B2 precipitation-strengthened refractory compositionally complex Ta-Mo-Ti-Cr-Al alloys (09/03/23)

Speaker & Affiliation:

​Prof. Martin Heilmaier
Aditya Birla Visiting Chair Professor, Department of Materials Engineering, IISc, Bengaluru
Professor, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany

When ?

09 February, 2023 (Thursday), 4.00 pm (IST)


KPA Auditorium, Department of Materials Engineering


Refractory compositionally complex alloys (RCCA) are promising candidates for high-temperature structural applications. Recently, Senkov and co-workers [1] reported a microstructure with coherent cuboidal (disordered A2) precipitates in an ordered B2 matrix yielding promising high-temperature strength in the Al-Mo-Nb-Ta-Ti-Zr system. However, the alloy lacks ductility at room temperature.

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 [2]. 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 observed in the Ta-free subsystem MoTiCrAl, Al has a significant effect on the ordering transition from A2 towards B2 [3]. However, in the case of MoTiCrAl, no multi-phase field was found after the homogenization. The situation changes by the addition of Ta, as a multi-phase field is thermodynamically predicted [4]. The microstructure of two compositions (high and low in Al) 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 found in the Al-lean alloy. The necessary reaction sequences are discussed in detail and provide information about the phase diagram 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 also discussed.


[1] O. N. Senkov, C. Woodward and D. B. Miracle, “Microstructure and Properties of Aluminum-Containing Refractory High-Entropy Alloys,” JOM, vol. 66, pp. 2030-2042, 2014.

[2] 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,” Corrosion Science, vol. 166, p. 108475, 2020.

[3] 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,” Journal of Alloys and Compounds, vol. 823, p. 153805, 2020.

[4] 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,” Acta Materialia, vol. 218, p. 117217, 2021.