(6.1)--Effects of solute Mg on grain bo机械工程材料机械工程材料.pdf

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1、Effects of solute Mg on grain boundary and dislocationdynamics during nanoindentation of AlMg thin filmsW.A.Soera,J.Th.M.De Hossona,*,A.M.Minorb,J.W.Morris Jr.c,E.A.StachbaDepartment of Applied Physics,Materials Science Center and Netherlands Institute for Metals Research,University of Groningen,Nij

2、enborgh 4,9747 AG Groningen,The NetherlandsbNational Center for Electron Microscopy,Lawrence Berkeley National Laboratory,One Cyclotron Road,MS 72,Berkeley,CA 94720,USAcMaterials Science Division,Lawrence Berkeley National Laboratory,One Cyclotron Road,MS 66,Berkeley,CA 94720,USAReceived 1 July 2004

3、;received in revised form 15 August 2004;accepted 24 August 2004Available online 23 September 2004AbstractUsing in situ nanoindentation in a transmission electron microscope(TEM)the indentation-induced plasticity in ultrafine-grained Al and AlMg thin films has been studied,together with conventional

4、 quantitative ex situ nanoindentations.Extensive grainboundary motion has been observed in pure Al,whereas Mg solutes effectively pin high-angle grain boundaries in the AlMg alloyfilms.The proposed mechanism for this pinning is a change in the atomic structure of the boundaries,possibly aided by sol

5、ute dragon extrinsic grain boundary dislocations.The mobility of low-angle boundaries is not affected by the presence of Mg.Based on thedirect observations of incipient plasticity in Al and AlMg,it was concluded that solute drag accounts for the absence of discretestrain bursts in indentation of AlM

6、g.?2004 Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.Keywords:Transmission electron microscopy;Dislocation dynamics;Grain boundary motion;Aluminum alloys;Plastic deformation1.IntroductionOver the last two decades conventional nanoindenta-tion has become a versatile technique for

7、 probing themechanical properties of materials with characteristicdimensions in the sub-micron regime 1,2.The resultsobtained from nanoindentation measurements havebeen limited for a long time to quantitative loaddisplacement data and ex situ analysis of indented spec-imens,lacking direct observatio

8、n of the induced plasticdeformation.Recently,direct observation of indenta-tion behavior has been accomplished through the tech-nique of in situ nanoindentation in a transmissionelectron microscope(TEM)3,4.In situ nanoindentation measurements by Minoret al.5 on polycrystalline Al films have provided

9、 exper-imental evidence that grain boundary motion is animportantdeformationmechanismwhenindentingultrafine-grained thin films.Grain boundary motion inmetals typically occurs at elevated temperatures,and isdriven by a free energy gradient across the boundary.The boundary mobility is greatly reduced

10、by the addi-tion of solutes 6.Winning et al.7 found that bothlow-angle and high-angle grain boundaries in pure Almove by an external shear stress at temperatures above200?C.This type of stress-induced grain boundarymotion(known as dynamic grain growth)has been con-sidered by many authors to be the m

11、echanism responsi-ble for the extended elongations obtained in superplasticdeformation of fine-grained materials 8.However,because of the high activation energy required for grainboundary mobility,it is not commonly considered an1359-6454/$30.00?2004 Acta Materialia Inc.Published by Elsevier Ltd.All

12、 rights reserved.doi:10.1016/j.actamat.2004.08.032*Corresponding author.Tel.:+31 503 634 898;fax:+31 503 634881.E-mail address:hossonjphys.rug.nl(J.Th.M.De Hosson).Acta Materialia 52(2004)57835790www.actamat-important deformation mechanism at room tempera-ture.The occurrence of grain boundary motion

13、 in roomtemperature deformation of nanocrystalline fcc metalswas anticipated recently by molecular dynamics simula-tions 9 and a simple bubble raft model 10,and con-firmed by in situ observations of grain coarseningduring indentation of nanocrystalline Al thin films 11and grain rotation during strai

14、ning of nanocrystallineNi thin films 12.The fact that in situ nanoindentationexperiments also showed grain boundary motion atmuch larger grain sizes(on the order of 0.1 lm)is attrib-uted to the highly inhomogeneous stress induced by theindenter.However,the high purity materials investi-gated by the

15、in situ nanoindentation technique to dateare less attractive for the design of advanced materials.Therefore,in this study we have focused on in situobservations of AlMg alloys,probing the influence ofsolutes on grain boundary motion.This is particularlyrelevant for understanding temperature effects

16、in super-plasticity of ultrafine and coarse-grained aluminum-based materials.Considerable research effort on AlMg alloys hasbeen devoted to understanding the pronounced,re-peated yielding that occurs during plastic deformationof these alloys.This phenomenon,known as the Port-evinLe Cha telier(PL)eff

17、ect or serrated yielding 13,leads to a negative strain rate sensitivity and is causedby interaction between dislocations and mobile soluteatoms 14.In recent years,the PL effect in AlMg hasbeen investigated in several deformation modes,includ-ing depth-sensing indentation 15,16.The associateddislocat

18、ion dynamics have been characterized by in situstraining in high-voltage electron microscopy 17,18andpulsednuclearmagneticresonance(NMR)experiments 19.We have conducted in situ nanoindentation measure-ments on Al and AlMg thin films.Preliminary resultsfrom these experiments have shown that solute Mg

19、 sig-nificantly affects the mobility of high-angle grain bound-aries and have also provided direct observation of solutedrag on moving dislocations 20.In this paper,we dis-cuss the influence of solute Mg on the observed indenta-tion-inducedplasticity.Theobservationsarequalitatively related to ex sit

20、u depth-sensing nanoinden-tation results from the same specimens.2.ExperimentalThe design of the in situ indentation stage for aJEOL 200 CX electron microscope used in this studyis extensively described in 4.Briefly,the single-tiltspecimen holder contains a Berkovich-type diamondindenter,which is bo

21、ron-doped in order to be electri-cally conductive in the TEM.The indenter is actuatedby a piezoceramic tube,which controls the movementin the indentation direction(perpendicular to the elec-tron beam)as well as the fine positioning on the sam-ple.The material to be indented is deposited as a thinfil

22、m onto a Si substrate with a sharp wedge-shapedprotrusion,which is prepared by bulk silicon microma-chining.At the top of the wedge,the film is electrontransparent and accessible to the indenter,as illustratedin Fig.1.Four AlMg films with Mg concentrations of 1.1,1.8,2.6 and 5.0 wt%and one high puri

23、ty(5 N)Al filmwere prepared by thermal evaporation.The substratewas kept at 300?C to establish a grain size of the orderof the layer thickness,which was 200300 nm for allspecimens.After evaporation,the substrate heatingwas switched off,allowing the specimen to cool downto room temperature in approxi

24、mately one hour.Be-cause the solubility level of Mg in Al is 1.9 wt%atroom temperature,b0and b precipitates were formedduring cooling in the 2.6 and 5.0 wt%Mg specimens.Although the attainable image resolution in the inden-tation setup was not high enough to resolve theseprecipitates,their presence

25、was confirmed by straincontrastanddistortedgrainboundaryfringes(Fig.2(a),which were not observed in the 1.1 and1.8 wt%specimens(Fig.2(b).On each of the specimens,34 in situ indentationswere carried out with maximum depths ranging from50 to 150 nm.In order to record the grain boundaryphenomena during

26、 each indentation,the specimen wasFig.1.Schematic of in situ indentation setup.Fig.2.Bright-field images of evaporated AlMg layers with 5.0(a)and 1.1(b)wt%Mg.5784W.A.Soer et al./Acta Materialia 52(2004)57835790tilted into such an orientation that both adjacent grainswere in a two-beam diffraction co

27、ndition.Ex situ nanoindentation measurements were carriedout on the same films away from the wedge.As in thein situ experiments,a pyramidal Berkovich tip was used.Load-controlled indentations were executed to maxi-mum depths of 50,100 and 150 nm at a targeted strainrate of 0.05 s?1,defined as loadin

28、g rate divided by load.At this strain rate the indenter velocity during loadingwas on the order of 2 nm/s,which is comparable tothe in situ measurements.3.Results3.1.In situ observation of grain boundary motionTo confirm the occurrence of grain boundary move-ment in pure Al as had been reported earl

29、ier 5,weperformed several in situ indentations near grain bound-aries in the pure Al film.Indeed,significant grain bound-ary movement was observed for both low-angle andhigh-angle boundaries.Fig.3 shows subsequent stagesof the loading part of an indentation near a high-angleboundary.After initial co

30、ntact(Fig.3(a)and plasticdeformation of grain B(Fig.3(b),both grain bounda-ries outlining grain B moved substantially(Figs.3(c)and(d),and the volume of grain B increased accord-ingly at the expense of the volume of the neighboringgrains.By comparing dark-field images taken beforeand after the indent

31、ation,the grain boundary shifts weremeasured to be 0.04 lm for the left boundary and0.22 lm for the right boundary.Qualitatively,the inden-tations on pure Al show that the grain boundary motionbecomes more pronounced with decreasing grain sizeand decreasing distance from the indenter to theboundary.

32、Similar grain boundary movement was never foundfor high-angle boundaries in any of the AlMg speci-mens,even when indented to a depth greater than halfof the film thickness.Fig.4 shows a sequence of imagesfrom an indentation on an Al1.8wt%Mg layer.At anindentation depth of approx.85 nm into grain B(F

33、ig.4(c),plastic deformation was initiated in grain Aeither by transmission across or nucleation at the grainboundary.However,no substantial grain boundarymovement was observed,indicating a significant effectof Mg on high-angle grain boundary mobility in thesealloys.Small grain boundary shifts(?10 nm

34、)that weremeasured occasionally can be attributed to displacementof the material under the indenter as a whole,with con-servation of grain volume,rather than to actual grainboundary motion.In contrast to high-angle grain boundaries,the mobil-ity of low-angle boundaries in AlMg was found to beless af

35、fected by the presence of Mg.Figs.5(a)and(b)show two grains in an Al5.0wt%Mg layer,separatedby a low-angle tilt boundary.The diffraction patternsfor both grains are shown in Fig.5(c).The grains sharethe same 1 1 2 zone axis,but are in different two-beamFig.3.Series of bright-field images from an in

36、situ indentation on Al,which is accommodated by movement of the grain boundaries(marked witharrows).The approximate indentation depth h is given in each image.W.A.Soer et al./Acta Materialia 52(2004)578357905785conditions due to their slight misorientation(?0.7?).Fig.5(d)shows the grains after an in

37、dentation to beboth in the same diffracting condition as the grain in(a).At the onset of plastic deformation,the boundarydisintegrated rapidly with the end result of the twograins becoming one.Fig.4.Series of bright-field images from an in situ indentation on Al1.8wt%Mg.No movement of the high-angle

38、 grain boundaries is observed.Fig.5.(a)and(b)Dark-field images of two grains of a Al5.0wt%Mg layer separated by a low-angle 1 1 0 tilt boundary;(c)diffraction patternshowing the 1 1 2 orientation of both grains;the cut-offis due to the in situ specimen geometry;(d)dark-field image after indentation.

39、5786W.A.Soer et al./Acta Materialia 52(2004)578357903.2.In situ observation of dislocation motionThe effect of Mg on dislocation propagation is par-ticularly visible during the onset of plasticity.While inpure Al the dislocations instantly spread across the en-tire grain(i.e.faster than our 30 frame

40、s per second vi-deo sampling rate)they advance more slowly and in ajerky type fashion in all observed AlMg alloys.InFig.4(b)for example,plastic deformation is alreadyvisible in the left part of grain B,while the right partof the grain is still undistorted.Fig.6 shows a se-quenceofimagesfromanindenta

41、tioninAl2.6wt%Mg.The arrows mark the consecutive positionswhere the leading dislocation line is pinned by solutes.Similar pinning of dislocations is visible during plasticdeformation of the grains adjacent to the indentedgrain,as in Figs.4(c)and(d):as the indenter ispressed into grain B,dislocations

42、 are repeatedly ar-rested in grain A.From these images,the mean jumpdistance between obstacles is estimated to be of theorder of 50 nm.Due to the single-tilt axis limitationof the indentation stage,the orientation of the slipplane relative to the electron beam is unknown;there-fore,the measured jump

43、 distance is a projection and alower bound of the actual jump distance.3.3.Quantitative ex situ nanoindentationThe extraction of mechanical properties from the exsitu indentation measurements was compromised bythe surface roughness and the grain size at shallowdepths and the film thickness at deeper

44、 depths.Recentnumerical studies 21,22 suggest that for a soft film ona hard substrate,the influence of the substrate maynot be appreciable until the depth exceeds one half ofthe film thickness.Still at these relatively high indenta-tion depths,the probed volume was not sufficiently largeto give reli

45、able hardness and modulus data.Most of thefilms showed considerable surface roughness due tocusps at the grain boundaries as illustrated by the scan-ning electron micrograph in Fig.7.This leads to anill-defined contact area during initial loading.Further-more,the size of the indents was on the order

46、 of theFig.6.Series of bright-field images showing jerky motion of dislocations during indentation of Al2.6wt%Mg.The time from the start of theindentation is given in seconds.Note the presence of a native oxide layer on the surface.Fig.7.Scanning electron micrograph of an indent of depth 150 nm inth

47、e as-deposited Al2.6wt%Mg film.W.A.Soer et al./Acta Materialia 52(2004)578357905787grain size,causing scatter in the indentation results dueto microstructural variations.Nonetheless,interesting qualitative differences inloading response between the specimens were observed.Many of the indentations of

48、 the pure Al film showedabrupt displacement bursts during loading up to a depthof around 70 nm,as illustrated in Fig.8(a).Between thebursts,the slope of the loading curve increases continu-ously.No such discontinuities were observed in indenta-tions of any of the AlMg films.Fig.8(b)shows loadingcurv

49、es of the Al2.6wt%Mg film.The initially softresponse of the first tens of nanometers is due to thesurface roughness as mentioned above.Only in theAl5.0wt%Mg specimen,having the highest Mg content,was the serrated yielding characteristic of the PL effectobserved(Fig.8(c).4.Discussion4.1.Grain boundar

50、y motionIdeally,direct measurements of the indenter-inducedstress at the boundary during our in situ nanoindenta-tions could be made to quantify the observed behavior.However,due to surface roughness,tip imperfectionsand the complicated specimen geometry,it is difficultto accurately measure or calcu