|Phase-field modeling of the evolution kinetics of porous metals during dealloying
|Shi, S. Q. (ME)
Hong Kong Polytechnic University -- Dissertations
|Department of Mechanical Engineering
|xvii, 123 pages : color illustrations
|Dealloying is the selective dissolution of one or more active elemental components of an alloy in corrosive solutions. It was initially studied as a key failure mechanism for engineering structures. Recently, it was found that dealloying can be used to create porous metals with pore sizes ranging from a few nanometers to a few tens of micrometers. This has attracted significant attention for applications in catalysis, supercapacitors, sensing and actuation, mass and heat transport, and battery electrodes. Therefore, it is essential to understand this process to prevent material failure and develop various porous metal structures. This work focuses on developing a comprehensive numerical model using a phase-field formulation for evolving nanoporous metal structures during corrosive dealloying. Firstly, a multi-phase-field model of chemical dealloying kinetics in binary alloys is developed to study the topology shape change of porous structures in one-dimension (1-D), two-dimension (2-D) and three-dimension (3-D). By introducing three phase-field variables to represent the phase constitution at the solid-liquid interfaces, the free energy of the precursor-porous clusters-electrolyte system is expressed as a function of field variables and their gradients. The thermodynamic driving force is treated as a function of the difference between the generalized and equilibrium chemical potentials at the interfaces. We propose that the dissolution of the less noble (LN) element leads to interface instability and triggers the nucleation and growth of porous clusters, which are essential for evolving porous structures. The dealloying velocity and porous structure morphology of the model show good overall agreement with experimental results by calibrating the activation energy of dissolution with the dealloying velocity. Moreover, the roles of some controllable dealloying parameters, including the chemical acid concentration, initial alloy composition, parting limit, and surface diffusion coefficient, are elaborated.
Secondly, a new comprehensive multi-phase-field (MPF) model is proposed to study topological porous patterns formed by spontaneously etching a bulk binary alloy that involves electrochemical reactions, bulk and surface diffusion, ion transport, applied electrode potential, and charge conservation. The governing equations for the alloy-porous cluster-electrolyte system account for a generalized Butler-Volmer electrochemical reaction and are in accordance with the classical nucleation theory. Based on a quantitative examination of the effects of electrode potential and precursor composition, the simulation results reproduce typical phenomena including passive surface dealloying, active porosity evolution, critical potential, and characteristic length scale in 2-D and 3-D. Finally, the multi-phase-field model is extended to simulate the corrosive dealloying process of alloys with complex structures. Precursors with various phase constitutions and compositions are designed and applied with different activation energies associated with intrinsic electrochemical properties. Several examples are presented to simulate the formation of novel porous structures with unimodal pores by investigating the effect of defect and reinforcement phases in the precursor. The model can also simulate the dealloying of a dual-phase binary alloy comprising a solid solution and intermetallics, or two types of intermetallics and the formation of a nested porous network.
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