EFFECT OF SOLIDIFICATION MICROSTRUCTURE ON STRENGTH AND DUCTILITY OF POWDER METALLURGICAL 2024-3FE-5NI ALUMINUM-ALLOY

Air atomised 2024Al-3Fe-5Ni, designated 3F5N, powders have been invest igated to characterise the rapidly solidified microstructure and to de termine the effects of the powder size on the microstructure and mecha nical properties of the consolidated material, with particular emphasi s on the improvement of the strength and ductility. Rapidly solidified powders of 3F5N alloy up to 150 mu m in diameter; consisted of four c haracteristic microstructures: microcellular alpha-Al, cellular alpha- Al, alpha-Al + Al9FeNi eutectic, and primary intermetallic structures. The intercellular regions also contained fine Al9FeNi phase. Powders less than 25 mu m undercooled significantly before solidification, yie lding a volume fraction of the microcellular microstructure as high as that of the eutectic. The microstructure of atomised powder was repro duced in the extrusions as bands containing intermetallic round partic les with different sizes, which are aligned parallel to the extrusion direction. The intermetallic particles after 623 K extrusion were fine r than those after 723 K, Rod shaped Al7Cu2Fe particles were observed by TEM mainly within the microcellular region of starting powders in t he materials extruded at 723 K, which precipitated during extrusion. T he elongation and the strength of the materials extruded at 623 K incr eased with a decrease in the size of the starting powder. This result is attributable to relatively uniformly dispersed fine, and round shap ed intel metallic particles which were mainly composed of Al9FeNi phas e. In contrast, the material extruded at 723 K showed higher tensile s trength and lower elongation with decreased starting powder size. Thes e phenomena could be explained by the precipitation of rod shaped part icles mid the coarsening of the intermetallic phase with a higher extr usion temperature. (C) 1997 The Institute of Materials.