It has been suggested that the decrease in strain rate sensitivity of flow stress, m, with a decreasing strain rate at low strain rates (Region I) in fine-grained superplastic materials results from a threshold stress for deformation. Despite the fact that the existence of the threshold stress is very well proved experimentally, the origin and nature of the threshold stress are still unexplained; the threshold stress for superplastic flow has been the source of great debate and has remained mainly as an adjusting factor for obtaining a stress exponent of 2. The grain size and temperature are sometimes recognized to be variables for threshold stress. Very limited data are available for the grain-size dependence of threshold stress. On the other hand, many experiments revealed that the threshold stress strongly decreases with increasing temperature. However, it is noted that the experimental data of the threshold stress were obtained in a rather narrow temperature interval, because the temperature at which superplasticity is normally observed lies in the relatively narrow range. To understand the origin of the threshold stress, it is, therefore, necessary to investigate both (i) the variation in the threshold stress with grain size and (ii) the variation in the threshold stress over a wide range of temperatures. Magnesium alloys are suitable for analyzing the threshold stress for superplastic flow compared with, e.g., aluminum alloys, because relatively fine-grained magnesium alloys exhibit superplasticity over a wide range of temperatures. In this study, the influence of the grain size and temperature on threshold stress for superplastic flow was examined using fine-grained Mg-Zn-Zr alloy and dilute Mg-Y alloy. The threshold stress was found to decrease with an increasing deformation temperature for both alloys. The threshold stress observed in Mg-Zn-Zr alloy was proportional to the reciprocal of the square root of the grain size at 473K, whereas it was independent of the grain size at 598K. The origin of the threshold stress was discussed through the interpretation of parametric dependencies.
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