Cooperative effect of Cr and Al elements on passivation enhancement of eutectic high-entropy alloy AlCoCrFeNi_(2.1)with precipitates

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)sam-ples,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)precipi-tates 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 precip-itates 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 unprece-dentedly 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.