Date of Award

2025

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering (PhD)

Administrative Home Department

Department of Materials Science and Engineering

Advisor 1

Paul Sanders

Committee Member 1

Timothy Langan

Committee Member 2

Stephen Kampe

Committee Member 3

Walter Milligan

Abstract

The 2xxx series of precipitation-strengthened aluminum-copper alloys are commonly used in high-temperature applications (up to 300°C) where lightweight metals are required. Scandium additions to aluminum-copper alloys have gained recent academic interest due to improved high-temperature mechanical properties, by forming coarsening- resistant Al3Sc dispersoids and stabilization of θ′ precipitates. However, gaps in knowledge about the effects of scandium present a challenge to successful commercialization of aluminum-copper-scandium alloys. This work seeks to explore these knowledge gaps and ultimately design and test a commercially feasible aluminum- copper alloy with scandium. To better understand the processing conditions required to avoid the detrimental W-phase (Al8Cu4Sc), castings were produced with a wide range of solidification rates and heat treatments. It was shown that the only feasible heat treatment is a single-step homogenization with copper below 4 wt%, and that higher copper or multi-step homogenization will lead to the formation of W-phase. W-phase results precipitate-free zones which lower the hardness of the resulting microstructure. Using a 3.5 wt% copper sample combined with a single-step homogenization, the effects of scandium in solution on precipitation of copper-rich precipitates are studied. Scandium in solution stabilizes θ″ and θ‴ precipitates over θ′ when aging at 160°C, which is supported by density functional theory calculations that show a significant reduction in the interaction energy of θ″, θ‴, and GP zones with the addition of scandium. Finally, an engineered alloy with low copper and scandium in solution is tested in comparison to AA2219 and is shown to have similar high-temperature strength while having a 60% increase in the fracture toughness. Transmission electron microscopy shows that θ′ precipitates in the scandium-containing alloy have larger aspect ratios and a preference for {100} semi-coherent interfaces. This work expands knowledge of the processing conditions required for a commercially feasible aluminum-copper-scandium alloy and demonstrates an alloy design strategy for this system.

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