Impurity-based, equilibrium-thickness, interfacial films have been widely observed at grain boundaries (GBs) [1], on free surfaces [2] and at phase boundaries [3], where they often control the microstructural evolution and materials properties. Somewhat recently, impurity-based, nanoscale, quasiliquid, intergranular films have been directly observed by HRTEM in Ni-doped W [4] and Mo [5] alloys, and the enhanced diffusion in these films explained a long-standing mystery regarding the origin of “solid-state activated sintering” [6, 7]. By extending bulk CalPhaD methods to model GBs, thermodynamic models were developed toconstruct a preliminary type of GB “phase” diagrams, “lambda diagrams”;the correctness and usefulness of these computed diagrams were systematically demonstrated [7-11]. As an example, a computed GB lambda diagram predicted that a retrograde solubility in a binary alloy could lead to a decrease in the GB diffusivity with increasing temperature, and this counterintuitive prediction was experimentally verified [11]. Furthermore, discrete interfacial phases, such as bilayers and trilayers, can also form [11]. Recent results on the role of a bilayer interfacial phase on liquid metal embrittlement [11] and the impurity effects on liquid metal corrosion [12] will also be presented. A focus of this presentation is to discuss the strategies for developing relevant thermodynamic models for high-temperature interfacial adsorption and disordering.
References:
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