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.
Refractory metal high entropy superalloys (RMHES) offer potentially superior strength at elevated temperatures and lower densities than Ni-based superalloys. However, concerns exist over their ductility as their microstructures comprise fine distributions of refractory metal solid solution precipitates within a Zr-and Ti-rich ordered matrix. Consequently, identifying methodologies to invert this arrangement is critical. Here, we show that removal of Al from the AlMo 0.5 NbTa 0.5 TiZr RMHES, enables a microstructure to be obtained comprising Zr-Ti-rich disordered precipitates within a refractory metal matrix. This observation represents a significant development for the field and may help guide future alloy design.
Refractory metal high-entropy superalloys (RSA), which possess a nanoscale microstructure of B2 and bcc phases, have been developed to offer high temperature capabilities beyond conventional Ni-based alloys. Despite showing a number of excellent attributes, to date there has been little consideration of their microstructural stability, which is an essential feature of any material employed in high temperature service. Here, the stability of the exemplar RSA AlMo0.5NbTa0.5TiZr is studied following 1000 h exposures at 1200, 1000 and 800 °C. Crucially, the initial nanoscale cuboidal B2 + bcc microstructure was found to be unstable following the thermal exposures. Extensive intragranular precipitation of a hexagonal Al-Zr-rich intermetallic occurred at all temperatures and, where present, the bcc and B2 phases had coarsened and changed morphology. This microstructural evolution will concomitantly change both the mechanical and environmental properties and is likely to be detrimental to the in-service performance of the alloy.
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