Thermodynamic Modeling of Grain Boundaries Complexions and Developing Grain Boundary Complexion Diagrams for Multicomponent Metallic Systems

Author(s): Zhou, Naixie | Advisor(s): Luo, Jian | Abstract: Grain boundaries (GBs) in crystalline materials can be treated as interfacial phases which are called complexions. Like bulk phases, GB complexions can undergo first-order or continuous transitions with varying thermodynamic potential and such GB complexion transitions can cause abrupt changes in structure and chemistry of GBs, thereby critically influencing a broad range of interfacial controlled materials’ properties such as sintering, grain growth, creep, embrittlement, electrical/thermal/ionic conductivity. Specifically, the presence of multiple dopants and impurities can significantly alter the GB complexion formation and transition. In the first part, a thermodynamic framework is developed to forecast the formation and stability of disordered premelting-like grain boundary complexions in multicomponent alloys to consider the interactions of multiple alloying elements. Subsequently, ternary and quaternary grain boundary diagrams have been computed and used to forecast the sintering behaviors of W–Ni–M (M = Fe, Co, Cr, Zr, Nb and Ti) and Mo–Si–B–M (M = Ni, Co and Fe) systems. In the second part, grain boundary adsorption transitions are studied using an Ising type lattice model. The GB complexion diagram is computed for the average general GBs in Bi-doped Ni. The predictions are calibrated with previously-reported density functional theory calculations and further validated by aberration corrected scanning transmission electron microscopy characterization results as well as prior Auger electron spectroscopy measurements. Subsequently, using the same model, a systematics of grain boundary adsorption transitions is derived. A normalized segregation strength is defined to represent the effects of mixing energy, solute strain and differential bonding energies as well as the misorientation for symmetrical twist boundaries, which is shown to be a dominant factor in controlling adsorption transition behaviors. This derived systematics of GBs exhibits phenomenological similarities to the cases of multilayer adsorption of inert gas molecules on the surfaces of attractive substrates, enriches the classical grain boundary segregation/adsorption theory. In the third part, bulk computational thermodynamics are extended to model binary poly/nanocrystalline alloys by incorporating grain boundary energies computed by a multilayer adsorption model. A new kind of stability diagram for equilibrium-grain-size poly/nanocrystalline alloys is developed. Computed results for Zr-doped Fe are validated by prior experiments and provide new physical insights regarding stabilization of nanoalloys and its relation to solid-state amorphization. In the final part, the effect of multicomponent alloying on the thermal stability of nanocrystalline alloys is studied. By introducing more alloying components, the grain boundary energy can be reduced more significantly via both bulk and grain-boundary high-entropy effects with increasing temperature at/within the solid solubility limit, thereby reducing the thermodynamic driving force for grain growth. Moreover, grain boundary migration can be hindered by sluggish kinetics. The theory is supported by numerical experiments and experiments in Ni systems.