The effect of Al on the formation and stability of a BCC-B2 microstructure in a refractory metal high entropy superalloy system

The development of refractory metal high entropy superalloys has received great interest due to their potential for high temperature structural applications. The formation of a two phase nanoscale microstructure, comprising bcc and ordered B2 phases, is thought to be fundamentally linked to the presence of Al. As such, determining the influence of Al concentration on the microstructural formation and thermal stability of these novel materials is critical for future alloy development. To elucidate this effect, the microstructural evolution of a series of alloys with systematically varying compositions from the Ti-Ta-Zr-Alx system have been studied following homogenisation and long term exposures at 1100, 900 and 700 degrees C. Nanoscale cuboidal morphologies were observed in all alloys, but ordering was only observed when the bulk Al content was 5 at.%. These results indicated that a common formation mechanism, thought to be spinodal decomposition, occurred prior to any B2 ordering. The results also showed that the B2 phase was only stable to relatively moderate temperatures, < 900 degrees C, eliminating order hardening as a high temperature strengthening mechanism. In addition, for the alloy with the greatest Al content, deleterious intragranular Al-Zr-rich intermetallics were observed following the long duration thermal exposures.