Creep behavior and microstructure stability of precipitation-strengthened A2+B2 refractory high entropy alloys

Refractory high-entropy alloys (RHEAs) strengthened by B2-ordered precipitates have emerged as promising candidates for next-generation high-temperature structural materials. However, their creep behavior and microstructural stability remain insufficiently investigated and understood. Systematic studies directly linking creep deformation to microstructural evolution in precipitation-strengthened A2+B2 RHEAs are still scarce. The present thesis addresses this gap through the first comprehensive experimental investigation of the polycrystalline alloy 27.3Ta-27.3Mo-27.3Ti-8Cr-10Al (at.%).
Despite its polycrystalline microstructure and non-close-packed crystal structure, the alloy exhibits minimum creep rates comparable to those of the state-of-the-art, single-crystal Ni-based superalloy CMSX-4 in the temperature range beyond 1000 °C and at stresses between 75 and 125 MPa. Notably, this performance is achieved very close to the precipitate solvus temperature $T_{\mathrm{solv}}$ (≈0.98 $T_{\mathrm{solv}}$), in contrast to ≈0.85 $T_{\mathrm{solv}}$ for CMSX-4. Compared to single-phase A2 and B2 RHEAs, the creep rate is reduced by three to four orders of magnitude. ... mehrDetailed post-creep microstructural analyses reveal, for the first time in an A2+B2 RHEA, rafting perpendicular to the loading axis in grains with ⟨100⟩ orientation close to the compression axis. Multi-side imaging of the raft microstructure reveals laterally continuous, separated plate-like structures oriented perpendicular to the loading axis. Furthermore, the following elementary creep deformation processes are observed at 1030 °C and stresses between 100 and 150 MPa: dislocation climb at low stresses and cross-slip of screw dislocations at high stresses. In the intermediate stress region, climb-controlled deformation, jogged screw dislocations, and the cutting of B2 precipitates by strongly coupled dislocation pairs are locally observed. Fully coherent A2/B2 interfaces are retained under all investigated creep conditions.
Finally, the effects of Re and Ru additions on phase stability, transformation behavior, and elemental partitioning are systematically examined with respect to their effects on solvus temperature. Both elements significantly increase the precipitate solvus temperature, thereby extending the thermal stability of the strengthening phase. Distinct partitioning characteristics of Re and Ru are identified, providing a mechanistic basis for optimizing alloy composition. Overall, this work establishes a coherent experimental framework for understanding creep deformation and microstructural stability in A2+B2 RHEAs and defines clear thermodynamic and microstructural targets for the development of creep-resistant, precipitation-strengthened, refractory alloys.