Characteristics of ZrAl intermetallic crystals during solidification of aluminum wrought alloys

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Zirconium is often added in aluminum wrought alloys to refine the grains, increase the strength and provide a better resistance to recrystallization. However, the addition of Zr may lead to the precipitation of undesirable zirconium intermetallic crystals and cause a number of casting process related problems and defects in the cast products. It is observed in some cases that interaction of Zr with other alloying elements (Fe, V, Ti and B) and poisoning effect of Zr on the Ti and TiB2 grain refinement have a great detrimental effect on the properties of final products. The main objective of this study is to better understand the effect of zirconium and its intermetallics on the fabrication processes and final product properties. Using thermodynamic calculation and physical experiments under controlled conditions, the formation of zirconium intermetallics and the solubility limit of zirconium in a typical 7xxx alloy will be investigated. The impact of the Zr level and the cooling condition on the nucleation, growth and morphology of zirconium intermetallic crystals will be examined using optical metallography and scanning and transmission electron microscopes. The mechanisms of the interaction of Zr with other alloying elements (Fe, V, Ti and B) in term of intermetallic formation, grain refinement and anti-recrystallization will be systematically studied. After the characterization of zirconium intermetallics in a typical alloy, the research will go through the 7xxx series alloy family to analyze the impact of complex Zr bearing intermetallics and to evaluate the resulted material properties. The information gained through this study can greatly help the aluminum industry to enhance the productivity and product quality for a number of important wrought alloys.