Unleashing the microstructural evolutions during hot deformation of as-cast AlCoCrFeNi2.1 eutectic high entropy alloy

Many applications with potential use of high entropy alloys (HEAs) involve thermo-mechanical processing. The principal aim of this study is to elucidate the deformation mechanisms of the as-cast AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA) at elevated temperatures. This investigation involves considering the interplay between phase transformations affecting microstructural evolutions and restoration mechanisms, which will be correlated with flow behavior. This correlation is investigated using in-depth characterization techniques coupled with flow curve analysis. The as-solidified microstructure consists of five phases, with disordered FCC and B2 phases dominating. Thermo-mechanical processing at elevated temperatures leads to the transformation of primary L12 precipitates into B2 precipitates. At high temperatures, unexpected deformation twinning occurs as a result of stress concentration caused by the B2 precipitates and Al depletion of the FCC matrix. In addition, the B2 precipitates promote strain accommodation within the FCC dendritic regions, leading to dendrite fragmentation and subsequent dynamic recrystallization (DRX) at 800 °C through continuous dynamic recrystallization (CDRX). Additionally, by increasing the deformation temperature to 1000 °C, DRX was suppressed, while dynamic recovery (DRV) was favored. This can be attributed to the dynamic dissolution of the disordered BCC and sigma (σ) phases, along with the dynamic transformation of the primary L12 precipitates, which effectively diminished the driving force of DRX.