Elucidating the microstructural development of refractory metal high entropy superalloys via the Ti–Ta–Zr constituent system

Abstract

The recently developed Refractory Metal High Entropy Superalloys have the potential to replace Ni-based alloys in very high temperature structural applications. However, the microstructures of these new alloys typically consist of refractory metal based solid solution precipitates within an ordered superlattice structured matrix, which is likely to compromise key properties such as toughness. As such, there is significant interest in inverting this arrangement, such that superlattice precipitates form within a disordered refractory metal matrix. Yet the mechanisms by which these microstructures form and how they might be modified with compositional variations are currently unclear. To elucidate these mechanisms, the microstructural evolution of a series of compositionally simpler alloys from the Ti-Ta-Zr system have been studied following long term exposures at 700, 900 and 1000˚C. Exposures of up to 1000 hours were used as a proxy to equilibrium and the resulting microstructures were analysed using advanced scanning and transmission electron microscopy methods. The microstructures of these alloys were found to predominantly contain one or two bcc phases, the lengthscale and morphology of which changed with exposure temperature. From these results it is established that the fine-scale microstructure, which is very similar to that widely reported in the more compositionally complex refractory metal high entropy superalloys, forms via spinodal decomposition during cooling. It is also shown, for the first time, how compositional modification can lead to a refractory metal solid solution based matrix. It is believed that these results provide key insights that can guide further development in the more complex systems that will be required for commercial applications.

Microstructural Degradation of the AlMo0.5NbTa0.5TiZr Refractory Metal High-Entropy Superalloy at Elevated Temperatures

“…The enrichment in refractory metal elements and reduction in Al is likely to stabilise a bcc structure [ 21 ]. In addition, since the formation of the nanoscale microstructure is primarily driven by the miscibility gap between Zr and refractory metals, the very low Zr content of the matrix phase is unlikely to result in such a phase separation [ 10 , 22 , 23 ]. These assertions are supported by other reports of the phase chemistries and structures identified in other RSA.…”

Observation of a refractory metal matrix containing Zr-Ti-rich precipitates in a Mo0.5NbTa0.5TiZr high entropy alloy

“…In contrast, recent work investigating the simplified constituent system, Ti-Ta-Zr, has shown that a microstructure consisting of a RM-rich bcc matrix with intragranular Ti-and Zr-rich bcc particles can be produced by varying the ratio of Ta:Zr [13]. These observations suggest that the study of simplified systems may facilitate the identification of routes by which microstructural control may be achieved in more complex RMHES.…”

“…These observations suggest that the study of simplified systems may facilitate the identification of routes by which microstructural control may be achieved in more complex RMHES. It should be noted that the alloys investigated in reference [13] did not contain Al and, therefore, the ordered B2 phase was not present in any of the microstructures produced. However, the observed elemental partitioning supports the hypothesis that compositional modification may be able to facilitate microstructural inversion in more complex RMHES.…”

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

“…However, despite significant global research activity, several key challenges remain in the development of commercially viable RSAs. Among these challenges, it is generally accepted that greater control over the microstructural formation is required in order to produce materials with a ductile bcc matrix phase and stable superlattice precipitates [2,5,[7][8][9][10][11][12].…”

“…The nano-scale microstructures in RSAs are believed to form via a spinodal decomposition during cooling to produce two coherent bcc phases, one of which undergoes an ordering transition to form the B2 phase [5,8,10,13,14]. Spinodal decomposition occurs when the second derivative of the Gibbs energy curve with composition within a miscibility gap is negative.…”