A Unique High-Temperature Deformation Mechanism in a CrMnFeCoNi Alloy

The Harmonic Structure (HS) design was implemented in a high-entropy CrMnFeCoNi alloy compact to study its deformation characteristics at elevated temperatures, with particular emphasis on comparison with the homogeneous (Homo) compacts. The HS compact was prepared by powder metallurgy, employing a mechanical grinding process with a planetary ball mill in an argon atmosphere. The rotational speed was set at 150 rpm, and the milling time was either 180 or 360 ks. The resulting powders were then exposed to spark plasma sintering at 1223 K for 1.8 ks under 50 MPa. Subsequently, the compacts were subjected to high-temperature compression tests at 1073 K or 1173 K, at varying initial strain rates over a range of temperatures. These tests were conducted after the sintering process was completed. Homo exhibited a work hardening at the initial stage of deformation, followed by a slight decrease in flow stress, which then remained nearly constant. In contrast, HS exhibited a distinctive softening in flow stress following initial work hardening. A thorough examination of the microstructure during the softening process revealed that adjacent Shell/Core units caused grain boundary sliding in the Shell region. Furthermore, each Core exhibited a rotation of approximately 2.3 degrees and a lateral displacement of 1.5 μm. Observation of the softening phenomenon during high-temperature deformation was confirmed through TEM analysis, revealing that this softening resulted from dynamic recrystallization within the Shell region. Consequently, dynamic recrystallization in the Shell was postulated, followed by rotation of the Shell-Core unit through grain boundary sliding of the UFG structure.