A new semi-empirical method based on a distorted tetragonal scheme for the structure prediction and alloy design of multiple-component alloys

High Entropy Alloys (HEA) are many-component (>4), near equiatomic compositions that by definition, forms disordered single phases, but are also known to form ordered phases such as C14 Laves, B2, Sigma etc. Understanding of the HEA phase stability is limited due to lack of knowledge of HEA thermodynamics as their stoichiometry is located at, or near the centre of their corresponding phase diagrams. Utilising HEAs in industrial and/or functional applications requires this understanding for successful design of HEA compositions. The original contribution of this thesis is the development of a simple, semi-empirical model that allows the prediction of some HEA properties (relative phase stability, hardness) that can be easily implemented. The model is developed through initial analysis of HEAs exploring the influence of the electronic density on phase stability utilising the Thomas-Fermi-Dirac equation for constrained neutral atoms; and the influence of a possible reduction in valence orbital degeneracy due to the breaking of symmetry from changes in phase. The hypotheses formed from this analysis are further compared with robust ab-initio calculations following the Rigid Band Approximation within Density Functional theory. The final two chapters examine the construction of the proposed model and its refinement; experimental validation of the model is performed through an engineering of a possible replacement for the Stellite family of Co-Cr alloys, which has been optimised for wear resistance. Experimental validation shows good results with respect to the predicted values.