An Investigation of the Miscibility Gap Controlling Phase Formation in Refractory Metal High Entropy Superalloys via the Ti-Nb-Zr Constituent System

Abstract

Refractory metal high entropy superalloys (RSAs) have been heralded as potential new high temperature structural materials. They have nanoscale cuboidal bcc+B2 microstructures that are thought to form on quenching through a spinodal decomposition process driven by the Ta-Zr or Nb-Zr miscibility gaps, followed by ordering of one of the bcc phases. However, it is difficult to isolate the role of different elemental interactions within compositionally complex RSAs. Therefore, in this work the microstructures produced by the Nb-Zr miscibility gap within the compositionally simpler Ti-Nb-Zr constituent system were investigated. A systematic series of alloys with compositions of Ti5NbxZr95−x (x = 25–85 at.%) was studied following quenching from solution heat treatment and long duration thermal exposures at 1000, 900 and 700 °C for 1000 h. During exposures at 900 °C and above the alloys resided in a single bcc phase field. At 700 °C, alloys with 40–75 at.% Nb resided within a three phase bcc + bcc + hcp phase field and a large misfit, 4.7–5%, was present between the two bcc phases. Evidence of nanoscale cuboidal microstructures was not observed, even in slow cooled samples. Whilst it was not possible to conclusively determine whether a spinodal decomposition occurs within this ternary system, these insights suggest that Nb-Zr interactions may not play a significant role in the formation of the nanoscale cuboidal RSA microstructures during cooling.

A review of metallic materials for latent heat thermal energy storage: Thermophysical properties, applications, and challenges

“…In the last years, much of the research efforts in phase change materials have mainly focused on the characterization of thermophysical properties of organic and salt hydrate; for metallic materials, the data found in the literature is incomplete and discrepant [46,47]. Recently, new promising utilizations of metals and alloys for thermal energy storage has appeared in different research areas: miscibility gap alloys [48][49][50][51][52][53][54][55][56], metal-organic framework and shape-stabilized PCMs [57][58][59][60][61], encapulation [62][63][64][65][66][67][68][69].The present work aims to provide an extensive and detailed compilation and analysis of the thermophysical properties of metals and alloys as phase change materials. The information presented includes the fundamental thermophysical properties, the temperature dependence of critical properties from the engineering design point of view, a summary of potential applications, and the main challenges of metallic PCMs.…”

Special Issue “Advanced Refractory Alloys”: Metals, MDPI

“…However, it is difficult to isolate the role of different elemental interactions within compositionally complex RSAs. In their manuscript, Tamsin E. Whitfield et al [8] investigated the microstructures produced by the Nb-Zr miscibility gap within the compositionally simpler Ti-Nb-Zr constituent system. They studied a systematic series of alloys with compositions of Ti 5 Nb x Zr 95−x (x = 25-85 at.%) following quenching from solution heat treatment and long duration thermal exposures at 1000, 900 and 700 • C for 1000 h. During exposures at 900 • C and above, the alloys had a single bcc crystal structure.…”