Fragmentation /spheroidization of constituent phases and correlated flow softening during thermomechanical processing of AlCoCrFeNi2.1 eutectic high entropy alloy

The present study explores the morphological evolutions of constituent phases during the hot deformation process of a promising as-cast AlCoCrFeNi 2.1 eutectic high-entropy alloy. Through which, a quasi-in-situ approach was employed using the Poliak-Jonas technique to carefully select interruption strains for conducting hot compression tests at 1000 °C. This approach allowed for tracking the sequence of microstructural evolutions. Furthermore, to understand the dominancy of shearing or thermally activated fragmentation mechanisms responsible for these evolutions, hot compression tests were conducted at different strain rates. Encountering an unexpected sharp softening behavior in the hot compression flow curve, its underlying micro-mechanisms were investigated using electron backscatter diffraction (EBSD) characterization and scanning electron microscopy (SEM) micrographs combined with detailed image processing and numerical calculations. It is shown that lamellar B2 eutectic regions underwent significant spheroidization with an increase in strain rate (from 12.6% ± 3.3%–64.5% ± 9%), while dendrites fragmentation was more pronounced in lower strain rates. These findings indicate that dynamic spheroidization and dendritic fragmentation mechanisms predominantly contribute to the aforementioned softening behavior observed in the alloy at high temperature.