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Abstract
The major objective of this thesis was to design new types of high entropy and compositionally complex alloys, and to understand the evolution of their microstructures during casting and secondary processing, and the subsequent mechanical properties.
The first series of alloys investigated were non-equiatomic, single-phase, high entropy alloys (HEAs) based on the multicomponent Al-Ti-Co-Cr-Fe-Ni system. By following the natural decomposition of the two-phase microstructure of Al2TiCoCrFeNi, it was found that, with increasing Ti content, a range of ordered body centred cubic (BCC) alloys (B2-type) that tend towards an L21 structure were generated. Phase stability and hardness relationships were discussed in terms of phase equilibria, structural ordering, lattice strain and thermodynamics. It was found that chemical enthalpy, topology and lattice strain play a key role in the ordering tendencies and mechanical response of these alloys.
The second series of HEAs investigated, based on the Mg-Li-Al-Zn quaternary system, involved the same iterative casting technique. After casting, three different crystal structures where identified; hexagonal, monoclinic and icosahedral quasicrystal, in the compositional regions of Mg34-35Li20-25Al19-20Zn22-25, Mg37-40Li25Al15-20Zn15-20, and Mg28-32Li6-8Al27-30Zn30-35, respectively. Well-documented thermodynamic guidelines for predicting the stability of single-phase fields were used for assessing their validity in predicting the compositional regions of single-phase stability in this Mg-Li-Al-Zn alloy system with varying degrees of success. The Ω ratio and lattice misfit were consistent with literature, however, calculations using the electronic structures fell outside the traditional guidelines, attributed to the lack of d-orbital electrons in the lightweight elements compared to that of the transition metals alloys whereby these parameters were derived for.
Finally the Mg-Sc binary alloy system was used to generate a range of compositionally complex ternary alloys containing one of the following rare earth (RE) elements, Y, Gd or Er. The Mg-Sc binary system was taken as the starting point due to the stable BCC phase region. It was found that with the addition of the third RE element that, the hardness was increased and the β-Mg (BCC) phase field is broadened allowing for the design of a cheaper and lighter alloy. Y was found to be the most successful at broadening this phase field.