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 Al0.7CoCrFeNi 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 L12 in FCC phase and an adjacent B2/BCC lamellar phase, was discovered to result in exceptionally high dynamic strength of 2150 MPa.