The assessment of chemical short range order in multi-principal element alloys using total scattering

'Short-range order' (SRO) - preferential atomic ordering behaviour deviating from a statistically random distribution of elements within a local region - is acknowledged across the literature as a contributor to the mechanical properties of crystalline materials. Multi-principal element alloys (MPEAs) may possess some form of statistical SRO, evidenced by their unique properties. Analytical complexities, however, have hindered efforts to quantify and describe atomic ordering. While local order has been identified in binary alloys, the efficacy of the method is not established for chemically complex systems.

This work seeks to develop a novel analytical method for the investigation of SRO in multi-principal element alloys (MPEAS) using total scattering. Firstly, neutron total scattering data is used to perform pair distribution function analysis on the CrCoNi MPEA. Modelling of the CrCoNi system to cryogenic temperatures demonstrates the local formation of the L1_ 2 structure.

Analysis of the CrFeCoNi MPEA is subsequently performed. This investigation is used to demonstrate the increase in analytical complexity with increased quantities of constituent elements, and provides a logical step towards the analysis of traditional 'high-entropy alloy' (HEA) systems. Through a neutron total scattering investigation at raised temperatures, it is shown that the CrFeCoNi alloy \textit{also} presents with a local L1_2 structure.

Finally, a novel framework for the investigation of SRO is presented. Building on the prior methods, this framework has been used to generate an associated software available for use across a range of crystal structures. Test cases are presented from a variety of material domains as a proof of concept.

In summary, this work shows the effectiveness of the total scattering and large-box modelling method in the investigation of atomic SRO in chemically complex materials. It is hoped that the successful development of novel, generalised analytical methods will provide the crystallographic community with the tools to investigate SRO more widely.