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 (e.g., Laves phase) preferentially 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. We report 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. The objective is to achieve a suitable multiphase microstructure of A2 matrix and B2 precipitates without significant grain boundary decoration. Thermodynamic calculations were used to predict specific transformation sequences of ordering and diffusion-controlled phase separation within this system. Systematically selected compositions were synthesized from bulk material by repetitive arc melting. The predicted reaction sequences were experimentally verified by means of differential scanning calorimetry (DSC) supported by post-mortem electron microscopy. The phase separation into an A2+B2 two-phase microstructure in RCCA has been speculated to be spinodal in nature with continuous chemical distribution during the separation. However, these reactions may instead occur as precipitation by nucleation and growth.
In order to unambiguously elucidate the distinct nature of the phase separation sequence in RCCA from the Ta-Mo-Ti-Cr-Al system, atom probe tomography (APT) and electron microscopy techniques (SEM and TEM) were applied to samples that were annealed over several orders of magnitude in time. In fact, the phase separation occurs via interfacial motion-controlled precipitation rather than spinodal decomposition. Thus, the requirements for controlled strengthening by superalloy-like microstructures are verified for the investigated alloy system.