A Novel Computational Thermodynamics Framework with Intrinsic Chemical Short-Range Order

TL;DR

This paper introduces an efficient hybrid computational thermodynamics framework, FYL-CVM, that accurately models chemical short-range order in multicomponent alloys within the CALPHAD formalism, balancing accuracy and computational cost.

Contribution

The novel FYL-CVM framework combines cluster variation method with CALPHAD using the FYL transform to efficiently incorporate SRO effects in multicomponent alloys.

Findings

FYL-CVM reproduces CVM phase diagrams with higher efficiency.

Incorporates vibrational, elastic, and electronic contributions.

Successfully applied to Cu-Au and Cu-Au-Ag systems.

Abstract

Chemical short-range order (SRO) provides new opportunities for tuning alloy properties, but conventional computational thermodynamics frameworks such as CALPHAD, based on Bragg-Williams mean-field approximations, cannot properly describe SRO or order-disorder transformations in multicomponent ($\geq$3) alloys. First-principles approaches combined with the cluster variation method (CVM) or cluster expansion method (CEM) can capture SRO but suffer from high computational cost. Here we present a hybrid CVM-CALPHAD framework with a thermodynamic solid solution model named as FYL-CVM, enabled by the Fowler-Yang-Li (FYL) transform to reduce the number of variables required in free-energy minimization. This achieves efficient modeling of SRO in multicomponent systems within the CALPHAD formalism. Benchmark tests on fcc AB binaries show that FYL-CVM reproduces CVM phase diagrams with much higher efficiency, while non-configurational contributions from vibrational, elastic, and electronic terms are also incorporated to capture their physical effects on order-disorder boundaries. Applied to the Cu-Au system, this method produces phase diagrams with experimental data in an efficient parameterization and elucidates the temperature-composition dependence of SRO parameters via the SRO diagram. Its applicability to ternary alloys is also demonstrated for the Cu-Au-Ag system. Overall, this framework strikes a balance between accuracy and efficiency, extends CALPHAD to account for chemical SRO, and enables a comprehensive physics-informed modeling of ordering phenomena. (This dissertation was submitted to the University of Virginia in 2023 as the author's doctoral research. For the original complete abstract, please refer to the PDF version.)