Optimizing nanoscale precipitates in Al alloys by microalloying with transition metals
Abstract
Funds are provided to explore the aluminum-rare-earth alloy systems, and to provide information for the design of alloys in this system.The investigators have proposed a two-pronged experimental and computational approach for their study that research builds on their extensive prior knowledge of aluminum-scandium alloys along with zirconium and/or titanium additions, and some exploratory research at Northwestern with hafnium. Their approach foregoes the use of expensive scandium by utilizing a novel tactic; which they base on using erbium as a nucleating agent and template, and silicon as an inoculant, to produce core-shell precipitates. Their proposed research involves three tasks.The first task is the production of alloy samples, and thermo-mechanical heat-treatments for further study. In addition, the investigators plan to utilize Thermocalc and Dictra (commercial thermo-kinetics codes) calculations of the expected microstructures, and will work to validate and refine the thermokinetic database for these elements in aluminum.The second task is the microstructural characterization of the temporal evolution of the core-shell nanoprecipitates. This task will elucidate the kinetics of nano-precipitate nucleation, growth, transformation, and coarsening. The task will rely principally on three-dimensional (3-D) atom-probe tomography (APT), which provides structural and chemical information on a sub-nanoscale. Additionally, they plan to use standard 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 that provide continuous information on the transformation kinetics. Of particular interest is the effect of silicon additions on the diffusion of the other alloying constituents. The investigators plan to use the Vienna ab initio simulation package (also known as VASP) to perform calculations of the silicon-metal-vacancy interaction energetics in an aluminum matrix to elucidate the mechanisms associated with silicon in accelerating the kinetics of precipitation.In the third task, the investigators plan to use two- and three-dimensional dislocation dynamics numerical models to create predictive models for yield stress at ambient temperature and creep threshold stress at elevated temperature. In particular, they plan to identify the critical microstructural information (that is, number density, mean radius, volume fraction of nanoprecipitates, mean edge-to-edge distance between nanoprecipitates, lattice mismatch, and dendrite dimensions) to both predict and optimize the strength of the alloys at elevated temperatures.The three tasks will correlate 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.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 12, 2016
- Source ID
- N000141612402
Entities
People
- David C. Dunand
Organizations
- Northwestern University
- Office of Naval Research
- United States Navy