Cooperative effect of Cr and Al elements on passivation enhancement of eutectic high-entropy alloy AlCoCrFeNi2.1 with precipitates

Abstract

In conventional corrosion-resistant alloys, precipitation usually reduces corrosion resistance severely by weakening passive films locally. In this work, we found that the aging-treated AlCoCrFeNi 2.1 samples, which have abundant nanosized L 12 and body-centered cubic (BCC) precipitates in the lamellar face-centered cubic (FCC) and B2 phases, displayed better corrosion resistance than solution-treated AlCoCrFeNi 2.1 samples without precipitates. In the AlCoCrFeNi 2.1 alloy, the FCC phase with L1 2 precipitates and the B2 phase with BCC precipitates were protected by passive films enriched with Cr and Al elements, respectively. Moreover, the Al-rich passive film of the B2 phase was less stable than the Cr-rich passive film of the FCC phase, so B2 phase dissolved preferentially. The Cr-rich passive film of the FCC phase remained stable with the formation of Al-rich L1 2 precipitates inside the phase because those precipitates with the size of ∼5 nm were too small to affect the composition of the Cr-rich passive film. The formation of Cr-rich BCC precipitates within the B2 phase increased the content of the Al element inside the phase, improving the stability of Al-rich passive film on the B2 phase. Furthermore, BCC precipitates with the size of ∼30 nm were protected by Cr-rich passive film, which could inhibit the expansion of corrosion pits. Thus, the corrosion resistance of eutectic high-entropy alloy AlCoCrFeN 2.1 was unprecedentedly enhanced by the precipitation of BCC precipitates. Our study could provide an attractive strategy for designing high-entropy alloys with excellent corrosion resistance and high strength.