Grain boundary engineering and its implications on corrosion behavior of equiatomic CoCrFeMnNi high entropy alloy

In the present study, strain-annealing based thermo-mechanical processing was employed to achieve a grain boundary engineered (GBE) microstructure in an equiatomic CoCrFeMnNi high entropy alloy (Cantor alloy) with a single phase, fcc structure. Cast, homogenized and recrystallized strips were cold rolled to 5%, 10% and 15% thickness reductions and annealed at temperatures from 1173 K to 1373 K for 1–6 h duration. Deformation twins were observed following cold rolling to 15%. From the deformed and annealed specimens, GBE microstructure was identified based on coincident site lattice (CSL) (Σ ≤ 29) boundary length fraction, number of twins per grain, triple junctions (TJs) character distribution and grain boundary plane orientations. Specimens rolled to 5% and annealed at 1223 K for 1 h exhibited GBE microstructure. The Σ3 fraction was enhanced from ~44% to ~62% in the GBE specimen with concurrent increments in TJs containing at least two CSL boundaries from ~20% to ~40% compared to as-recrystallized (AR) specimen. Potentiodynamic polarization studies revealed that the GBE specimen exhibited lower corrosion rate in both 0.1 M and 0.6 M NaCl solutions as compared to AR counterpart. The GBE specimen also displayed better passive film resistance due to higher polarization and charge transfer resistance, as evaluated from electrochemical impedance spectroscopy studies.