High Strain Rate Response of Al0.7CoCrFeNi High Entropy Alloy: Dynamic Strength Over 2 GPa from Thermomechanical Processing and Hierarchical Microstructure

Dynamic mechanical behavior of a dual-phase, precipitation-hardenable, near-eutectic, hierarchical high entropy alloy Al 0.7 CoCrFeNi was investigated using split-Hopkinson pressure bar. Cast alloy was processed via cold rolling to 30% reduction and annealing at two different temperatures, 1100 °C/5 min and 580 °C/24 h. The microstructure consists of a fine FCC + B2 lamellar structure which looked similar for all three conditions under scanning electron microscope but demonstrated very disparate flow stresses. X-ray diffraction measurements were used to analyze stored dislocation density, i.e. micro-strain in the individual phases i.e. FCC and B2 , induced by the cold work and the extent of its recovery after each heat treatment. Transmission electron microscopy was used to characterize the naano-scale features in all the three heat treatment conditions. Low temperature annealing at 580 °C/24 h, with limited recovery of cold work coupled with precipitation of L1 2 in FCC phase and an adjacent B2 / BCC lamellar phase, was discovered to result in exceptionally high dynamic strength of 2150 MPa.