Annealing twin formation study of cold-drawn copper wires frome EBSD measurements

Annealing twin formation and recrystallization study of cold-drawncopper wires from EBSD measurementsT.Baudin ⁎,A.L.Etter,R.PenelleUniversitéde Paris Sud,ICMMO,UMR CNRS 8182,Laboratoire de Physico-Chimie de l'Etat Solide,bâtiment 410,91405Orsay Cedex,FranceReceived 29August 2006;accepted 18September 2006AbstractThe crystallographic texture and microstructure of an electrolytic tough pitch copper have been investigated by Electron Back Scattered Diffraction (EBSD)after cold wire drawing (reduction in area between 52%and 94%)and after primary recrystallization.The material presents a deformation texture composed of major 〈111〉and minor 〈100〉fibers.The evolution of the quality index of the Kikuchi patterns shows that the stored energy is lower in the 〈100〉fiber than in the 〈111〉fiber.Then,after recrystallization,the volume fraction of the 〈100〉fiber increases at the expense of the other texture components.The study of the grain boundary nature shows that the recrystallization twin fraction decreases with increasing strain.It is shown that this evolution is the consequence of the grain size reduction.©2006Elsevier Inc.All rights reserved.Keywords:Copper;Wire drawing;Recrystallization;Twin;EBSD1.IntroductionThe recrystallization development of copper (or alloys with similar recrystallization behavior such as Ni alloys)is now quite well known.Indeed recent studies [1–3]have examined and explained different steps of the recovery and recrystallization processes and their influence on the microstructure and the texture development.So,it is now well known that to grow,a nucleus needs to have a sufficient size that can be attained by dis-location cell recovery,see Ref.[4].For copper,when the strain amount is low,all grains,i.e.all orientations,have the same probability to grow,and the recrystallization texture becomes isotropic because of the twinningmechanism which involves the formation of new orientations.For a high amount of strain produced by cold rolling,the {100}〈001〉cube orientation dynam-ically recovers during deformation.Then,this compo-nent quickly develops at the expense of the deformed matrix.Consequently the recrystallization texture is essentially composed of the cube component plus its twin orientation.In all cases,when the nucleus has reached a “critical ”size,it grows at the expense of the deformed matrix by bulging.This is possibly due to the large stored energy difference between the nucleus and the matrix [5].Let us finally note that a nucleus can twin during the first stages of its growth [6]and several twin generations may be observed before complete recrystal-lization [7,8].This short-term recrystallization behavior clearly shows the great importance of the twinning mechanism,which is still not well understood.Materials Characterization 58(2007)947–952⁎Corresponding author.Tel.:+33169157983.E-mail address:Thierry.Baudin@lpces.u-psud.fr (T.Baudin).1044-5803/$-see front matter ©2006Elsevier Inc.All rights reserved.doi:10.1016/j.matchar.2006.09.009In the present study,EBSD measurements have been performed on wires after different cold drawn reductions and also after recrystallization.This study focuses on the twin boundary volume fraction in order to understand the formation of the recrystallized microstructure.2.Experimental detailsThe material classified as Electrolytic Tough Pitch (ETP)contains a minimum copper concentration of 99.99%,i.e.the impurities level is less than100ppm(the impurity content of this material is given in Table1).The copper wires were industrially hot rolled(8mm diameter)and then cold drawn.The identity,the section reduction,the final diameter after deformation and the true strain of each wire are given in Table2.After working,the copper wires were annealed at260°C for10min in an oil bath.The EBSD measurements on the deformed and re-crystallized samples were performed on an SEM (Scanning Electron Microscope)equipped with a W-filament.The OIM™software was used to analyze the orientation maps.3.Results and discussion3.1.Deformed specimensThe wire-drawn texture in copper and some alloys can be described as a combination of the〈111〉and〈100〉fibers[9].By neutron diffraction experiments,Gerber et al.[9]have shown from orientation distribution func-tion calculations that the intensity of the〈111〉fiber increases with increasing strain.The〈100〉fiber is less intense than the〈111〉fiber,but the intensity related to the〈100〉ideal position tends to increase with increasing strain.These results are consistent with those described in the literature.Indeed,it has been shown that the〈111〉fiber is a stable orientation in the deformed state[10], while the〈100〉component is obtained as the result of a combined dynamic recovery and recrystallization pro-cess[11].Gerber et al.[9]have then assumed that the volume fraction of recovered and/or recrystallized〈100〉grains in the deformed state increases for the highest strain level.This hypothesis is verified from the volume fraction calculated from EBSD measurements(Fig.1a). This figure shows the increase of the〈100〉fiber volume fraction when the strain amount increases and Fig.1b shows that the white points(good Quality Index—QI) correspond to the recovered or recrystallized grains of the〈100〉fiber.Let us note that the quality of the Kikuchi patterns is too bad to estimate the〈111〉fiber volume fraction.The QI distribution can be used to estimate relative values of the stored energy,which is the driving force for recrystallization[4],in the two different fibers.Fig.2 shows that the QI factor is greater for the〈100〉fiber than for the〈111〉fiber(with a15°spread).Then,the stored energy is lower in the first fiber than in the second.This result has already been observed from neutron diffraction measurements[12].Finally,it is interesting to note that the stored energy difference(Fig.2)between these two fibers increases with the amount of strain.This gives to the〈100〉grains the ability to grow at the expense of the matrix during recrystallization,especially after high strain.3.2.Recrystallized specimensFig.3shows the microstructural evolution along the radius of the different wires.The grain size is calculated excluding twins and it appears that the grain size de-creases when the strain amount increases(Fig.4a). Moreover,no significant change(small increase for the A sample)of the grain size is observed along the sample radius(Fig.4b)except for the last point of each curve which corresponds to points at the periphery of the wire. In this particular area,a shear strain can modify the recrystallization mechanisms and thus the grain size.Table1Chemical composition of copper given in ppm(in weight)ElementC O Si S Cl Fe Co Ni Zn As Se Sn Pb2.31700.033.20.02 2.80.030.090.170.010.030.030.26 Table2Diameter,section reduction and true strain of the A,B,C and D cold-drawn wiresSample name Diameter after reduction(mm)Section reduction(%)TruestrainA 5.54520.73B 4.2472 1.27C 2.5790 2.27D 1.9394 2.84948T.Baudin et al./Materials Characterization58(2007)947–952The development of recrystallization textures in wire-drawn fcc metals has been reviewed by several authors [9,13].At high reduction level,the wire-drawn texture evolves during recrystallization to a major 〈100〉fiber.The EBSD measurements (Fig.5a)allow us to verify the increase of the 〈100〉fiber volume fraction as a function of the strain amount.Let us note that no particular evolution of the 〈100〉fiber volume fraction was ob-served along the wire radius for the A and B specimenswhile the 〈100〉fraction of C and D specimens increases (Fig.5b).However,let us further remark that this increase is only due to the value estimated at the wire periphery which can be submitted to a different strain path than the other points.The microstructures present many twins character-ized by Σ3grain boundaries.A decrease of the twin fraction is observed when the strain amount increases (Fig.6a).The change of twin volume fraction along the radius is presented in Fig.6b.Regarding the three last Figs.4a,5a and 6a ,it is worth trying to link the twin evolutions with the grain size and/or the 〈100〉fiber fraction changes.Indeed,first,it can be seen that the decrease of twin fraction as a function of the reduction section could be associated with the decrease of the grain size but also might be related to the increase of the 〈100〉fiber fraction.In order to separate the influence of these two para-meters,their evolutions along the specimen radii can be useful.Indeed,at a first glance,it appears that the twin evolution (Fig.6b)is easier to correlate to the grain size (Fig.4b)than to the 〈100〉fiber fraction (Fig.5b).This assumption is further supported if we focus on the specimens A and C at 0.5mm from the wire center.In these two areas,the 〈100〉fiber fraction is similar (18%),but the grain size is quite different with 10.2μm diameter for A and 8.4μm for C.Consequently,it appears that the drop of Σ3grain boundary fraction (about 43%to 50%)between these two areas is related to the grain size.Once more,the decrease in twinning at the end of the curves corresponds fairly well to the grain size decrease.For each sample,the small (b 10μm)and large (N 10μm)grains were separated in the microstructure.For each grain partition,the twin fraction was calculated.As an example,Fig.7shows,for the center of the D sample,the orientation maps ({hkl}plan distributioninFig.2.Quality index evolution for the two 〈100〉and 〈111〉fibers.(a)Sample A and (b)sampleD.Fig.1.(a)V olume fraction of the 〈100〉fiber as a function of the section reduction.(b)QI map of the D sample and {001}pole figure of the main white points.949T.Baudin et al./Materials Characterization 58(2007)947–952the wire section)of the complete microstructure and for the two associated partitions.Inside these microstruc-tures,it becomes easy to determine the Σ3volume fraction (Fig.8).For all the samples,the Σ3volume fraction is lower in the small grains of the microstructure than in the large grains as expected above.The important point of this study is that the twin fraction is mainly distributed in the large grains.Indeed,it appears that large grains favor twin formation.This conclusion could be used to develop isotropic textures often desired from an industrial point of view to improve physical properties.4.ConclusionsThe results of the present study allow us to discuss some important conclusions which can help in the understanding of the microstructure and the re-crystallization texture formation in cold-drawn copper alloys.During deformation,the 〈100〉grains dynamically recover.In these conditions,the stored energy difference,estimated from the EBSD measurements,between the 〈100〉grains and the matrix (especially the 〈111〉grains)becomes large and favors the nucleation of this texture component during the annealing treatment.Consequently,the minor 〈100〉fiber present in the strained material develops preferentially during the re-crystallization process at the expense of the other tex-ture components.During recrystallization,twinning is an important mechanism which affects the texture development.For a low reduction,all the texture components can develop and then each grain can twin,creatingnewFig.3.Microstructure evolution along the radius of the different samples analyzed by EBSD.950T.Baudin et al./Materials Characterization 58(2007)947–952orientations which increase the random part of the texture.On the contrary,if the reduction is high,the 〈100〉grains grow preferentially and the final texture is mainly composed of this component plus its twin 〈122〉and a random part.The present results show that the Σ3grain boundary fraction decreases with the increasing strain amount.Then,an isotropic texture,which is often industrially desired to obtain isotropic physical properties,can be achieved by recrystallization of low strained specimens.Indeed,this twin volume fraction increase is also directly linked to the grain size increase as shown in the present study.However,it is often necessary to produce flat sheets.Consequently,one can consider the possibility to develop a normal grain growth after primary recrystal-lization,in order to obtained large twinned grains favor-ing the isotropic texture development.AcknowledgementsThe authors are grateful to the Sociétéde Coulée Continue du Cuivre (SCCC)and SociétéLensoise du Cuivre (SLC)for providing the wire-drawn copper.The Ministère de la Recherche Française is acknowledged for the financial support of this project through the Réseau National Matériaux et Procédés (RNMP).References[1]Julliard F.,Ph.D.Thesis,Etude des mécanismes de recristallisa-tion dans l'Invar,alliage Fe –36%Ni.Universitéde Paris XI,Orsay,France;2001.[2]Gerber Ph.,Ph.D.Thesis,Etude des liens entre hétérogénéités dedéformation et mécanismes de recristallisation.Application au cuivre et ses alliages.Universitéde Paris XIII,Villetaneuse,France;2002.[3]Jakani S.,Ph.D.Thesis,Effet des impuretés sur les mécanismesde recristallisation du cuivre tréfilé.Universitéde Paris XI,Orsay,France;2004.[4]Humphreys FJ,Hatherly M.Recrystallization and relatedannealing phenomena.Elsevier;2004.[5]Etter AL,Mathon MH,Baudin T,Branger V ,Penelle R.Influenceof the cold rolled reduction on the stored energy and the recrystallization texture in a Fe –53%Ni alloy.Scr Mater 2002;46:311–7.[6]Julliard F,Baudin T,Penelle R.Formation de la texture derecristallisation cubique dans l'alliage Fe –36%Ni élaborépar voie lingot et par coulée continue en bandes minces.Arch Metall 2000;45:33–45.[7]Gottstein G.Annealing texture development by multipletwinning in fcc crystals.Acta Metall 1984;32:1117–38.[8]Baudin T,Julliard F,Penelle R.Développement de la texture derecristallisation par maclage multiple dans l'Invar (Fe36%Ni).Rev Métall 2003;2:193–202.Fig.5.Evolution of the 〈100〉fiber volume fraction after recrystal-lization (a)as a function of the section reduction and (b)along the radius of the differentsamples.Fig.4.Grain size evolution (a)at the center of the samples with section reduction and (b)along the radius of the different samples.951T.Baudin et al./Materials Characterization 58(2007)947–952[9]Gerber Ph,Jakani S,Mathon MH,Baudin T.Neutron diffractionmeasurements of deformation and recrystallization textures in cold wire-drawn copper.ICOTOM 14,Leuven,Belgium,July 11–15,2005.Mat Sci Forum 2005;495–497:919–24.[10]Dillamore IL,Roberts WT.Preferred orientation in wrought andannealed metals.Metall Rev 1965;10:271–380.[11]Inakazu N,Kaneno Y ,Inoue H.Fiber texture formation andmechanical properties in drawn fine copper wires.ICOTOM 10,Clausthal,Allemagne,September 20–24,1993.Mat Sci Forum 1994;157–162:715–20.[12]Jakani S,Mathon MH,Benyoucef M,Gerber P,Baudin T,de Novion CH.Impurities effects on the stored elastic energy in cold-drawn copper wires.J Neutron Res 2004;12:249–54.[13]Park H,Lee DN.The evolution of annealing textures in 90Pctdrawn copper wire.Metall Mater Trans A Phys Metall Mater Sci 2003;34:531–41.Fig.6.Evolution of Σ3grain boundaries measured (a)at the center of each sample and (b)along the radius of the differentsamples.Fig.7.Orientation maps (distribution of {hkl}plan)of (a)the center of sample D,(b)the small grains and (c)the largegrains.Fig.8.Evolution of Σ3grain boundaries measured in small and large grains of all specimen centers.952T.Baudin et al./Materials Characterization 58(2007)947–952。