An inverse Hall-Petch relationship during room-temperature compression of commercially pure magnesium

The effect of grain size and strain rate on the room-temperature compression of pure magnesium was investigated. Samples with equiaxed grains and grain size ranging over 1.4–88 µm were obtained by extrusion at different temperatures. The extruded samples were compressed at strain rate between 1 × 10−2 s−1 and 1 × 10−6 s−1. In microstructures with grain size greater than 15 µm, the deformation mechanism was dominated by twinning; while in those with grain size smaller than 8 µm, the dominant deformation mechanism was dislocation slip. In addition, significant grain-boundary sliding was observed with decreasing grain size and strain rate. The Hall-Petch relationship broke down with mean grain size smaller than a critical grain size, which increased with decreased strain rate. Further, the deformation mode (compression or tension) had a negligibly small effect on the critical grain size for changes in the dominant deformation mechanisms and the Hall-Petch breakdown. The changes in deformation mechanisms with grain size were understood in terms of the strain-rate sensitivity exponent m. When the grain size decreased from 88 µm to 1.4 µm, m increased to approximately 0.3 from 0.01. The increase in m with grain refinement was consistent with enhanced grain-boundary sliding, in addition to dislocation slip and the suppression of deformation twinning. It thereby reduced the asymmetry of the tension-compression yield strength. •Inverse Hall-Petch relationship (H-P) is seen in ultrafine grain size pure Mg.•Critical grain size for break-down of H-P is independent of deformation mode.•Deformed structures show grain boundary sliding (GBS) in low-strain rates.•GBS strain contribution to overall strain increases with decrease in grain sizes.