UC San Diego

High entropy alloys (HEAs) demonstrate remarkable properties which are useful in critical engineering applications. Recent studies report that NbMoTaW exhibits many of the properties similar to HEAs, including high hardness, high electrical resistance, and high thermal stability. In turn, selecting and implementing the NbMoTaW alloy would improve components in power and nuclear applications. This Master’s thesis will discuss vacancy formation and diffusion behavior in NbMoTaW, along with atomistic simulations that can capture that phenomena. DFT and MD simulations as implemented in VASP and LAMMPS, respectively, calculated vacancy formation energies for the NbMoTaW HEA using an SQS structure. These vacancy formation energy calculations show that Nb and Ta vacancies are the most energetically favorable and illustrate some discrepancies between the MD and DFT simulations. In addition, DFT simulations used the climbing image NEB method as implemented in VASP calculated migration enthalpies in the Nb-W alloy. These calculations used a set of structures which have different atomic compositions in the local environment. The composition changes in the set of structures surrounded the diffusion path and varied in the spatial distance from the diffusing atom. DFT simulations using the climbing-image NEB method as implemented in VASP showed that a change in composition five nearest neighbors away from the diffusion path had a non-negligible effect on the migration enthalpy, however the error peaked at three nearest neighbors.