Unexpected partial dislocations within stacking faults in a cold deformed Mg-Bi alloy

C He and Y Zhang and CQ Liu and Y Yue and HW Chen and JF Nie, ACTA MATERIALIA, 188, 328-343 (2020).

DOI: 10.1016/j.actamat.2020.02.010

Stacking faults and the partial dislocations associated with them in a cold-rolled Mg-3wt%Bi alloy have been systematically characterized using transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy. Intrinsic stacking faults I-2 and I-1 are both detected. The I-2 fault results from dissociation of a basal (a) dislocation into two Shockley partial dislocations and exists in both matrix and 10 (1) over bar2 tension twins. Most of the I(1 )faults are from dissociation of (c + a) dislocations, but a small fraction has a structure similar to that of growth I-1, and they might result from condensation of vacancies, interstitial precipitation, or coalescence of (c + a) ribbons. The I-1 faults are distributed densely in 10 (1) over bar2 tension twins but sparsely in the magnesium matrix. They are either bounded by Frank partials or linked to twin boundaries. Unexpectedly, Shockley partials are often found lying at the edge of steps within the I-1 faults. Three cases are categorized in terms of the number and the sign of the Shockley partials located in each single I-1 fault: one Shockley partial, two Shockley partials having the same sign, and two Shockley partials having opposite signs. The fault bounded by two Shockley partials is explicitly I-1 but not I-2, which is different from the dissociation of the basal (a) dislocation. Based on the geometric analysis, two possible mechanisms are proposed to explain the origin of Shockley partials formed in the I-1 faults. The mechanism in which basal dislocations react with the bounding Frank partial of I-1 is more likely to operate, and this mechanism is supported by molecular dynamics simulations. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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