The Hf-Mo-Nb-Ta-Ti-W-Zr system of compositionally complex alloys have been studied in this work by means of density functional theory modelling techniques in order to understand the usage of these novel alloy systems within the nuclear sector. Especially, this endeavour has sought to meet the materials design criteria for an interlayer material between zirconium-based fuel claddings, used in current light water nuclear reactors, and current chromium coatings, using compositionally complex alloys that are characterised as compositing four or more metallic elements in close to equal ratios.
An analysis of the particular environment at the nuclear fuel cladding surface – characterised by its high temperature, corrosiveness and irradiation – has prompted meticulous materials design criteria for combining chromium coatings to zirconium-based claddings. This design criterion is put in place to ensure these two materials can stay separated for a greater period of time should a loss-of-cooling nuclear accident occur. This analysis suggested two equiatomic refractory alloys as a suggested starting point for an interlayered coating system: MoNbTaW and MoNbTaTiW.
With regard to the fundamental thermophysical properties of compositionally complex alloys, atomistic simulation has revealed for the first time a straightforward mechanism for thermal expansion in these alloys, which can be directly related to their constituent elements. By means of alloys in the Mo-Nb-Ta-Ti-W system, it is possible to precisely tailor the thermal expansion response of an interlayer alloy so as to optimise expansion between zirconium-based fuel claddings and coating materials. The role of vacancy defects in these alloys, and their consequences for expansion behaviour are also explored. An in-depth analysis of vacancy defects in the Hf-Mo-Nb-Ta-Ti-W-Zr system has revealed the role configurational entropy plays in the formation of intrinsic defects, and an alternative mechanism to explain the higher concentration of vacancies observed in complex solid solution alloys is suggested. This argument rests upon the chemically disordered environment which occurs around each vacancy site, and may be generalisable to all alloy systems, even more dilute ones. The findings indicate that compositionally complex alloys possess a substantially higher number of vacancies than conventional alloy systems and pure metals, which could have impacts on the radiation damage of compositionally complex alloys.
Regarding the testing of compositionally complex alloys in real zirconium-based alloy coating systems, a pathway is laid out to explore properties such as eutectic reaction temperatures, oxidation, spalling phenomena, interdiffusion, and chemical reactions, in laboratory-scale testing. It is shown that an adequate assessment of these coating systems is possible on a small scale and can aid future efforts to develop and optimise interlayered materials in zirconium fuel cladding coating systems.