Masters Theses
Date of Award
12-2024
Degree Type
Thesis
Degree Name
Master of Science
Major
Materials Science and Engineering
Major Professor
Gerd Duscher
Committee Members
Gerd, Duscher, Eric Lass, Jon-Erik Mogonye
Abstract
Demand for light-weight high performance alloys for high temperature applications in aerospace, automotive, and military industries has propelled aluminum alloy modification research. Alloying with rare earth element, cerium (Ce) and appropriate amounts of additional alloying elements forms the aluminum matrix form a high-symmetry, isotropic Al-Ce-Cu-Ni-Mn phase, rhombicuboctahedron (RCO) which precipitates out of the aluminum alloy as a primary phase during solidification. This complex intermetallic enhances the performance of the alloy through uniform load sharing and improved high-temperature strength retention of the alloy. However, there are significant castability challenges due to RCO solidification behavior dependence on cooling rate strongly influencing the size, distribution, and morphology. This study investigates the effectiveness of the developed Flux Growth Casting (FGC) method on improving the castability and tunability of RCO-containing aluminum alloys, with a focus on eliminating cooling rate dependence and enhancing the microstructural properties. The FGC technique involves undercooling the melt to optimize RCO particulate formation, resulting in higher particle density and area percentage of RCO while minimizing dendritic growth. A series of Ce rich aluminum castings were enhanced with varying microstructure modification mechanisms: undercooling, inoculant addition, and alloying elements such as Cu, Mn, and Zn. Laboratory-scale experiments showed that FGC successfully enhanced microstructure control. However, scaling the method to industrial casting revealed challenges due to differences in melt cooling rates, particularly from the dissolution temperature to the nucleation hold temperature. To overcome this, an "ice cube method" was proposed, using molten aluminum to rapidly cool the melt, achieving the desired cooling rate and nucleation range. Through controlled supercooling and precise alloying, the research demonstrates the ability to produce cooling rate-independent microstructures with enhanced structural predictability and performance. This study also explored the behavior of inoculants (Gd, TiB2, Nb) in RCO alloys, revealing that these elements were primarily absorbed by the RCO phase, where they acted as getters rather than promoting grain refinement. The results suggest that the RCO phase plays a crucial role in impurity scavenging, improving the alloy’s mechanical properties.
Recommended Citation
Counce, Emilee Hannah, "Advancements in Tunability of High-Symmetry, Intermetallic Phase Formation in Al-Ce alloys: Influence of Undercooling and Alloying. " Master's Thesis, University of Tennessee, 2024.
https://trace.tennessee.edu/utk_gradthes/12846
