Connections between the basal I1 "growth" fault and ⟨c plus a⟩ dislocations

Recently published experimental results have suggested a connection between the I1 basal stacking fault and the non-basal (1123) (i.e. < c + a >) dislocation, particularly in Mg-Y and Mg-Y Zn alloys. Deformed Mg-Y alloys contain more < c + a > dislocations and I1 faults than pure Mg, and the I1 fault energy has been shown to be significantly reduced by the addition of Y solute atoms using ab initio modeling. The TEM evidence of a possible connection between the two crystal defects is reanalyzed to reveal: (i) a non-planar dissociation of the pyramidal < c + a > edge dislocation into the basal plane, resulting in an I1 fault bounded by (2023) partial dislocations; and/or (ii) a new source mechanism involving nucleation of the pyramidal < c + a > dislocation from a pre-existing I1 fault. The former reaction is energetically favorable, but non-conservative. The latter is shown to be energetically conceivable for a wide range of fault geometries. The concepts hypothesized in this paper provide explanations for the frequent TEM observation of rectilinear, edge < c + a > dislocations in Mg alloys, as well as a possible explanation for the yield strength anomaly observed in Mg and Mg alloy single crystals. Implications for radiation damage in hexagonal close packed metals are also suggested. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.