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

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 I2 and I1 are both detected. The I2 fault results from dissociation of a basal 〈a〉 dislocation into two Shockley partial dislocations and exists in both matrix and {101¯2} tension twins. Most of the I1 faults are from dissociation of 〈c + a〉 dislocations, but a small fraction has a structure similar to that of growth I1, and they might result from condensation of vacancies, interstitial precipitation, or coalescence of 〈c + a〉 ribbons. The I1 faults are distributed densely in {101¯2} 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 I1 faults. Three cases are categorized in terms of the number and the sign of the Shockley partials located in each single I1 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 I1 but not I2, 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 I1 faults. The mechanism in which basal dislocations react with the bounding Frank partial of I1 is more likely to operate, and this mechanism is supported by molecular dynamics simulations.