Significance of Si impurities on exceptional room-temperature superplasticity in a high-purity Zn-22%Al alloy

Recent numerous studies demonstrated the advantages of producing bulk metals with submicrometer grain sizes which provide the opportunity to demonstrate improved mechanical characteristics including superplastic properties. Besides the effort, although the impurity may cause low ductility due to grain boundary segregation, there are limited studies to date on the influence of general impurities upon flow behavior of conventional superplastic materials. Accordingly, the present report demonstrates the significance of Si impurity on superplastic properties in an ultrafine-grained high-purity Zn-22%Al eutectoid alloy at room temperature. The alloy was prepared to include different levels of Si contents up to 1500 ppm in the high-purity alloy and the consistent fine grain sizes of ~0.60 µm were introduced through a series of solutionizing followed by cold rolling. Tensile testing showed an occurrence of excellent room-temperature superplasticity and the maximum elongation of 500% was recorded at an optimal superplastic strain rate of 1.0×10−3 s−1 in the alloy with less Si. Increasing Si contents reduced ductility without changing the strain rate sensitivity, thereby implying the consistency in the deformation mechanism for superplastic flow but the difference in the fracture mode. The present analysis estimates a threshold stress and demonstrate the validity of applying the conventional superplastic relationship for depicting the room-temperature superplastic flow in the high-purity Zn-22%Al alloy. Moreover, the separate fracture modes are proposed for the alloy with increasing Si impurity contents by taking fractographs after superplastic elongations.