Alloy development for the enhanced stability of Omega precipitates in aluminum-copper-magnesium-(silver) alloys

This research involved a combined analytical and experimental approach to the design of an age-hardenable Al-Cu-Mg-Ag alloy for moderate temperature application. The applied methodology involved the complimentary techniques of thermal analysis, calculated phase diagrams, analytical microscopy and quantitative microstructural characterization. The objective of this research was to exploit several avenues for enhancing the coarsening resistance and thermal stability of the O phase through careful control of the alloy chemistry and processing. Differential thermal analysis (DTA) coupled with conventional and analytical transmission electron microscopy (TEM) techniques were implemented to refine the calculation of the Al-rich corner of the quaternary Al-Cu-Mg-Ag phase diagram for subsequent alloy development. Quantitative energy dispersive spectroscopy (EDS) demonstrated that Ag preferentially partitioned to S-phase for all conditions investigated, which ultimately led to a concomitant loss of O precipitates. The elimination of S-phase precipitation and limiting the alloy Si content proved to enhance the nucleation and thermal stability of the O phase.; Several O-dominated microstructures were manipulated through various thermo-mechanical processing techniques in order to evaluate the O nucleation density, particle size and thermal stability as a function of alloy composition and processing conditions. The long-term stability of O plates was found to coincide with high levels of Ag and moderate Mg additions, with the latter limiting the competition with S-phase precipitation. Several alloys were found to be dominated by O precipitation, which remained stable through long-term isothermal and double-aging heat treatments. This enhanced thermal stability of O plates is a significant improvement over the previous generation of Al-Cu-Mg-Ag alloys in which O plates dissolved sacrificially at long aging times for moderate aging temperatures. The competitive microstructural evolution of Al-Cu-Mg-Ag alloys proved to be sensitive to trace additions of Mg, Ag and Si.