Deformation and rate controlling mechanisms in fine-grained magnesium

The effect of grain size and deformation temperature on the deformation and rate-controlling mechanisms of commercially pure magnesium (CP-Mg) has been evaluated using a combination of mechanical response analysis, microstructure and texture studies, dislocation network analysis, and crystal plasticity modelling of work hardening. The effect of grain size on mechanical properties varies between room and cryogenic temperatures. Twinning and slip are dominant deformation modes in coarse-grained Mg at 298 K. A critical grain size of approximately 3 μ m marks a transition in deformation mechanisms, characterized by reduced slip and twinning activity and the promotion of grain boundary-mediated deformation coupled with intense dynamic recovery. At 78 K and 4 K, twinning and slip remain the primary deformation modes across all grain sizes, with dislocation-twin and dislocation-dislocation interactions acting as the primary work-hardening processes. Suppression of thermal activation, which facilitates deformation at 298 K, leads to more effective dislocation storage at cryogenic temperatures. Strain rate sensitivity measurements were conducted to understand the rate-limiting mechanisms during plastic flow of CP-Mg. The cross-slip of basal 〈 a 〉 and 〈 c + a 〉 dislocations characterized by activation volume of Δ V ∗ ≈ 16 b 3 and activation distance d ≈ 0 . 25 b is identified as the rate-controlling process in fine-grained Mg at 298 K during all stages of deformation and also occurs in coarse-grained magnesium at high strains. Transmission electron microscopy (TEM) studies provide independent data on the state of the dislocation microstructure, complementing the results of crystal plasticity modelling, and establishing a link between grain size and the mechanical properties of Mg between 4 K and 298 K.