Thermodynamics of Disordered Crystalline Alloys: Multiphysics Atomic-scale Simulations

無序晶態合金的熱力學研究: 多物理原子尺度模擬

Student thesis: Doctoral Thesis

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Award date27 Dec 2023


Lattice distortion is commonly observed in both binary and multi-principal element alloys that have varying atomic radii; this phenomenon is thought to facilitate the formation of single-phase solid solutions. Using high-entropy alloys (HEAs) as an example, we investigate the influence of varying inter-atomic distances on the stabilization and control of their structural, mechanical, and thermodynamic properties. To achieve this, we employ a combination of statistical mechanics analysis and molecular dynamics simulations on simplified 1D and 2D systems, as well as a 3D crystal model with harmonic inter-atomic bonds and variable inter-atomic lengths. We comprehensively capture the impact of this inter-atomic length dispersion (representing static lattice distortion) and temperature fluctuations (representing dynamic lattice distortion) on fundamental and universal thermodynamic, structural, and elastic characteristics through a unified effective temperature. We present a novel scaling law for HEAs that establishes a relationship between both factors. This scaling law reveals that curves depicting various degrees of local lattice distortion collapse into a single curve when plotted against the effective temperature. As a result, lattice distortion significantly enhances the stability of HEAs by elevating the effective temperature. This means that increasing the temperature further reinforces the stability of a solid solution in comparison to the tendency towards phase separation or ordering. Local lattice distortion can be manipulated in HEAs to create a material with a zero coefficient of thermal expansion (the Invar effect) by incorporating anharmonic bond features.

On the other hand, recently emerging new definition of HEAs as multi-principal element alloys are prone to develop local chemical inhomogeneities, e.g., chemical order/clustering and/or compositional undulation. To model the structural evolution of complex HEAs, a flexible pairwise-energy model is developed. Lattice Monte Carlo (MC) simulations, coupled with discrete atomic pair energies, are extensively conducted to investigate thermodynamic atomic structures of NiCoFe-based alloys. The results of the simulations are consistent with reported experimental data and provide detailed insights into phase decomposition, species segregation/preferences, and chemical order/disorder. The simulations also offer microscopic views of atomic segregation by incorporating small atom (e.g., Boron) at grain boundaries. By employing Cahn’s wetting theory, the observed phase transformation processes, ranging from perfect to partial/pre-wetting, are explained and validated thermodynamically by the model. The current approach has the potential to be extended to a variety of HEA systems with constant or rather low mismatch lattice structures.