Modelling high entropy alloys with density functional theory

The aim of this thesis is to study several High Entropy Alloys (HEAs) with seven low-activation elements, Cr, Fe, Mn, Ta, Ti, V, W in order to find new compositions of HEAs for use as nuclear materials. The main focus is to be able to predict how likely an alloy system can form a single phase solid solution. The work forms part of a joint project involving UK experimental groups at Manchester, Sheffield and Oxford Universities and an Indian modelling group from the Bhabha Atomic Research Centre (BARC) in Mumbai.

We investigate equiatomic binary, ternary and quaternary systems by Density Functional Theory (DFT) calculations, special quasi random structures (SQS) have been generated for combinations of the 7 low activation elements. Using modified Hume-Rothery parameters and other criteria, predictions of single phase are made. These predictions were given to our experimental collaborators to make quaternary alloys and investigate their structure using X-ray diffraction. Computational X-ray diffraction spectra were generated for comparison. The quaternary alloy TaTiVW was predicted by computational methods to be single phase and this was verified experimentally. Other properties such as lattice constant and bulk modulus were obtained, and they were found to be in agreement with Vegard’s law. Atomic spacing distributions were investigated to evaluate how much the systems were deformed from the strict BCC structure.

A final aim of the project was to begin to understand the role of defects in the materials, by determining the migration pathways and energy barriers of point defects (vacancies and interstitials) in a model system. It was found that V-V and V-Ti dumbells were the favoured interstitial defects and Ta vacancies were preferred. Some preliminary comparisons were made with molecular dynamics simulations of low energy collision cascades with good agreement.