The effect of grain size on the threshold stress for superplastic flow in aluminum alloys

Abstract

Superplastic behavior of dispersion strengthened aluminum alloys and discontinuously reinforced aluminum composites at high strain rates has been interpreted by incorporating the concept of threshold stress, and the threshold stress exhibits strong temperature dependency. The threshold stress data in literatures fit well with an Arrhenius-type plot, and however the origin and exact meaning of the thermally activated process have not been fully understood. In this paper, a possibility of structural dependency of the threshold stress for superplasticity has been investigated. By considering the grain size as a structural factor, it is demonstrated that the threshold stress data for many superplastic aluminum alloys with different grain sizes and alloying chemistry fall into a narrow band in a modulus compensated threshold stress vs, grain size correlation. This analysis leads to a phenomenological equation with the grain size exponent of -1.1 for the threshold stress behavior in superplastic aluminum alloys.