Microstructure evolution and mechanical properties of atmosphere plasma sprayed AlCoCrFeNi high-entropy alloy coatings under post-annealing

Abstract In this work, atmosphere plasma spraying (APS) was employed to fabricate thick AlCoCrFeNi high entropy alloy (HEA) coatings from commercial HEA powders. Effects of annealing treatments on the phase transformation, microstructure and mechanical properties of HEA coating was systematically investigated. Results showed that the as-sprayed HEA coating exhibited typical lamellar microstructures with some oxidation strips. The as-sprayed coating was composed of two BCC phases-Fe-Cr rich (A2) and Al-Ni rich phase (B2). The transformation from BCC to FCC structure (A1) occurred during post-annealing, which became more apparent with increasing temperature in the range of 600-900℃. The rod-shape A2 phase was embedded in the B2 matrix and merged to grow larger with increasing temperature. EPMA analysis revealed that the diffusion of Al and Cr made A2 phase of (Fe, Cr) separate from B2 phase of (Al, Ni), accompanying with the formation of A1 phase (Fe, Ni). Microhardness and nano-indentation test showed that the hardness and elastic modulus of as-annealed coatings reached the peak value at 600℃ (576 HV) and 700℃ (232.4 GPa), respectively, then decreased with further increasing temperature (800-1000℃). The mechanical performance showed close relationship with the volume fraction change of A2 and B2 phase.