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High-Entropy Alloys (HEAs) are a new class of metallic materials based on the combination of multiple principal elements, often with the intention to form a concentrated solid solution. It has been suggested that these solid solution phases could offer enhanced mechanical properties, due to fluctuations in their local atomic environments. However, at present, the thermodynamic stability of the solid solution phases which form in HEAs is poorly understood. This study aims to improve our understanding of phase stability in HEAs and investigate possible differences in the mechanical behaviour of dilute and concentrated solid solutions. Firstly, a systematic series of experiments is presented, which establish the effect of Co and Fe on the phase equilibria of the widely studied CrMnFeCoNi system. Both Co and Fe were found to stabilise the A1 matrix relative to the A2 and D8b phases at elevated temperatures but did not prevent the formation of ordered phases at 500°C. Alongside literature data, the results show that stable single-phase alloys are rare in the CrMnFeCoNi system. Secondly, a systematic assessment of the AlxCrFeCoNi alloy system showed that Al promotes the formation of B2, D8b and A2 phases, and improved our understanding of phase stability in this system, which has strong potential for developing alloys with desirable mechanical properties. In each study, experimental observations were used to test the fidelity of thermodynamic predictions from the latest CALPHAD databases. Whilst the predictions generally provided a close approximation of the observed phase equilibria, inaccuracies were common, especially at lower temperatures. Phase stability investigations also served to identify thermodynamically stable alloys, suitable for the investigation of the Portevin-Le Chatelier (PLC) effect - a discontinuous yielding phenomenon widely attributed to the repeated pinning of dislocations by mobile solute atoms over certain regimes of temperature and strain-rate. Under mechanical testing, the effect is generally associated with serrations in the materials flow curve and the localisation of strain into discrete bands. Serrated flow has been observed in several HEAs but the associated strain localisation behaviour has not yet been reported, so it is uncertain whether this behaviour is similar to the PLC effect in other alloy systems. Thus, a series of alloys based on the stable equiatomic quaternary alloy, CrFeCoNi, were selected to investigate the effect of compositional complexity on the PLC effect. Tensile testing, combined with digital image correlation, enabled simultaneous recording of the materials stress and local strain response across a range of temperatures. The results showed close similarities between the alloys at each temperature, indicating that compositional complexity did not have a dominant influence on the PLC effect. This suggested that the dislocation-solute interactions in the complex, concentrated solid solutions were not substantially different from those of more dilute solutions. Calorimetric investigations also revealed evidence of a varying degree of short-range order among the alloys, the influence of which warrants further investigation.

Over the years `short-range order' (SRO), whereby the local atomic arrangement differs from that of a random distribution, has been used to explain physical phenomena such as thermodynamic discontinuities, increased strength, anomalous electrical resistivity and magnetic variations in a host of alloys. However, due mainly to experimental difficulties and the complexity of the calculations required for the analysis of diffuse scattering, such work has been largely abandoned and hence quantification and assessment of SRO is notably sparse in the literature. The recent development of reverse Monte-Carlo (RMC) methods for the analysis of total scattering data has opened a promising route for the assessment of a material's local environment and has already provided important insights into a host of complex chemical systems, including liquids, network glasses, nano-materials, functional oxides and metal organic frameworks. The work presented in this thesis focuses on the development of a new methodology for the analysis of local structural effects in metallic systems using total scattering, and the first systematic application to the study of alloys. The simulation of total scattering data from a range of model structures is used to show that the information content of total scattering functions, in particular the pair distribution function (PDF), is sufficiently high to allow the assessment of different types and degrees of short-range order. This is supported by a demonstration of how such orders can be quantified from large box models, produced by fitting total scattering data using the RMC algorithm, with the mathematical analyses outlined. This culminates in a proposed methodology for the analysis of SRO in alloys. Having developed this analytical methodology it is subsequently applied to a number of interesting alloy systems. To demonstrate the efficacy of this methodology it was first applied to the study of a sample of Cu$_{3}$Au - the classically cited case example of a system demonstrating SRO prior to an ordering transition. This experiment provides new insight into this well characterised transition, and also demonstrates the significance of data processing errors on the generation of artefacts in large box modelling. The technique is also applied to the study of the industrially important family of nickel superalloys, assessing ordering in the gamma-phase alloy Ni-Cr and the sublattice orderings occurring in L1$_{2}$ alloys. Next, the use of the technique for the analysis of local strains exhibited in a lattice is presented. A series of models is used to demonstrate how the PDF is expected to change under variations in local strain caused by increased concentration of atomic substitution and variation in atomic radii. This is subsequently used to study the characteristic high-entropy alloy (HEA) CrMnFeCoNi. Through analysis of the PDF, it is demonstrated that the level of local strain exhibited in this alloy is not significantly different from those of other related compositionally simpler alloys. This result is highly significant as it challenges one of the core principles of the field - that the lattices of HEAs are necessarily highly strained. Finally, the energetics of ordering reactions are briefly considered and used to justify some of the observed transformations presented in the earlier work. Together, the body of work in this thesis shows how the total scattering technique can be used to provide valuable insight into a host of interesting local phenomena occurring in alloy systems. It is hoped that this will open up a new field of study into these effects, and ultimately guide the creation of new alloys based on these structure-property relationships.

Radiation-induced segregation (RIS) was studied in Fe-Cr-Ni alloys irradiated with protons to better understand the mechanisms causing changes in grain boundary chemistry and to improve the ability to predict RIS in austenitic Fe-Cr-Ni alloys. Ni-18Cr, Ni-18Cr-9Fe, Ni-18Cr-0.08P, and Fe-20Cr-9Fe were irradiated with 3.2MeV protons at temperatures from 200$\sp\circ$C to 500$\sp\circ$C and to doses from 0.1 to 3 dpa. Grain boundary chemistry was measured using both Auger electron spectroscopy (AES) and scanning transmission electron microscopy with energy dispersive x-ray spectroscopy (STEM/EDS). The significant driving mechanism far segregation in Fe-Cr-Ni alloys is shown to be the inverse Kirkendall (IK) mechanism, specifically the coupling between alloying elements and the vacancy flux. The inclusion of interstitial binding effects to RIS models results in poor agreement between model predictions and segregation measurements, severely overpredicting the measured Ni enrichment and Fe depletion. Grain boundary segregation is unique for each bulk alloy composition in that the amount and the rate of segregation differs for alloys irradiated under the same conditions. Kinetic parameters must be known for each alloy to accurately predict segregation, but the kinetic parameters in Fe-Cr-Ni alloys at low temperature are not well studied. Additionally, short range ordering interactions are important in determining the segregation in all Fe-Cr-Ni alloys. Ordering enthalpies must be included in RIS models to correctly describe the segregation process. Therefore, to develop a predictive RIS model, a method for calculating diffusivities from the bulk composition that includes ordering enthalpies was developed. The Perks (IK) model has been modified to account for composition dependent segregation kinetics by calculating the migration energy using pair interaction potentials, ordering enthalpies, and the local concentration. Based on segregation measurements from seven different alloys, irradiated to five different temperatures and six different doses, this model is in better agreement with the segregation measurements than the original Perks model. Using a single set of input parameters, the modified IK model predicts the measured segregation of Fe, Cr, and Ni with an error less than about 5 at%.