Effect of solidification microstructure on strength and ductility of powder metallurgical 2024–3Fe-5Ni aluminium alloy

Abstract

Air atomised 2024Al-3Fe-5Ni, designated 3F5N, powders have been investigated to characterise the rapidly solidified microstructure and to determine the effects of the powder size on the microstructure and mechanical properties of the consolidated material, with particular emphasis on the improvement of the strength and ductility. Rapidly solidified powders of 3F5N alloy, up to 150 μm in diameter, consisted of four characteristic microstructures: microcellular α-Al, cellular α-Al, α-Al + Al9 FeNi eutectic, and primary intermetallic structures. The intercellular regions also contained fine Al9 FeNi phase. Powders less than 25 μm undercooled significantly before solidification, yielding a volume fraction of the microcellular microstructure as high as that of the eutectic. The microstructure of atomised powder was reproduced in the extrusions as bands containing intermetallic round particles with different sizes, which are aligned parallel to the extrusion direction. The intermetallic particles after 623 K extrusion were finer than those after 723 K. Rod shaped Al7CU2 Fe particles were observed by TEM mainly within the microcellular region of starting powders in the materials extruded at 723, which precipitated during extrusion. The elongation and the strength of the materials extruded at 623 K increased with a decrease in the size of the starting powder. This result is attributable to relatively uniformly dispersed, fine, and round shaped intermetallic particles which were mainly composed of Al9FeNi phase. In contrast, the material extruded at 723 K showed higher tensile strength and lower elongation with decreased starting powder size. These phenomena could be explained by the precipitation of rod shaped particles and the coarsening of the intermetallic phase with a higher extrusion temperature.