Local Al-Cu-Sc ternary phase transformation degrades corrosion performance of Sc-microalloyed Al-Cu alloy

Abstract

Rare earth element Scandium (Sc) has been widely used as a micro-alloying element for aluminum (Al) alloys. The resultant Al₃Sc phase is believed to help resist corrosion. However, when it comes to the Sc-microalloyed Al-Cu system, the complex phase transformation has rarely been studied, leaving its impacts on corrosion behavior controversial. In this paper, A206 and A206 microalloyed with Sc are fabricated to study the phase transformation’s effects on corrosion at peak-aged state (T6). Our results find that Al-Cu-Sc based eutectic phases form at the grain boundaries (GBs) after solidification. Then, these GB phases turn to stabler but adjacent θ-Al₂Cu, W-Al₈Cu₄Sc, and other complex phases after heat treatment. Due to the large difference in diffusion rates of Sc and Cu during heat treatment, the Al₃Sc forms and distributes inhomogeneously inside the grains but near the GBs. Cu distributes more evenly within the grains, while its θ′-Al₂Cu can preferentially precipitate at the interface of Al and Al₃Sc after aging. The abovementioned phase transformation, even with the resultant beneficial nanoscale Al₃Sc, degrades Sc-microalloyed Al-Cu’s corrosion performance, because the closely-located complex GB phases and the unevenly distributed Al₃Sc favorably nucleating interfacial θ′-Al₂Cu inside the grain can accelerate the intergranular corrosion (IGC), promoting the corrosion-induced cracks. Our methodology by multiscale experimental characterization and atomic-scale simulation validation critically evaluates the reasons behind the unpredictable corrosion performance in Al-Cu alloy with Sc by a clear phase distribution picture, instead of a mere phase composition consideration, which can enlighten better microalloying designs facing corrosive environment applications.