Abstract
High-entropy alloys (HEAs) offer exceptional tunability in microstructure and properties through thermal processing. This study investigates the phase evolution, mechanical properties, and corrosion behaviour of equiatomic AlCoCrFeNi HEA under varying annealing conditions. X-ray diffraction (XRD) and scanning electron microscopy (SEM) reveal a homogeneous body-centred cubic (BCC) structure with B2 ordering in the as-melted alloy. Annealing at 800 °C induces multiphase segregation (BCC1 + BCC2 + face-centred cubic (FCC) + sigma (σ)), while annealing at 1000 °C eliminates the σ phase, stabilizing a BCC1 + BCC2 + FCC microstructure. Subsequent aging of the 1000 °C-annealed sample at 800 °C increases the FCC fraction while suppressing σ-phase precipitation, indicating enhanced phase stability. Nanoindentation measurements demonstrate high strain rate sensitivity (m = 0.1 ± 0.03) in the as-melted and 1000 °C-annealed states, promoting damage tolerance. The 1000 °C-annealed alloy exhibits an optimal balance of hardness (8.14 ± 1.66 GPa) and elastic modulus (268.4 ± 48.5 GPa). Electrochemical testing in 3.5 wt% NaCl solution reveals enhanced passivation behaviour in heat-treated samples, with the lowest corrosion current (0.1 and 0.2 nA/cm²) and highest pitting potential (126 and 143 mV) in the 1000 °C annealed and aged conditions. However, scanning Kelvin probe (SKP) and atomic force microscopy (AFM) reveal increased electrochemical heterogeneity and surface roughness in annealed samples, indicating latent susceptibility to localized attack. Immersion testing in 6 wt% FeCl₃ confirms this, with corrosion rates increasing from 4.49 mm/year in the as-melted state to 17.62 mm/year in the 800 °C annealed sample due to microgalvanic effects driven by phase segregation and topographical disparities.
