Fracture behavior of high-entropy alloys: Resistance to fracture from strain hardening and softening

Atomic structure and electronic state influence deformation mechanisms in traditional and high-entropy alloys (HEAs). In HEAs, nature and scale differ from those of traditional alloys due to lattice distortion and variations in local chemistry resulting from large concentrations of multiple principal elements. In CrCoNi, a face-centered cubic (fcc) HEA, dislocation dissociation, nanotwinning, and transformation-induced plasticity are promoted at cryogenic temperatures (<77 K). In Nb₄₅Ta₂₅Ti₁₅Hf₁₅, a body-centered cubic (bcc) HEA, screw dislocations, twinning, and kink band formation are activated at temperatures ranging from 77 to 1,473 K. These deformation mechanisms impart exceptionally high fracture resistance in CrCoNi and Nb₄₅Ta₂₅Ti₁₅Hf₁₅. However, their tensile stress-strain curves differ significantly at these temperatures; while CrCoNi exhibits extensive strain hardening, Nb₄₅Ta₂₅Ti₁₅Hf₁₅ demonstrates nearly elastic, perfectly plastic stress-strain behavior. Understanding the origin of the high fracture resistance of these alloys, despite their contrasting stress-strain behavior, would enable the discovery of HEAs suitable for applications in extreme environments.