Negative Strain Rate Sensitivity in Two-Phase Nanocrystalline CoCrFeMnNi High-Entropy Alloy with Broader Grain Size Distribution Studied by Nanoindentation

Two-phase microstructure was realized in CoCrFeMnNi system using a combination of ball milling and spark plasma sintering at 1273 K. Microstructure consisted of finer average grain size with broader grain size distribution, complex solid solution phases (a major face-centered cubic phase and a minor body-centered cubic phase) with very close lattice constants, dislocations and nano twins. A high hardness of 6.3 GPa and a negative strain rate sensitivity (with no associated serrated flow) of − 0.0206 were observed. Various interactions between dislocations and grain boundaries/interphase boundaries/twin boundaries, prevailing atomic level complex chemistry in the lattices, interfaces and dislocation cores might be the reasons for the observed flow characteristics. Dislocation–solute atom interactions observed in conventional dilute alloys that have resulted in negative strain rate sensitivity and dynamic strain aging may not be operating in the present alloy system under the given set of test conditions.