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

• Published in: Journal of alloys and compounds Vol. 818; p. 152935
• Main Authors: Whitfield, T.E., Pickering, E.J., Christofidou, K.A., Jones, C.N., Stone, H.J., Jones, N.G.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 25.03.2020; Elsevier BV
• Subjects: Alloy systems; Chemical precipitation; Complex systems; Coordination compounds; Entropy; Exposure; Heat resistant alloys; High entropy alloys; High temperature; High-temperature alloys; Metallurgical constituents; Microstructure; Morphology; Nickel base alloys; Phase transitions; Precipitates; Solid solutions; Spinodal decomposition; Structured matrices; Superalloys; Superlattices; Tantalum; Thermodynamic properties; Titanium; Zirconium; High entropy alloys; High-temperature alloys; Microstructure; Thermodynamic properties; Phase transitions
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2019.152935

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 h 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. [Display omitted] •Phase equilibria of Ti–Ta–Zr alloys studied as proxy to RM high entropy superalloys.•Nanoscale phase morphology similar to RMHES produced in simplified Ti–Ta–Zr system.•Single bcc phase at high temperatures; two bcc phases at intermediate temperatures.•Nanostructures formed by spinodal decomposition due to Zr and Ta miscibility gap.•Refractory metal matrix phase can be achieved by compositional variations.