Effect of Al content on microstructure and mechanical properties of atmosphere plasma sprayed AlxCoCrFeNi high-entropy alloy coatings under post-annealing

In this work, AlxCoCrFeNi (x = 0.2, 0.5, 1.0) high-entropy alloy (HEA) coatings were fabricated by atmosphere plasma spraying (APS) and then annealed at 600, 800, 1000 °C under Ar atmosphere. The as-sprayed HEAs exhibited lamellar microstructure of alloy phase with oxide strips. The Al content showed a positive and negative correlation with the formation of BCC (A2/B2) and FCC (A1) in the coatings, respectively. A symbiotic relationship between FCC and oxides was first proposed. During annealing, the transformation from mixed BCC to A2/B2 in HEA coatings was dominated by Al content, thus leading to unique microstructure at different temperatures. The hardness of as-annealed HEAs was determined by the morphology and distribution of Al-rich BCC phase. The HEA coating with higher Al content (x = 1.0) could keep high hardness under elevated annealing temperatures. While the coating with lower Al content (x = 0.2) showed stability both in microstructure and mechanical properties.