Microstructure and Creep Properties of Al-Sc Alloys Micro-alloyed with Lanthanides (Yb or Gd) and Transition Metals (Ti or Zr)Public Deposited
This thesis examines the effects of micro-alloying additions to Al-Sc alloys on the microstructure, coarsening resistance and creep properties. The overarching goal of this research is to develop castable, creep-resistant aluminum alloys which can be used at temperatures in excess of 300?C. Successful high-temperature application of aluminum based alloys offers a lower cost and lower weight alternative to other materials commonly used at high temperatures, including titanium- and nickel-based alloys. To this end, this aims to improve the properties of the Al alloys by adding various alloying elements in small quantities, on the order of several hundred atomic parts per million, to aluminum. The thesis begins by focusing on additions of Ti to Al-Sc. Ti is a slow diffuser in Al, and it will be shown that it improves the coarsening kinetics of the precipitate phase at 300?C. Since these alloys are coarsening resistant, it is found that they can be aged and crept at temperatures of up to 425?C. The properties displayed are similar to those of Al-Sc-Zr alloys studied previously. The examination of Ti additions is followed by a study of the additions of lanthanide elements. These elements are of interest since they are known to increase the lattice parameter of the precipitate phase, which could potentially lead to improved creep resistance. Initially, binary Al-Yb alloys are studied to obtain some fundamental knowledge of the behavior of Yb in Al. Subsequent additions of Yb to Al-Sc result in improved creep resistance. A similar improved creep resistance is observed for additions of Gd to Al-Sc. Finally, this dissertation concludes with the study of Al-Sc-Yb-Zr alloys. Since the goal of this research is to obtain a creep-resistant as well as coarsening resistant alloy, both a slow diffusing element (Zr) and an element which improves the creep resistance (Yb) are added. The quaternary alloys are found to maintain the creep resistance and coarsening resistance of the Al-Sc-Yb and Al-Sc-Zr alloys, respectively, which points to opportunities for future research in this area.