B2 Precipitation-Strengthened Refractory Compositionally Complex Ta-Mo-Ti-Cr-Al alloys

Refractory compositionally complex alloys (RCCA) are promising candidates for high-temperature structural applications [1]. Many of the reported alloys consist of A2 or B2 phases with additional intermetallic phases often located at 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<br/>attributed to an ordered B2 matrix phase with disordered A2 precipitates.<br/>Following along above conceptual path we report here on the current status of our investigations within the Ta-Mo-Ti-Cr-Al system which exhibits a promising combination of strength and oxidation resistance at elevated temperatures [3]. The objective is to attain a suitable multi-phase microstructure of A2 matrix and B2 precipitates. Thermodynamic calculations were employed with an in-house database to predict specific transformation sequences of ordering and diffusion-controlled phase separation within this system. As already observed in the Ta-free subsystem MoTiCrAl, Al has a significant effect on the ordering transition of A2 towards B2 [4]. However, in the case of the MoTiCrAl system, no multi-phase phase field has been predicted and confirmed after the homogenization. This changes by the addition of Ta, as a multi-phase field is predicted [5]. To minimize the formation of inherently brittle Laves phase, the Cr concentration was adjusted accordingly [6]. 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; in contrast, an A2 matrix with B2 precipitates was determined in the Al-lean alloy. The necessary reaction sequences are discussed in detail and provide information about the phase diagram structure necessary to reproducibly obtain the desired two-phase microstructures. Microstructural peculiarities, such as segregation at planar defects as well as their implications on the mechanical properties will be discussed.<br/><br/>References:<br/>[1] D. B. Miracle, M.-H. Tsai, O. N. Senkov, V. Soni and R. Banerjee, Refractory high entropy superalloys (RSAs), <i>Scripta Materialia, </i>vol. 187, pp. 445-452, 2020<br/>[2] O. N. Senkov, C. Woodward and D. B. Miracle, Microstructure and Properties of Aluminum-Containing Refractory High-Entropy Alloys, <i>JOM, </i>vol. 66, pp. 2030-2042, 2014<br/>[3] B. Gorr, F. Müller, S. Schellert, H.-J. Christ, H. Chen, A. Kauffmann and M. Heilmaier, A new strategy to intrinsically protect refractory metal based alloys at ultra high temperatures, <i>Corrosion Science, </i>vol. 166, p. 108475, 2020<br/>[4] S. Laube, H. Chen, A. Kauffmann, F. Müller, B. Gorr, J. Müller, B. Butz, H.-J. Christ and M. Heilmaier, Controlling crystallographic ordering in Mo–Cr–Ti–Al high entropy alloys to enhance ductility, <i>Journal of Alloys and Compounds, </i>vol. 823, p. 153805, 2020<br/>[5] S. Laube, S. Schellert, A. Tirunilai, D. Schliephake, B. Gorr, H.-J. Christ, A. Kauffmann and M. Heilmaier, Microstructure tailoring of Al-containing compositionally complex alloys by controlling the sequence of precipitation and ordering, <i>Acta Materialia, </i>vol. 218, p. 117217, 2021<br/>[6] F. Müller, B. Gorr, H.-J. Christ, H. Chen, A. Kauffmann, S. Laube und M. Heilmaier, Formation of complex intermetallic phases in novel refractory high-entropy alloys NbMoCrTiAl and TaMoCrTiAl: Thermodynamic assessment and experimental validation, <i>Journal of Alloys and Compounds, </i>vol. 842, p. 155726, 2020