Characterization and Modification of Microstructure in Al-Co-Cr-Fe-Ni Eutectic High-Entropy Alloys

Licentiate thesis, 2019

The recently emerging high-entropy alloys (HEAs) present a novel alloying strategy, significantly expanding the scope of metal alloy design. HEAs are defined as alloys consisting of at least four principal elements in equi- or near equiatomic ratios, with a possibility of small amounts of alloying elements. For the past 15 years, most researchers focused on HEA compositions forming simple FCC or BCC solid solutions. However, such compositions have been found to suffer from a trade-off of strength and ductility. Many of the FCC HEAs can provide high ductility at room and elevated temperatures, however, they are unable to withstand high loads and their tensile strength is unimpressive. On the other hand, BCC HEAs are usually characterized by high strength, with unfortunately low ductility. Therefore, for HEAs to be truly considered as potential structural materials, alloys with a good balance of strength and ductility must be developed.

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 improve mechanical properties. The first such alloy, AlCoCrFeNi2.1, with an FCC+BCC lamellar microstructure remains as the most studied of EHEAs. Despite much work put into its characterization, much remains to be understood.

The first part of this work focuses on the as-cast microstructures of the eutectic and near-eutectic compositions of the AlCoCrFeNix 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.

The second part of this work studies the recrystallization process in the cold-rolled AlCoCrFeNi2.1 alloy. Development of the microstructure at various stages of recrystallization and the associated changes in mechanical properties are studied. Distinctive behavior of the two constituent phases during recrystallization, with varying rates of recrystallization and grain growth is observed. These findings show there is a possibility to further improve the EHEAs via careful thermo-mechanical processing.