Tailoring A2/B2 Microstructures of Refractory Compositionally Complex Alloys (RCCA) for Assessing Creep Properties

Refractory compositionally complex alloys (RCCA) are promising candidates for high-temperature structural applications. 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 plastic deformability at room temperature, the proper formation of a strengthening phase is crucial. 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 [1]. The objective is to attain a suitable multi-phase microstructure of A2 matrix and B2 precipitates without significant grain boundary decoration. Thermodynamic calculations were employed to predict specific transformation sequences of ordering and diffusion-controlled phase separation within this system. These predictions were experimentally verified. The microstructure of alloys with high Al concentration exhibited a B2 matrix with A2 precipitates; in contrast, an A2 matrix with B2 precipitates was determined in Al-lean alloys [2]. Additionally, it maintains a stable ultrafine particle microstructure even after prolonged exposure to elevated temperatures (1000°C) [3]. The phase separation into an A2+B2 two-phase microstructure in RCCA has been speculated as being spinodal in nature with continuous chemical distribution during the separation. However, phase separation in the system Ta-Mo-Ti-Cr-Al, occurs via interface motion-controlled precipitation as verified by atom probe tomography and electron microscopy techniques. Thus, the requirements for controlled strengthening by superalloy-like microstructures are verified in a certain compositional range, e.g. in 82(Ta-Mo-Ti)-8Cr-10Al (in at.%). We therefore present the current status of investigations into the creep behavior of this B2 precipitation strengthened A2 alloy: Compression creep tests were conducted at elevated temperatures close to the solvus temperature and above, with varying constant true stresses to unveil the creep deformation behavior and underlying mechanisms. Subsequently, SEM/TEM were employed to examine the deformed microstructures at different creep strains. The discussion will encompass the impact of the coherent interface between the matrix and precipitates.<br/>[1] B. Gorr, F. Müller, S. Schellert, H.-J. Christ, H. Chen, A. Kauffmann and M. Heilmaier, Corrosion Science, 166 (2020), 108475<br/>[2] S. Laube, S. Schellert, A.S. Tirunilai, D. Schliephake, B. Gorr, H.-J. Christ, A. Kauffmann and M. Heilmaier, Acta Materialia, 218 (2021), 117217<br/>[3] S. Laube, A. Kauffmann, S. Schellert, S. Seils, A.S. Tirunilai, C. Greiner, Y. M. Eggeler, B. Gorr, H.-J. Christ and M. Heilmaier, Sci. Techn. Adv. Mater., 23 (2022), 692