The role of alloying elements on the microstructure and thermal stability of Refractory Metal High Entropy Superalloys

Environmental targets require lower emissions, which necessitate increased operating efficiency for future generations of aeroengines. Consequently, higher operational temperatures will be needed, beyond the capabilities of current nickel-based superalloys. Therefore, new high temperature alloys are being investigated, including refractory metal high entropy superalloys (RSAs), based on the AlMoNbTaTiVZr system. These alloys comprise nanoscale order-disordered B2+bcc microstructures, similar to nickel-based superalloys, and have promising high temperature compressive yield strengths and competitive densities. However, RSAs typically have a ordered B2 matrix and can form Al-Zr-rich intermetallic phases, limiting room temperature ductility. This thesis aimed to develop understanding of the contributions of different elements to RSA microstructures, through the systematic study of constituent systems, which will aid the design of future RSAs. The nanoscale microstructures in RSAs are believed to form due to the miscibility gaps between the refractory metals and Zr. To investigate the contributions of different bcc+bcc miscibility gaps to RSA microstructures key simplified systems were studied. Nanoscale morphologies, like those in RSAs, are shown to form via a spinodal decomposition in the TaTiZr system, Chapter 4, albeit comprising of disordered bcc phases, but similar morphologies were not observed within the NbTiZr system, Chapter 5. Furthermore, compositional modifications in the TaTiZr system produced a refractory metal rich bcc matrix phase, indicating a potential route to produce more ductile RSAs. Through varying the ratio of refractory components in the NbTaTiZr and MoTaTiZr systems, Chapter 6, Nb was observed to lower the bcc+bcc solvus temperatures while Mo raised the solvus temperatures. The primary role of Al in RSAs was believed to be the ordering of the B2 phase. To investigate this premise, in Chapter 7, Al was added in a TaTiZr alloy with a Ti-Zr-rich matrix, analogous to complex RSAs, which demonstrated sufficient Al content can induce B2 ordering. However, B2 precipitates were only observed at relatively low temperatures of ∼700˚C, raising concerns for high temperature mechanical properties. The effect of Al was seen to be more complex in Chapter 8, where additions of Al into a TaTiZr alloy with a Ta-rich matrix increased the propensity for forming the nanoscale basketweave structure. In Chapters 8 and 9, where Al was removed from AlMoNbTaTiZr alloys, Al was observed to impact the volume fractions of the bcc phases formed. Critically, Al was also associated with the formation of intermetallic phases, the most prevalent of which is an Al-Zr-rich intermetallic related to the binary Al4Zr5 phase, which are believed to be deleterious to the mechanical properties. In Chapter 10, Mo suppressed some of the intermetallic phases in complex RSAs but a greater fraction of the Al-Zr-rich intermetallic formed. Microstructural stability of RSAs is critical to retain advantageous properties during high temperature service. Throughout this work, homogenised alloys were exposed to long duration thermal exposure at sub-solvus temperatures. In both simplified (Chapters 4-9) and complex RSAs (Chapter 10), the homogenised microstructures were not thermally stable but exhibited significant precipitate coarsening and many alloys formed additional phases. These studies highlight some of the challenges faced by RSAs and the potential for microstructural optimisation.