Precipitation on stacking faults in Mg-9.8wt%Sn alloy

In this work, we have systematically investigated precipitation of beta'-Mg3Sn phase on intrinsic stacking faults I-1 and I-2 in a Mg-9.8 wt%Sn alloy using aberration-corrected scanning transmission electron microscopy. All observed I-1 faults are generated by the dissociation of c + a perfect dislocations and bounded by Frank partial dislocations having a Shockley component. Precipitation of beta' on I-1 involves a shear of 1/3 <01<(1)over bar>0>(alpha), similar to its formation directly from the alpha-Mg matrix. The beta' phase often nucleates at one end of an I-1 fault due to the interaction between shear strain fields of beta' and the Shockley component of the Frank partial at that end, and subsequently grows towards the other end of the fault. When the beta' reaches to the other end, the Shockley partial bounding the lengthening end of the beta' reacts with the Frank partial bounding the fault, generating an a perfect dislocation that can glide away from the precipitate and the fault. The observed I-2 faults are generated by the dissociation of a perfect dislocations and bounded by Shockley partials. Precipitation of beta' on I-2 does not need a shear of 1/3 <01 -1 0>(alpha), since the pre-existing I-2 fault already provides an ABCA four-layer structure of beta'. Thickening of the beta' that has already formed on the I-2 involves the successive occurrence of three crystallographically equivalent shears of 1/3 < 01 -1 0>(alpha) on every second (0002)(alpha) plane of the alpha-Mg matrix. Although this thickening mechanism is similar to that of the beta' formed directly from the alpha-Mg matrix, an a perfect dislocation will be produced when the beta' is thickened to eight layers, and it can again glide away from the precipitate and the fault. (C) 2020 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.