Thermodynamics and Kinetics of Aluminum Alloys

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There is growing interest in and demand for lightweight, age-hardenable alloys such as aluminum (Al) alloys in the transportation sector. This interest is due to the potential for reducing vehicle mass and thereby improving fuel economy. Precipitation microstructures improve the mechanical properties of materials, such as yield stress, by impeding the motion of dislocations. That is, the distribution and shape morphology of precipitates have a significant impact on the materials’ properties. Therefore, understanding the mechanisms of microstructure evolution is very important to material design. The microstructure of the precipitate is strongly influenced by the interfacial stability, which is closely connected to the atomic-scale crystal structure of the precipitate phase, the matrix phase, and the interface between the two phases. For this reason, it is vital to understand the thermodynamic and kinetic behavior of strengthening precipitate phases at a variety of length scales for designing structurally strong Al alloys. This work aims to understand phase stability and the growth/coarsening mechanism of Al-based precipitates. We utilize computational methods (i.e., first-principles calculation based on density functional theory (DFT) and phase-field model (PFM)) and develop analytical theories to investigate the thermodynamics and kinetics of a variety of Al precipitates at various length-scales. In this study, we: 1) investigate the energetically-favored interfacial structure at coherent and semi-coherent Al2Cu (θ')//α-Al interfaces for the growth and coarsening studies; 2) search for the lowest energy crystal structure of the ternary (Al-Li-Cu) T_1 phase in order to resolve controversial topic found in many experimental studies; 3) perform atomic-scale DFT calculations of defect properties, solute partitioning, and interfacial stability of the Al3Cu2Mg9Si7 (Q) precipitate to explain Q-phase off-stoichiometry in experiments, find a potential solute to make the Q-phase energetically stable for coarsening resistance, and to derive a low-energy structure for future growth and coarsening studies; 4) examine the growth of equilibrium Al2Cu (θ') morphology using the phase-field method with parameters supplied by first-principles calculations in order to elucidate fundamental physics for the high aspect ratio of θ^' precipitates in experiments of binary Al-Cu alloys; and 5) develop a general theory of phase coarsening (Ostwald ripening) for a prolate spheroidal particle in a multicomponent alloy by accounting for the anisotropic effects of interfacial energy and geometry in order to analyze anisotropic shape effects on the growth and coarsening law. These thermodynamic and kinetic studies at atomistic- and micro-scales will move the field of structure metal alloys forward, shedding light on fundamental issues in design of strengthening precipitates in Al alloys.

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  • 01/29/2019
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