Defect-characterized phase transition kinetics

Phase transitions are a common phenomenon in condensed matter and act as a critical degree of freedom that can be employed to tailor the mechanical or electronic properties of materials. Understanding the fundamental mechanisms of the thermodynamics and kinetics of phase transitions is, thus, at the core of modern materials design. Conventionally, studies of phase transitions have, to a large extent, focused on pristine bulk phases. However, realistic materials exist in a complex form; their microstructures consist of different point and extended defects. The presence of defects impacts the thermodynamics and kinetics of phase transitions, but has been commonly ignored or treated separately. In recent years, with the significant advances in theoretical and experimental techniques, there has been an increasing research interest in modeling and characterizing how defects impact or even dictate phase transitions. The present review systematically discusses the recent progress in understanding the kinetics of defect-characterized phase transitions, derives the key mechanisms underlying these phase transitions, and envisions the remaining challenges and fruitful research directions. We hope that these discussions and insights will help to inspire future research and development in the field

Experimental and computational investigation of multi-principal element alloys

Multi-principal element (MPE) alloys, unlike traditional alloys, consist of five or more principal elements with near equi-atomic compositions creating a large new compositional space for exploring new alloy possibilities. However, designing MPE alloys with the desired phases, microstructures and properties is challenging task, and there is a demand for basic research for a better understanding of structure-processing-property relations in these alloys. In this Ph.D. research, different computational models and experiments were integrated to study phase formations, and mechanical properties of different MPE alloys. Density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations were used to determine crystal structures, phase stability, and plastic deformation mechanisms. A modified thermodynamic approach was developed to calculate the phase diagrams of MPE alloys, and the accuracy of this approach was tested against commercial software. Experimental casting and characterization, and literature data were used to validate modeling predictions. The phase diagram calculations of AlFeCoNiCu HEA showed coexistence of two phases at room temperature and stabilization of one phase above 1070 K at the equiatomic composition. The characterization experiments confirmed the crystal structures and composition of phases. To investigate the plastic deformation mechanisms and ductilities of CoCrFeNi-based HEAs, unstable and intrinsic stacking fault and unstable twinning energies were determined by DFT calculations. Finally, the effects of interstitial carbon on the phase formations in AlxFeCoCrNiCu HEAs were investigated, showing formation of different possible carbides and inter-granular graphite --Abstract, page iv