Computational Study of High Entropy Alloys: Stability, Lattice Distortion and Mechanical Properties

First-principles calculation of solid state structures can accurately predict properties of microscopic systems, based on density functional theory (DFT). In this thesis, we focus on the first-principles investigation of refractory high entropy alloys (HEAs). HEA is a new type of material containing multiple types of element, and has excellent mechanical properties, especially at high temperature. The research on the Band structure theory of Burgers distortion discovers that a pseudo-gap in electronic density of state, a band gap opening at high symmetry k-point in band structure, and

drop in energy of an occupied bonding state vs. increased energy of an empty antibonding state can explain how Burgers distortion lowers the energy. The study of a

thermodynamics model on the stability of BCC-based structures with different short range ordering of ternary alloys consisting of Al and transition metals shows that a

continuous order-disorder phase transition of BCC-based structures can be predicted with good accuracy compared with experiment. Our study of elasticity of refractory

HEAs prove that the inclusion of BCC/HCP elements can increase lattice distortion, and mechanical properties of HEAs such as shear modulus, ductility can be designed

by varying the composition of BCC/HCP elements. DFT can be accurate but computationally expensive for large scale system. Hybrid molecular dynamics (MD) Monte Carlo (MC) simulation can be used to simulate and find ordering in larger systems. Our study of an equimolar refractory HfNbTaZr HEA using hybrid MC/MD finds short range clustering of Hf-Zr, Nb-Ta pairs and larger segregation of Ta and Zr elements. Configurational and vibrational entropies are analyzed to favor phase mixing at high temperature.