Hall-Petch Breakdown in Fine-Grained Pure Magnesium at Low Strain Rates

The effects of grain size and strain rate on tension behavior at ambient temperature were investigated for several extruded magnesium with an average grain size in the range between 1 and 20 mu m. In quasi-static strain rate regimes (1 x 10(-2) to 10(-4) s(-1)), the activation volume was similar to 20b (3) (b is the Burgers vector), which suggested that the major contribution of deformation was cross-slip and/or multiple slips. In contrast, in low strain rate regimes (< 1 x 10(-4) s(-1)), the ductility increased with grain refinement, and the maximum elongation-to-failure was 230 pct for a grain size of 1.2 mu m and a strain rate of 1 x 10(-5) s(-1); such ductility was never observed in magnesium at room temperature. In addition, the activation volume was also reduced to similar to 5b (3). Observations of the deformed surface revealed plentiful traces of grain boundary sliding. This mechanism played an important role in deformation. As a result, while the yield strength was aligned on the Hall-Petch relation in quasi-static strain rate regimes, an inverse Hall-Petch effect was observed in low strain rate regimes.