Refractory complex concentrated alloys (RCCA) are promising candidates for high-temperature structural applications [1]. Many of the reported alloys consist of solidifying A2 or B2 phases with additional complex (Laves) phases at the grain boundaries. However, to achieve good mechanical performance at elevated temperatures as well as sufficient ductility at room temperature, the proper formation of a strengthening phase is crucial. Recently, Senkov and co-workers [2] reported a microstructure with coherent cuboidal precipitates and promising high-temperature strength in the Al-Mo-Nb-Ta-Ti-Zr system. However, the alloy lacks ductility at room temperature, which could be attributed to an ordered B2 matrix phase with disordered A2 precipitates. This microstructure can be inverted by an appropriate heat-treatment; however, this limits the working temperature to approximately 600 °C [3]. Therefore, the composition has to be carefully designed, in order to favour the A2 crystal structure as the matrix over the whole temperature region of interest.
We report here on the current status of our investigations in the Ta-Mo-Ti-Cr-Al system. Thermodynamic calculations were employed with an in-house database, to further strengthen the understanding of the phase formation and phase transitions within this system. To minimize the formation of inherently brittle Laves phase, the Cr concentration was adjusted [4]. The calculations predicted a significant effect of varying Al concentration on phase transition and, correspondingly, on the room temperature phase composition and distribution [5]. The microstructure of two compositions (high and low in Al, respectively) were investigated experimentally by scanning and transmission electron microscopy, while the phase transitions were determined by means of differential scanning calorimetry.  The microstructure of the alloy with high Al concentration exhibited a B2 matrix with A2 precipitates; however, an A2 matrix with B2 precipitates was determined in the Al-lean alloy. Microstructural peculiarities, such as segregation at planar defects as well as their implications on the mechanical properties will be discussed.
[1] B. Gorr et al., “A new strategy to intrinsically protect refractory metal based alloys at ultra high temperatures,” Corrosion Science, vol. 166, p. 108475, 2020.
[2] O.N. Senkov et al., “Microstructure and Properties of Aluminum-Containing Refractory High-Entropy Alloys,” JOM, vol. 66, pp. 2030 – 2042, 2014.
[3] V. Soni et al., “Microstructural Design for Improving Ductility of An Initially Brittle Refractory High Entropy Alloy,” Scientific Reports, vol. 8, p. 8816, 2018.
[4] F. Müller et al., “Formation of complex intermetallic phases in novel refractory high-entropy alloys NbMoCrTiAl and TaMoCrTiAl: Thermodynamic assessment and experimental validation,” Journal of Alloys and Compounds, vol. 842, p. 155726, 2020.
[5] S. Laube et al., “Microstructure tailoring of Al-containing compositionally complex alloys by controlling the sequence of precipitation and ordering,” Acta Materialia, vol. 218, p. 117217, 2021.