A re-evaluation of super-high ductility mechanism in superplastic uniaxial tension

Abstract

Existing mechanical theories fall short in explaining the origin of delocalization behavior and the associated fluctuation phenomena observed in superplastic quasi-stable tension, which typically results in extraordinarily high fracture elongation. This study introduces a novel phenomenological model that accounts for the stick-slip effect arising from discontinuities in grain boundary sliding (GBS) during superplastic deformation. Experimental observations from uniaxial tension tests on a Zn-5Al superplastic alloy at 230 °C validate the accuracy of the simulations, which demonstrate static-kinetic coordinated deformation during each serrated fluctuation. Additionally, the simulations indicate that the stick-slip effect triggers dynamic delocalization behavior in superplastic tension, elucidating the nucleation and growth of additional micro-necks. Although this dynamic delocalization disrupts the initially stable flow, it significantly mitigates the development of macro-necks during unstable plastic flow, potentially leading to quasi-stable tensile deformation devoid of prominent macro-necks as the stick-slip effect intensifies. Notably, this study identifies for the first time the phenomenon of stress reduction during superplastic tension with jerky flow, attributable to an oscillatory stress induced by the GBS stick-slip effect superimposed on a static stress backdrop. The combined effects of GBS-induced stick-slip behavior and high strain rate sensitivity contribute to the extraordinarily large elongations observed in superplastic tension. Overall, the phenomenological model developed in this work not only clarifies the mechanical reasons behind specific phenomena in superplastic uniaxial tension, often overlooked by conventional mechanical analyses, but also proposes a new approach to enhancing superplastic deformation.