Fe alloy has been subjected to non-equilibrium container-less solidification using a 6.5 m drop tube. Spherical samples were collected and sieved into 7 sizes fractions ranging from 850 μm to 53 μm, with the estimated cooling rates being 150 to 11000 K s -1 respectively. XRD analysis was employed on all droplet size fractions for identification and evolution of the phases, showing that Al, Al 6 Fe and Al 13 Fe 4 were formed for all sizes while Al 5 Fe 2 was observed only in droplets ≤ 150 μm in diameter. Microstructural evaluation was conducted by using SEM and optical microscopy, showing that droplet larger than 300 µm in diameter exhibited distinct morphologies; microcellular, dendritic α-Al with inter-dendritic Al 13 Fe 4 eutectic and an Al-Al 6 Fe eutectic region. With increasing cooling rate, the Al-Al 6 Fe region disappears. EDX analysis reveals that increasing the cooling rate increased the dissolved Fe content in α-Al from 0.37 wt% Fe to 1.105 wt% Fe, and correspondingly the eutectic fraction decreased from 49.7 vol.% to 26.7 vol.%. Measurement of the lamellar spacing allowed the eutectic growth velocity and interfacial undercooling to be calculated, wherein the Al-rich boundary of the eutectic coupled zone could be reconstructed. This shows a coupled zone skewed significantly towards the intermetallic side of the eutectic. In order to understand the effect of non-equilibrium the solidification on the mechanical properties micro hardness of the droplets was measured. The micro-hardness has risen from 55.3 HV 0.01 to 66.5 HV 0.01 for ≥ 850 μ m and ≤ 75 μ m droplets, respectively.