Microstructure development and mechanical properties of cast and thermo-mechanically treated eutectic high-entropy alloys

Abstract: The recently emerging high-entropy alloys (HEAs) present a novel alloying strategy, significantly expanding the scope of metal alloy design. Single-phase HEAs, nevertheless, suffer from the strength-ductility trade off as seen in conventional metallic materials. A possible solution is found in the recently developed eutectic HEAs (EHEAs), which borrow the concept of using lamellar structures as in-situ composites to balance mechanical properties. The first such alloy, AlCoCrFeNi2.1, with an FCC(L12) + BCC(B2) lamellar microstructure, remains the most studied EHEA. Despite much work put into its characterization, much remains to be understood. For example, more efforts have been given to optimize the mechanical properties while less are given to quantitatively describe the microstructure. Various thermo-mechanical treatments have been used to modify the mechanical properties of the AlCoCrFeNi2.1 alloy, however, previous studies mainly focused on the fully recrystallized materials, while a clear understanding of the recrystallization process is still missing, and the potential of partial recrystallization remains to be explored. The first part of this thesis work focuses on the as-cast microstructures of the eutectic and near-eutectic compositions of the AlCoCrFeNi2.1 system. Quantification of the phase volume and lamellar spacing is performed as a function of the Ni content. Orientation relationship and misorientation angle-axis changes in the five investigated alloys are also studied, with the previously unknown dependency of misorientation angle on the Ni content revealed. Some attention is also given to irregular microstructures in eutectic and near-eutectic compositions, which have not been discussed in previous studies. In the second part of this thesis work, a systematic study of the recrystallization process and the correlation between microstructure and mechanical properties in the thermo-mechanically treated AlCoCrFeNi2.1 alloy is undertaken. Distinctive behavior of the constituent phases during recrystallization, with varying rates of recrystallization and grain growth are observed, providing new insights to the recrystallization process in this alloy. Furthermore, it is shown that by controlling the annealing temperature and time, hetero-deformation induced hardening could lead to abnormal hardening in the as-rolled alloy, providing a new strategy to achieve high-strength with acceptable ductility in EHEAs.