Two-Phase Dendrite and Bimodal Structure in an Al-Cu-Ni Alloy: Their Roles in Hardness

With the increasing interest in developing Al-Cu-Ni alloys for use as structural material comes the need to understand more about their processing–microstructure–properties relationships. In this work, an unsteady-state directional solidification technique was used to investigate the influence of growth (v) and cooling (T˙) rates on the microstructure evolution of an Al-15Cu-5Ni alloy (wt.%) and analyze the consequent effects on hardness. The results reveal two-phase dendrites with a trefoil morphology in which the angle between the dendritic arms is around 120°. In addition, the interdendritic regions are shown to be composed of a bimodal structure including irregular block-type (Al7Cu4Ni) and lamellar-type (α-Al + Al2Cu) IMCs. A mechanism is proposed to describe the microstructure formation and evolution as well as experimental equations to represent the growth of dendritic arm spacings in terms of v and T˙. Finally, it is discussed why Al7Cu4Ni and Al2Cu IMCs are shown to have more impact on hardness of the studied alloy than the length scale of two-phase dendrites.