An investigation of the miscibility gap controlling phase formation in refractory metal high entropy superalloys via the Ti-Nb-Zr constituent system

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 Ti5NbxZ95-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 hours. 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.