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

In this work,we have systematically investigated precipitation ofβ’-Mg3Sn phase on intrinsic stacking faults I1 and I2 in a Mg-9.8 wt%Sn alloy using aberration-corrected scanning transmission electron microscopy.All observed I1 faults are generated by the dissociation of c+a perfect dislocations and bounded by Frank partial dislocations having a Shockley component.Precipitation ofβ’on I1 involves a shear of 1/3<0110>α,similar to its formation directly from theα-Mg matrix.Theβ’phase often nucleates at one end of an I1 fault due to the interaction between shear strain fields ofβ’and the Shockley component of the Frank partial at that end,and subsequently grows towards the other end of the fault.When theβ’reaches to the other end,the Shockley partial bounding the lengthening end of theβ’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 I2 faults are generated by the dissociation of a perfect dislocations and bounded by Shockley partials.Precipitation ofβ’on I2 does not need a shear of 1/3<01-10>α,since the pre-existing I2 fault already provides an ABCA four-layer structure ofβ’.Thickening of theβ’that has already formed on the I2 involves the successive occurrence of three crystallographically equivalent shears of 1/3<01-10>αon every second(0002)αplane of theα-Mg matrix.Although this thickening mechanism is similar to that of theβ’formed directly from theα-Mg matrix,an a perfect dislocation will be produced when theβ’is thickened to eight layers,and it can again glide away from the precipitate and the fault.