镁合金

Transmission Electron Microscopy Investigation of͗cϩa͘Dislocations in Mg and␣–Solid Solution Mg-Li Alloys S.R.AGNEW,J.A.HORTON,and M.H.YOOThe ductility of Mg alloys is limited due to a shortage of independent slip systems.In particular,c-axis compression cannot be accommodated by any of the easy slip or twinning modes.Basal-textured samples of pure Mg and Mg-15at.pct Li were examined for the presence of͗cϩa͘dislocations by post-mortem transmission electron microscopy(TEM)after a small deformation,whichforced the majority of grains to compress nearly parallel to their c-axes.A higher density and moreuniform distribution of͗cϩa͘dislocations is found in the Li-containing alloy.Because the1/3͗1123͘{1122}pyramidal slip mode offers five independent slip systems,it provides a satisfying explanationfor the enhanced ductility of␣–solid solution Mg-Li alloys as compared to pure Mg.The issue of͗cϩa͘dislocation dissociation and decomposition remains open from an experimental point of view.Theoretically,the most feasible configuration is a collinear dissociation into two1/2͗cϩa͘partialdislocations,with an intervening stacking fault on the glide plane.It is speculated that Li additionsmay lower the fault’s energy and,thereby,increase the stability of this glissile configuration.I.INTRODUCTION such as the viscoplastic self-consistent approach,[3]simula-tions can be used to assess the activity of the different A MAJOR problem facing hcp metals,such as magne-deformation modes.The objective of the current study[4] sium,is limited low-temperature formability.The dominant has been to use deformation-texture data to probe the slip mode in all hcp metals has the Burgers vector1/3͗1120͘effects of solid-solution alloying on the balance between (or,͗a͘),whether the primary slip plane is basal(e.g.,cad-the deformation modes.The results should help guide mium,zinc,or magnesium)or prismatic(e.g.,titanium or efforts to develop wrought magnesium alloys with better zirconium).Even if both slip planes operate,there is still no forming characteristics.way to accommodate strains along the c-axis.Deformation In a previous article,[4]comparisons of experimental and twinning can help alleviate the problem;[1,2]however,twin-simulated textures suggested that1/3͗1123͘(or,͗cϩa͘) ning modes are unidirectional.Mechanical twins in hcp crys-dislocations play a critical role in the deformation-texture tals are divided into two categories:those that provide tensile evolution of magnesium alloys.Furthermore,a connection strains along the c-axis and those that provide compressive was drawn between an apparent increase in the activity of strains.There is only one major twinning mode in magne-this deformation mode and the increased ductility observed sium,the{1012}tension twin;hence,the crux of the problem in alloys containing10to15at.pct Li.The purpose of the is c-axis compression.If a grain is forced to deform under current transmission electron microscopy(TEM)study is to c-axis compression,a large plastic incompatibility will investigate the dislocation structures of deformed Mg and develop between neighboring grains,and plastic instability Mg-Li for evidence that supports(or refutes)the previously and/or fracture will ensue.cited simulation results.In addition,clues to help explain In addition to focusing on c-axis compression,special why Li additions might enhance͗cϩa͘slip were also attention is drawn to crystallographic texture.When poly-sought.crystalline materials are plastically deformed and theaccommodation occurs by the physical mechanisms of slipand twinning,the crystallites rotate to preferred orienta-II.BACKGROUNDtions.This texture is a“fingerprint”of the imposed defor-mation geometry and the deformation modes that sustainedHauser et al.[5]were the first to report the dramatic the plasticity.With computer models for texture evolution,improvement in Mg ductility due to Li additions.Since thattime,numerous reports have been made regarding the benefitof Li to improving the ductility of Mg.[6–11]In many of thesereports,however,the Li additions resulted in a substantial S.R.AGNEW,Assistant Professor,is with the Department of MaterialsScience and Engineering,University of Virginia,Charlottesville,V A,Con-volume fraction of the soft Li-rich bcc␤phase.In the current tact e-mail:sra4p@ J.A.HORTON and M.H.YOO,Senior study and that of Hauser et al.,the alloys are completely Research Scientists,are with the Metals and Ceramics Division,Oak Ridge hcp␣phase.It was originally concluded that the increased National Laboratory,Oak Ridge,TN37831.ductility of Mg-Li alpha–solid solution alloys was connected This article is based on a presentation made in the symposium entitledwith an increased incidence of͗a͘Burgers-vector disloca-“Defect Properties and Mechanical Behavior of HCP Metals and Alloys”at the TMS Annual Meeting,February11–15,2001,in New Orleans,tions gliding on{1010}prism planes.[5]Single-surface slip Louisiana,under the auspices of the following ASM committees:Materials traces in polycrystalline samples were cited as evidence of Science Critical Technology Sector,Structural Materials Division,Elec-more uniform nonbasal slip in the Li-containing alloys,as tronic,Magnetic&Photonic Materials Division,Chemistry&Physics ofopposed to pure Mg,where nonbasal slip traces were isolated Materials Committee,Joint Nuclear Materials Committee,and TitaniumCommittee.to regions of stress concentration,such as grain corners.This conclusion has been cited repeatedly throughout the temperature and below.[25]In this orientation,the Schmid factors for ͗c ϩa ͘slip on second-order pyramidal planes literature.[2,7,12,13]and for ͗a ͘slip on prismatic planes are similar (0.446and 0.433,respectively).Hence,the fact that the slip-trace analy-A.Nonbasal Slip of ͗a ͘Dislocationsses indicated ͗c ϩa ͘slip,rather than prismatic ͗a ͘slip,is significant.In an hcp material with easy basal slip,if the In order to explain their observation of enhanced prismatic critical resolved shear stress (CRSS)for ͗c ϩa ͘slip is lower ͗a ͘slip,Hauser et al.[5]cited a decrease in the axial ratio than that of prismatic ͗a ͘slip,there is essentially no stress (c /a )of Mg as a function of Li content (from 1.624to condition for which the latter slip system will be favored 1.607at the solid-solubility limit.)This change is primarily over the former.associated with a decrease in the (0002)lattice spacing,rather than an increase in the {1010}planar spacing.Hence,it was argued that the Peierls stress for basal slip would C.Dissociations of ͗c ϩa ͘Dislocationsincrease relative to prism slip.These analyses do not account A dislocation with a Burgers vector as large as ͗c ϩa ͘for the role of stacking faults.More recently,Couret and has a strong tendency to reduce its energy by dissociation Caillard have published a series of reports [14,15,16]on non-or decomposition.Previous authors have recognized this fact basal slip of ͗a ͘dislocations in basal-slip-dominated hcp and proposed a number of possibilities.[27–31]Three possible metals,such as magnesium and beryllium.Their findings dissociation reactions are as follows:emphasize that such nonbasal slip is better understood as thermally activated “cross slip”of basal dislocations,since 1/2[1123]→1/6[2023]ϩSF (1122)ϩ1/6[0223][1]͗a ͘dislocations prefer to dissociate into Shockley partials c ϩa →[c /2ϩp 1]ϩSF (1122)ϩ[c /2ϩp 2]lying in the basal plane.In fact,an early study of nonbasal slip of ͗a ͘dislocations in pure Mg single crystals pulled in 1/3[1123]→␩/3[1123]ϩSF 1122ϩ(1Ϫ␩)/3[1123][2]tension perpendicular to the c -axis emphasized that prismatic slip was only unambiguously identified for samples tested c ϩa →␩[c ϩa ]ϩSF (1122)ϩ(1Ϫ␩)[c ϩa ]above 180ЊC.[17]Even in Mg-Li ␣–solid solution alloy 1/2[1123]→1/3[1010]ϩSF (0001)ϩ1/3[0113][3]single crystals,Quimby et al.[18]only observed extensive prismatic slip above 130ЊC.However,at 380ЊC,only pris-c ϩa →p 1ϩSF (0001)ϩ[c ϩp 2]matic slip occurred,despite a favorable orientation for basal slip.All of these findings stress the fact that prismatic slip Frank and Nicholas [27]suggested the Shockley-type disso-is a thermally activated process.ciation of Eq.[1]on the second-order pyramidal slip plane,based on a hard-sphere model.Rosenbaum [28]was first to recognize that this dissociation may be sessile,due to the B.Pyramidal Slip of ͗c ϩa ͘Dislocationscorrugated nature of the slip plane,and proposed a zonal dislocation model.The collinear splitting indicated by Eq.Early deformation studies of magnesium (e.g.,References [2]was obtained by atomistic simulation studies in hcp 6through 8)did not mention a dislocation with a Burgers metals.[29,30]Recent work on generalized stacking faults [31]vector as large as ͗c ϩa ͘,which is (1ϩc 2/a 2)1/2(i.e.,1.907confirms the earlier result of a symmetric dissociation with for pure Mg)times the interatomic spacing in the close-␩ϭ1/2.The nonplanar dissociation of Eq.[3],along the packed directions.It was emphasized that ͗a ͘dislocations [1100]direction,was also obtained by atomistic simulation could cross slip from the basal plane onto first-order studies,[29,30,32]where the second partial splits further on prism and {1011}pyramidal planes at elevated temperatures the (1121)plane to form a tension-twin nucleus.No direct and in regions of high stress concentration.[17]A number of experimental evidence for any of these reactions has been twinning modes were also discussed in the early studies of reported.Mg deformation,including {1011}and {1013}compression twins,[19]as well as the {1012}tension twin,which is the most common twinning mode in magnesium and its alloys. D.Deformation TexturesEarly reports of Mg single-crystal plasticity in hard orien-tations emphasized deformation twinning,such as {1011}Throughout this report,an emphasis is placed on what can be gleaned from deformation-texture analysis.Even in compression twins during c -axis compression.[19,20,21]How-ever,more recent reports that include TEM characterization early discussions of the Mg-Li system,qualitative analyses were made of deformation textures.In the written discussion unanimously cite the significance of ͗c ϩa ͘slip.[22–25]It was observed during the single-crystal experiments that fracture of the article by Hauser et al.,[5]Pearson noted that a hot (425ЊC)strip–extruded Mg -4.6wt pct Li alloy exhibited originated at voids formed at twin boundaries,[21]and twin-ning only narrowly preceded fracture in tests below about a plane-strain compression texture similar to that of metals characterized by significant prismatic slip,like Ti and Zr.200ЊC.[19,23]Since we are usually ultimately interested in polycrystal deformation rather than single-crystal deforma-(Evidence of the same was given in a recent investigation of a hot-rolled two-phase (hcp ϩbcc)Mg-Li alloy,where tion,it is noted that the tendency to mechanically twin may be suppressed in polycrystalline materials,since the stress the “Ti-or Zr-type”rolling texture was revealed in the hcp phase.[10])There,the peak texture component consists of required to activate twinning has been shown to depend more strongly upon grain size than slip.[26]basal poles inclined ϳ30Њfrom the sheet normal direction toward the transverse direction .However,when the alloy Ando and Tonda recently performed two surface-slip-trace analyses on single crystals of pure Mg and Mg-Li was subsequently cold rolled,the texture began to return to one more characteristic of Mg,with basal poles inclined byalloys strained in tension along a ͗1210͘direction at roomthin sections were ground using600-grit SiC paper to anapproximate thickness of200␮m prior to punching out3mm discs.Final thinning was performed using a double-jetelectropolisher(Struers Tenupol-2)with a solution of450mL of ethanol,50mL of perchloric acid,and25mL ofbutyl cellosolve cooled toϽϪ30ЊC.Some of the sampleswere polished using a solution of6pct sulfuric acid inmethanol cooled toϪ15ЊC.After perforation,the sampleswere rinsed in a series of alcohol baths.The gиbϫuϭ0invisibility criterion was used todistinguish the Burgers vectors of the different dislocationspresent in the samples.Table I shows the values of gиbfor all the perfect dislocations observed in hcp crystals forlow-index reflections near the zone axes used in this study. Fig.1—Inverse pole figure(equal area projection)of pure Mg compressed Notably,the gϭ[0002]diffraction condition was used to to a true strain of0.3shows a preferred orientation having basal poles near search for dislocations having a c component.(Because Mg the compression axis.and Mg-Li solid-solution alloys are nearly elastically iso-tropic,there are few residual contrast issues.[33])By observ-ing a number of grains within a number of TEM foils,the onlyϳ20Њto the sheet normal direction toward the roll-general characteristics of the dislocation structures could ing direction.be ascertained.It is currently concluded that,while the CRSS of theprismatic͗a͘slip mode may be decreased relative to basalIV.RESULTS͗a͘slip in Mg alloys containing Li,[18]prismatic slip is stillsubstantially more difficult at room temperature.Based upon A.Dislocation Structures in Mg-Lithe recent quantitative texture analyses,[4]the͗cϩa͘slipmode appears more important during room-temperature By virtue of the crystallographic texture,the gϭ[0002]diffraction condition could be utilized in nearly all the grains compression than prism͗a͘slip.During elevated-tempera-ture deformation,the role of prismatic slip is likely to be within all the samples observed.In all of the Mg-Li samplesobserved,most of the grains had arrays of dislocations exhib-substantial,as evidenced by the texture of the strip-extrudedMg-Li alloy[5]and the early single-crystal experiments per-iting strong contrast for gϭ[0002].Figure2represents a formed on Mg-Li.[18]typical dislocation structure observed showing a large array Again,the purpose of this TEM investigation is to deter-of either c or͗cϩa͘dislocations.These dislocations have mine whether or not direct experimental evidence of thea strong tendency to align with the basal plane.However, dislocation structures bears out the results of the simulations there are a number of loop segments which have portions of texture evolution,which have suggested that͗cϩa͘slipout of the basal plane;thus,the c character of the dislocation on{1122}second-order pyramidal planes may be more is unambiguous,with gиbϫuϷ0for any͗a͘dislocationlying out of the basal plane.important than cross slip of͗a͘dislocations onto prismplanes during compression at ambient temperatures.[4]Figure3demonstrates a typical Burgers-vector analysisof the͗cϩa͘dislocations identified in this study.Thedislocations of interest are labeled A through F in the sche-III.EXPERIMENTAL matic illustration(Figure3(a)).Dislocation A is visible inPolycrystalline samples were used for this TEM study,the center of Figure3(b)(gϭ[0002]).Since this dislocation following the previous study of texture evolution.[4]It is is invisible for gϭ[1011](Figure3(c)),it is a͗cϩa͘rather than c type,but there are still four possible Burgers understood that grain boundaries may be important sourcesand/or sinks for dislocations.Hence,the slip-mode selections vectors,according to Table I.All contrast is weak for the may be affected by the presence of crystalline interfaces.gϭ[1010]condition(Figure3(d))but the͗cϩa͘dislocation Cast samples of pure Mg and Mg-15at.pct Li were(A)is still visible;therefore,it must be either bϭ1/3[2113] compressed approximately20pct and subsequently recrys-or1/3[1123].Again,if it were a c type,it would be invisible tallized in an Ar-purged furnace at475ЊC for1hour.After for this diffraction condition.For gϭ[1101](Figure3(e)), annealing the samples to produce strain-free material,theythe͗cϩa͘dislocations are visible,so they may be identified were again compressed to specified plastic strains(␧pϳ1as bϭ1/3[1123].As a final check,it can be seen thatthe dislocations exhibit strong contrast in the gϭ[1011] to3pct)along the same compression axis used previously.The texture generated during cold working(Figure1)condition(Figure3(f)).Similar to the images published by Paton and Backofen,[35] ensured that many of the grains were favorably oriented toexhibit͗cϩa͘slip during the final compression.In other in Ti,there is a junction pair of c,͗a͘and͗cϩa͘dislocations words,the majority of grains had orientations close to theshown in the upper half of the images in Figure3.Only the critical orientation where c-axis compression is demanded.parts containing a c component,labeled B and C,are visiblein Figure3(b).In Figure3(d),only the dislocations having Thin sections containing the compression axis were cutfrom the compressed samples using electrodischarge an a component,labeled C and D,are visible.The͗cϩa͘machining.Due to the preferred orientation,this foil orienta-segment,labeled C,is invisible in Figure3(c).Therefore,tion enabled a gϭ[0002]diffraction condition to be it too is a bϭ1/3[1123]dislocation,according to Table I.Table I.The gؒb Values for Perfect Dislocations in the Hexagonal Close-Packed Crystals Close to the[1210]and[1213] Zone Axes;the g Vectors Indicated are Reciprocal Lattice Vectors of the Plane with the Same Four-Index Notation[34]* Zone AxisMode b g[0002]b c d f e͗a͘1/3[1120]011Ϫ101 1/3[1210]00001Ϫ11/3[2110]0Ϫ1Ϫ11Ϫ10͗cϩa͘1/3[1123]201Ϫ2Ϫ12 1/3[1213]2Ϫ10Ϫ1001/3[2113]2Ϫ2Ϫ10Ϫ211/3[1123]Ϫ2210101/3[1213]Ϫ21012Ϫ21/3[2113]Ϫ20Ϫ120Ϫ1 c[0001]2Ϫ10Ϫ111 *Along with the g vectors,the letters labels in the column titles correspond to the individual images within Fig.3.In the current study,the purpose of examining pure Mgsamples was primarily to serve as a point of reference fordrawing conclusions about the effect of Li additions.Withthat view in mind,distinctions in the͗cϩa͘and͗a͘disloca-tion configurations with those observed in the solid-solutionalloy were of particular interest.The gϭ[0002]diffractioncondition was again used to search for dislocations with ac component.Some dislocations did appear in contrast forgϭ[0002];however,there were distinctions with the dislo-cation microstructures observed in the Mg-Li alloy.In general,the overall density of dislocations with a ccomponent was low relative to the͗a͘dislocations.Themajority of grains did not have the uniform arrangementsof͗cϩa͘dislocations observed in the Mg-Li alloy,likethe one in Figure2.Specifically,͗cϩa͘dislocations wereoften closely associated with deformation twins,as foundearlier by Morozumi et al.[24]Figure4shows an ensembleof͗cϩa͘dislocations emanating from the tip of a mechani-cal twin.It is commonly observed that activating deforma-Fig.2—Typical dislocation microstructure in Mg-Li solid solution alloy tion twinning requires a significant stress concentration and after compression of textured samples where the grains’c-axes are all closethat severe accommodation requirements are placed upon the to the compression axis.Images like this one,formed using gϭ[0002],demonstrate the high density of dislocations containing a c component surrounding matrix crystal.The close relationship between observed in most grains of every sample.deformation twins and͗cϩa͘dislocations suggests that asignificant stress concentration is necessary to activate͗cϩa͘slip as well.A small number of͗cϩa͘dislocations The͗cϩa͘dislocation,discussed in the previous paragraphwere found among high-density dislocation tangles primarily and labeled A in the schematic drawing,is also decomposedcomposed of͗a͘dislocations(Figure5).Again,observations into c and͗a͘parts labeled E and F in the drawing.such as this suggest that the high stresses associated with a In addition to͗cϩa͘dislocations,there also was a highdislocation tangle may be required to activate͗cϩa͘slip. density of the͗a͘primary slip dislocations observed in theA final evidence that͗cϩa͘slip may be more difficult in Mg-Li alloy.As mentioned in Section II,previous explana-pure magnesium than in Mg-Li alloys is the presence of tions of the improved ductility of Mg-Li solid-solution alloysloop debris within the grains containing͗cϩa͘dislocations. emphasized slip of͗a͘dislocations on the prism planes.[5]Prismatic loops will result during deformation if dislocation No strong evidence for this conclusion could be found,sincedipoles decompose or jogs on a screw dislocations are forced tilting experiments showed the vast majority of͗a͘disloca-through the crystal.In either event,these represent cases of tions to lie in the basal plane.high stresses that seem to coincide with the activity of͗cϩa͘dislocations in pure Mg.The distinctions between B.Dislocation Structures in Pure Mg these microstructures and those observed in the Mg-Li alloysuggest that the motion of͗cϩa͘dislocations is more Although the earliest studies of Mg plasticity failed todifficult in pure magnesium.realize the significance of a pyramidal͗cϩa͘slip mode,There were no prismatic͗a͘dislocations observed and, TEM studies conducted during the1970s verified the pres-unlike the Mg-Li samples,some of the basal͗a͘dislocations ence of dislocations with a͗cϩa͘Burgers vector in c-axis-compressed single crystals[22,23]and strained polycrystals.[24]were widely dissociated,with a stacking fault between theFig.3—Images from a series of diffraction conditions outlines a typical Burgers vector analysis emphasizing the presence of͗cϩa͘dislocations in the Mg-Li alloy.In addition,the propensity to break into c and͗a͘dislocations is exhibited.partial dislocations(e.g.,Figure5(c)).The basal stacking-slip modes are elusive.However,the dislocation arrange-fault energy was not determined in this study.Recent experi-ments observed are suggestive.Most grains in the Mg-Li mental[15]and calculated[36,37]values for the stacking-fault samples contained uniform arrays of͗cϩa͘dislocations. energy in pure Mg verify that it is low(30to50mJ/m2).Some͗cϩa͘dislocations were observed in pure Mg sam-ples;however,they were present in fewer of the grains(despite a very similar texture in the two types of samples), V.DISCUSSION and they were always associated with microstructure featuresindicative of a high-stress configuration,such as deformation The enhancement of͗cϩa͘dislocation activity in Mg-twins,dislocation tangles,and loop debris.Li alloys over pure Mg,which was suggested previouslyRegarding the dissociation of͗cϩa͘dislocations,Stohr based on distinctions in their deformation-texture evolu-and Poirier[22]concluded that͗cϩa͘dislocations dissociate tions,[4]is supported by the current TEM observations.Trulyquantitative comparisons of dislocation densities for hard according to the Shockley-type reaction of Eq.[1].However,(a )(b )Fig.4—(a )A slip band composed of ͗c ϩa ͘dislocations emanating from a mechanical twin is visible for g ϭ[0002].(b )Basal ͗a ͘dislocations are visible in the lower-right-hand corner for g ϭ[1011].contrary to Eq.[1],they concluded that the faults were on the basal plane.Vacancies and self-interstitials in hcp crystals frequently precipitate on the basal plane as faulted loops bound by 1/6͗2023͘-type partial dislocations.[38]These are the same partials involved in the dissociation of Eq.[1],where the fault is on a second-order pyramidal plane.Stohr and Poirier indicated that their partial-dislocation Burgers-Fig.5—Dislocation tangle containing ͗c ϩa ͘dislocation (visible for (a )vector analysis was not conducted on a ͗c ϩa ͘dislocation,g ϭ[0002],but invisible for (b )g ϭ[1101])and the presence of a high but on dislocation loops lying in the basal plane.This was density of ͗a ͘dislocations suggest that high stress levels are required to done because they observed ͗c ϩa ͘dislocations to decom-activate or move ͗c ϩa ͘dislocations in pure Mg.The arrow simply points pose into loops under the “heating”by the electron beam.at a fiduciary mark to guide the eye.It is important to realize that these loops are not necessarily directly related to ͗c ϩa ͘dislocations,but may simply be the result of vacancy or self-interstitial condensation.The supersaturation of point defects may be the result of the amount of strain applied prior to TEM examination)or elec-tron irradiation.The threshold energy for knock-on damagedeformation (although this seems unlikely,given the smallgetic incentive for͗cϩa͘edge dislocations to decomposeinto c and͗a͘parts,there will be some energy reduction ifit is understood that the͗a͘dislocation can further dissociateinto Shockley partials on the basal plane,according to Eq.[4].1/3[1123]→[0001]ϩ1/3[1010]ϩSF(0001)ϩ1/3[0110][4]cϩa→cϩp1ϩSF(0001)ϩp2Evidence of this possibility can be found in Figure3,wherejunctions of c,͗a͘,and͗cϩa͘dislocations are observed.This decomposition into the basal plane may explain why the͗cϩa͘dislocation lines tended to lie along the intersection ofthe second-order pyramidal plane and the basal plane.Sinceno Shockley-dissociated basal dislocations were observedin the Li-containing alloy,it is suggested that the decomposi-pure Mg.Nonplanar dissociations such as those of Eqs.[3]and[4]represent locking configurations of the typedescribed by Couret and Caillard.[16]Dislocations whichadopt such configurations essentially have a very largePeierls–Nabarro friction stress.If the Li additions indeedreduce the tendency to decompose into the basal plane,thisis another possible explanation for the increased observationof͗cϩa͘dislocations.VI.CONCLUSIONSTextured polycrystalline samples of pure Mg and Mg-15at.pct Li were examined for the presence of͗cϩa͘disloca-tions by post-mortem TEM after(1to3pct)deformation,which forced the majority of grains to compress along theirc-axis.The higher density and more uniform distribution of Fig.6—Prismatic loops formed in the microscope(operating at150kV)͗cϩa͘dislocations in the Li-containing alloy support the and evolved with time(from image(a)to(b)).These images demonstrate previous conclusion that Li additions promote the activity the tendency for point defects to condense on a pre-existing defect,rather of these dislocations.Because the1/3͗1123͘{1122}pyrami-than in the surrounding matrix,as shown by denuded zones above anddal slip mode offers five independent slip systems,it pro-below basal faults.The contrast produced by the faults for gϭ[0002]isa result of bϭ1/6͗2023͘prismatic loops,which decorate it.vides a satisfying explanation for the enhanced ductility ofMg-Li␣–solid solution alloys as compared to pure Mg.Areview was made of the experimental and theoretical studiesof the possible dissociation and decomposition reactions that of Mg has been measured to be10eV(accelerating voltage͗cϩa͘dislocations may undergo.It is concluded that this ofϳ125keV).[39]remains an open question from an experimental point of In the current study,pre-existing defects were shown to view.Theoretically,the most appealing configuration for strongly interact with point defects and their resultant col-understanding glissile͗cϩa͘dislocations is a collinear lapse into faulted prismatic loops.Under electron irradiation dissociation into two1/2͗cϩa͘partial dislocations with an in the transmission electron microscope operating at150intervening stacking fault on the{1122}glide plane.If this kV,loops formed throughout the microstructure,except in is the case,Li additions may lower this nonbasal stacking-fault energy and,thereby,increase the stability of this glissile denuded zones around basal faults(Figure6).It is suggestedconfiguration.Another possibility is that the Li additions that Stohr and Poirier were observing this type of artifact,increase the effective stacking-fault energy of the basal and that͗cϩa͘dissociation remains an open issue fromstacking faults.Indeed,basal faults were observed in the the experimental point of view.Based on the theoreticalpure Mg samples,but not in the Mg-Li alloys.This would calculations,[29,30,31]a collinear dissociation on the glidealso reduce the incentive for͗cϩa͘dislocations to decom-plane appears most feasible in metals where͗cϩa͘disloca-pose into the basal plane.Although this latter possibility tions are known to be glissile.One possible explanation forwould also promote prismatic cross-glide of͗a͘dislocations, the enhancement of͗cϩa͘slip is that the{1122}stacking-no strong evidence was found to support this mechanism, fault energy for this glissile dissociation is lowered by Liwhich was formerly suggested as responsible for the solid-solution atoms.enhanced ductility of Mg-Li alloys.In addition to dissociation reactions,there is also thepossibility that͗cϩa͘dislocations will decompose intoACKNOWLEDGMENTStwo perfect dislocations of the c and͗a͘type.This possibilityhas previously been invoked to explain the infrequent TEM This research was sponsored by the Laboratory Director’s observation of͗cϩa͘dislocations,despite the macroscopicResearch and Development Fund at the Oak Ridge National。