Effect of rapid solidification on the microstructure and mechanical properties of hot-pressed Al-20Si-5Fe alloys

The aim of this work is to study the effect of cooling rate and subsequent hot consolidation on the microstructural features and mechanical strength of Al-20Si-5Fe-2X (X=Cu, Ni and Cr) alloys. Powder and ribbons were produced by gas atomization and melt spinning processes at two different cooling rates of 1 x 10(5) K/s and 5 x 10(7) K/s. The microstructure of the products was examined using optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The particles were consolidated by hot pressing at 400 degrees C/250MPa/1h under a high purity argon atmosphere and the microstructure, hardness and compressive strength of the compacts were evaluated. Results showed a profound effect of the cooling rate, consolidation stage, and transition metals on the microstructure and mechanical strength of Al-20Si-5Fe alloys. While microstructural refining was obtained at both cooling rates, the microstructure of the atomized powder exhibited the formation of fine primary silicon (similar to 1 mu m), eutectic AI-Si phase with eutectic spacing of similar to 300 run, and delta-iron intermetallic. Supersaturated Al matrix containing 5-7 at.% silicon and nanometric Si precipitates (20-40 nm) were determined in the microstructure of the melt-spun ribbons. The hot consolidation resulted in coarsening of Si particles in the atomized particles, and precipitation of Si and Fe-containing intermetallics from the supersaturated Al matrix in the ribbons. The consolidated ribbons exhibited higher mechanical strength compared to the atomized powders, particularly at elevated temperatures. The positive influence of the transition metals on the thermal stability of the Al-20Si-5Fe alloy was noticed, particularly in the Ni-containing alloy. (C) 2009 Elsevier Inc. All rights reserved.