Microstructures with extraordinary dynamic work hardening and strain rate sensitivity in Al0.3CoCrFeNi high entropy alloy

Al0.3CoCrFeNi is an FCC-based high entropy alloy (HEA) that can display a wide spectrum of mechanical properties. Cold work and precipitation are good strengthening mechanisms in this HEA, and their effect on dynamic work hardening and strain rate sensitivity (SRS) was investigated. Dynamic deformation testing using split-Hopkinson pressure bar (SHPB) at 2 x 10(3) s(-1) and quasistatic tensile deformation at 10(-3) s(-1) were conducted on thermomechanically processed conditions. The HEA behaved like a conventional FCC alloy with higher flow tresses and work hardening in the dynamic regime. All the conditions showed substantial dynamic work hardening due to the low stacking fault energy (SFE) of the HEA. Precipitates enhanced twinning, as observed from post-SHPB deformed microstructures, and resulted in an extraordinary work hardening rate > 2200 MPa. Cold work introduced large-scale deformation twins and these suppressed further twin nucleation during testing, and a lower work hardening of 1000 MPa was observed. SRS was highly microstructure sensitive and varied from an exceptional m = 0.063 in the cast condition, where the biggest flow stress contributor is solute strengthening of short-range nature and can be thermally activated. Introduction of long-range athermal incoherent precipitates reduced SRS slightly. Cold work, which introduces much higher density of athermal defects, dislocations and large-scale deformation twins, drastically reduced SRS to m = 0.006. Upon annealing the rolled material, partial recovery of cold work and precipitation occurred together and coupled to result in a moderate SRS, m = 0.02.