Stress corrosion cracking of a superplastic and nonsuperplastic Zn-22.3Al alloy in 3% NaCl solution

Through appropriate heat treatments, a Zn-22.3wt%Al(Zn-22.3Al) alloy can be prepared in both superplastic and nonsuperplastic specimens. It has been f ound that the superplastic Zn-22.3Al alloy possesses a very fine microduple x structure, while the nonsuperplastic alloy has a lamellar duplex structur e with locally coarsened second phases. The very different microstructures of both specimens result in different corrosion and stress corrosion cracki ng (SCC) behaviors in 3% NaCl solution. In addition, the fractographs of bo th the superplastic and nonsuperplastic Zn-22.3Al specimens after SCC tests under various anodic applied potentials have been compared. Through the ob servations, a mechanism for the SCC in this case was proposed to show that the cracks proceeded with successive processes of oxide film rupture and Zn -Al matrix tearing. Such a mechanism is more evident for the fractography o f nonsuperplastic specimens, on which a series of parallel strips inserted with dimple-bands can be obviously found.