Atomic-resolution Electron Microscopy Studies On Reactions Between Dislocations And Twin Boundaries In A Mg Alloy

Hexagonal-close-packed(HCP)magnesium(Mg)alloys usually exhibit poor ductility.Slip and twinning are the two important plastic deformation modes in Mg alloys.Basal(a)slip is activated firstly in Mg alloys,due to its lowest critical resolved shear stress required,but it can only supply two independent slip systems.<c+a>slip not only can supply five independent slip systems,but also can accommodate strain along the c-axis.However,<c+a>slip usually occurs at the later stage of plastic deformation at relatively high flow stresses for Mg alloys.In addition,twinning can accommodate deformation along the c-axis,and reorientate basal planes to promote basal slip in the twins.But,twin boundaries(TBs)can serve as barriers to the motion of slip dislocations,which may lead to local stress concentration and microcrack nucleation.In-depth understanding of the mechanisms of dislocation slip,deformation twinning,dislocation-TB reactions and evolutions of TBs,as well as plastic relaxation mechanisms at TBs in Mg alloys,should have implications in improvement their mechanical properties.In the present work,crystal defects,such as dislocations,grain boundaries(GBs)and TBs,produced in a Mg-9.0Gd-3.0Y(wt.%)alloy by multidirectional impact forging were characterized and analyzed in details using both atomic-resolution aberration-corrected scanning transmission electron microscopy and the theory of interfacial defects.We have investigated reactions of slip dislocations with TBs or symmetric tilt GBs(STGBs)near twin orientations,evolutions of fine structures of TBs and GBs,as well as their effects on the plastic deformation and mechanical properties of Mg alloys,based on atomic-resolution experimental results.Reactions of isolated basal(a60)dislocations with {1012} TBs created steps with residual dislocations at TBs and multiple twinning dislocations(TDs),while only one TD was produced from the reaction between a {1011} TB and one basal<a60>dislocation.Glide of TDs created by dislocation-TB reatcions drove migration of TBs.A step with a b±1/±1-type residual dislocation could be generated by either reactions of a basal<a60>dislocation from one side of a {1011} TB with another basal<a60>or<c+as>dislocation from the other side,or through dissociation of an incoming basal<a60>or<c+as>dislocations at a {1011} TB.Reactions of low-angle GBs(LAGBs)composed of basal<a60)dislocations with {1012} and{1011} TBs generated either basal-prismatic/prismatic-basal(BP/PB)interfaces or asymmetric tilt GBs with abundant facets/steps.BP/PB interfaces themselves or their reactions with basal<a60>dislocations could emit TDs,activating the migration of BP/PB interfaces and TBs.Loss of TB coherency due to dislocation-TB reactions may thus facilitate the migration of {1012} TBs,rather than impede their migration during plastic deformation of Mg alloys,which may retard premature nucleation and propagation of microcracks at deformation TBs.A step with a b±1/±1-type residual dislocation may develop into one with a b±p/±p-type dislocation by absoption or emission of TDs.Steps with b±p/±q-type or btp/±p-type interfacial dislocations could absorb or emit basal<a60>,<c+a60>or(2023)/6 Frank partial dislocations,producing steps with b±pSF/±q-type or b±pSF/±p-type interfacial dislocations and three-layer basal stacking faults anchored at TB steps.These reactions can relax lattice strain at the steps,which may facilitate TB migration,and play an important role in improving the ductility of Mg alloys.Reactions between TBs and two LAGBs composed of basal(a60)dislocations coming respectively from the twin and matrix could produce STGBs near twin orientations with b±3/±2-type,b-1/1-type or b-1/1-type dislocations.The Burgers vectors of b-1/1-type or b-1/1-type dislocations are perpendicular to the STGBs,which may pin them to some extent.Interestingly,migration of STGBs could occur by the glide of multiple TDs resulted from reactions of the STGBs with incident basal<a60>dislocations.Moreover,STGBs near the {1012} twin orientation could evolve into {1012} TBs by either GB sliding or reacting with arrays of incident basal<a60>dislocations.Migration and evolution of STGBs could recover the mobility of those GBs,and thus benefit the ductility of Mg alloys.