Order Phenomena and Mechanical Properties in Refractory High Entropy Alloys of the System Ta-Mo-Cr-Ti-Al

Refractory high entropy alloys, especially those from the Ta-Mo-Cr-Ti-Al system, exhibit a promising combination of strength and oxidation resistance at elevated temperatures [1]. However, at room temperature the equiatomic alloys possess a B2-ordered crystal structure and, therefore, lack ductility. Apart from this, significant amounts of brittle C15 Laves phase (TaCr₂) are formed at intermediate temperatures [2]. In the current presentation, investigations on ordering phenomena and the impact on mechanical properties are presented. Specifically, by systematically reducing the Al concentration in the Laves phase-free subsystem of alloys, Mo-Cr-Ti-<i>x</i>Al, a disordered, solid solution with A2-type crystal structure was found [3] which case of MoCrTi-3Al (in at.%) existed as a single-phase without any indications of an order-disorder transformation. Consequently, compression tests show plastic deformability at room temperature for this alloy as compared to equiatomic MoCrTiAl (with B2 crystal structure).<br/>However, for adequate high temperature (creep) strength, the formation of two-phase microstructures consisting of a disordered matrix with ordered precipitates deems desirable. Thermodynamic calculations were performed to identify a compositional range in which a two-phase A2+B2 microstructure may be obtained [4]. Depending on Al concentration, phase transitions were determined by differential scanning calorimetry and TEM. Different microstructures including B2 precipitates within an A2 matrix as well as A2 precipitates alongside planar faults inside a B2 matrix are discussed with respect to their corresponding effects on mechanical properties.<br/>[1] B. Gorr et al., A new strategy to intrinsically protect refractory metal based alloys at ultra high temperatures, Corr. Sci. 166 (2020), 108475<br/>[2] H. Chen et al., Crystallographic ordering in a series of Al-containing refractory high entropy alloys Ta-Nb-Mo-Cr-Ti-Al, Acta Mater. 176 (2019), 123<br/>[3] S. Laube et al., Controlling crystallographic ordering in Mo–Cr–Ti–Al high entropy alloys to enhance ductility, J. All. Comp. 823 (2020), 153805<br/>[4] S. Laube et al., Microstructure tailoring of Al-containing compositionally complex alloys by controlling the sequence of precipitation and ordering, Acta Mater. 218 (2021), 117217