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 pm were obtained by ex-trusion at different temperatures. The extruded samples were compressed at strain rate between 1 x 10-2 s-1 and 1 x 10-6 s-1. In microstructures with grain size greater than 15 pm, the deformation mechanism was dominated by twinning; while in those with grain size smaller than 8 pm, the dominant deformation mechanism was dislocation slip. In addition, significant grain-boundary sliding was observed with de-creasing 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 de-creased from 88 pm to 1.4 pm, 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.(c) 2022 Elsevier B.V. All rights reserved.