Optimizing nanoscale precipitates in Al alloys by microalloying with transition metals

We propose to undertake fundamental physical metallurgy studies of the microstructural evolution and resulting mechanical properties of aluminum micro-alloyed to form Al3X nano-precipitates containing Group IVB elements (Ti, Zr and Hf) in a coarsening-resistant shell and Er in a early-nucleated core that provides creep resistance via lattice mismatch, while adding Si as an inoculant and an accelerant for diffusion of Ti, Zr, and Hf. Combining a synergistic two-pronged experimental and computational approach, this research builds on our extensive prior knowledge of aluminum alloys containing Sc along with Zr and/or Ti additions, and some exploratory research at Northwestern with Hf. This new methodology foregoes the use of expensive Sc by utilizing a novel tactic based on Si as an inoculant and Er as the nucleus for core-shell precipitates. The proposed research involves three tasks: (1) Alloy fabrication and thermo-mechanical heat-treatment; (2) Microstructural characterization of the temporal evolution of the nanoprecipitates: collaboration with Dr. K. E. Knipling, NRl. This task will elucidate the kinetics of nano-precipitate nucleation, growth, transformation and coarsening, relying principally on three-dimensional (3-D) atom-probe tomography (APT), which provides nanostructural and chemical information on a sub-nanoscale. Additionally, we will use optical microscopy, scanning electron microscopy and transmission electron microscopy to study the complete hierarchy of length scales, in parallel with electrical conductivity and microhardness measurements. This task will be complemented utilizing first-principles calculations concerning the role of Si in accelerating the kinetics of precipitation utilizing the Vienna ab initio simulation package. (3) The third task involves an investigation of the strength of the newly developed alloys, both at ambient and elevated temperatures, in conjunction with two- and three-dimensional dislocation (ParaDiS) dynamics numerical models for calculating yield stress at ambient temperature and creep threshold stress at elevated temperature. In particular, the critical microstructural information (number density, mean radius, volume fraction of nanoprecipitates, mean edge-to-edge distance between nanoprecipitates, lattice mismatch and dendrite dimensions) will be used to both predict and optimize the strength of the alloys at elevated temperatures. The three tasks will be integrated by correlating the processing (casting and heat-treatments), microstructures (precipitates, grain boundaries and dendrites) and properties (yield and creep strengths), according to the fundamental paradigm of materials science and engineering. i.e., the relationships between micro-/nano-structures and physical and mechanical properties. For all tasks, we will model the results using analytical and computational strength models, and additionally utilizing Thermocalc and Dictra calculations of the expected microstructures wherever possible. The new Al superalloys are strengthened by coarsening- and creep-resistant Al3(Ti,Zr, Hf,Er) (L12 structure) nanoprecipitates with core-multishell structures that are stable up to at least 425oC for long periods of time ranging from weeks to months, which are nucleated at a very high number density, 1022-1023 m-3, with the aid of Si and Er. We will select the alloys’ compositions and heat-treatment conditions, guided by experimental results, modeling phase transformation theory, first-principles calculations, using VASP as well as Thermocalc and Dictra calculations. The APT experimen