Transitions in mechanical behavior and in deformation mechanisms enhance the strength and ductility of Mg-3Gd

Samples of Mg-3Gd (wt.%) were prepared by accumulative roll-bonding followed by annealing at different temperatures to produce samples with average grain sizes ranging from 3.3 mu m to 114 mu m. The samples were tensile tested at room temperature to characterize their strength and ductility, both of which were found to be significantly affected by transitions in mechanical behavior and deformation mechanisms. These transitions occurred with decreasing grain size and are described by: (i) a transition in the mechanical behavior from continuous flow to discontinuous flow associated with a yield point phenomenon, and (ii) a transition in the deformation mechanisms from (a) slip and twinning to (a) and < c +a > slips. The dislocation structures and deformation twins in the tensile samples have been characterized by transmission electron microscopy and electron backscatter diffraction, respectively. Dislocations of < a > and < c + a > type were identified based on two-beam diffraction contrast experiments. The results reveal that < a > dislocations and tension twins dominate in the samples with grain sizes larger than 10 pm, while < a > and < c +a > dislocations dominate in the samples with grain sizes smaller than 5 pm. In parallel, a consistent trend for both the strength and ductility to increase with decreasing grain size is observed. The appearance of a yield point phenomenon at small grain sizes has a significant effect on both strength and ductility, illustrated by an increase in boundary (Hall-Petch) strengthening and an increase in the total elongation to 36.6%. These results demonstrated a positive effect of a superposition of the transitions on both the strength and ductility of Mg-3Gd. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.