Cu-segregation at the Q′_α-Al interface in Al

Cu-segregation at the Q0/a-Al interface in Al–Mg–Si–Cu alloy Kenji Matsuda a,*,Daisuke Teguri b,Yasuhiro Uetani c,Tatsuo Sato d,Susumu Ikeno aa Faculty of Engineering,Toyama University,3190,Gofuku,Toyama930-8555,Japanb Toyama University,3190,Gofuku,Toyama930-8555,Japanc Toyama Prefectural University,5180,Kosugi,Toyama939-0398,Japand Graduate School of Science and Engineering,Tokyo Institute of Technology,2-12-1,O-okayama,Meguro-ku,Tokyo152-8552,JapanReceived15March2002;received in revised form1May2002;accepted5August2002AbstractCu segregation was detected at the Q0/a-Al interface in an Al–Mg–Si–Cu alloy by energy-filtered transmission electron microscopy.By contrast,in a Cu-free Al–Mg–Si alloy no segregation was observed at the interface between the matrix and Type-C precipitate.Ó2002Acta Materialia Inc.Published by Elsevier Science Ltd.All rights reserved.Keywords:Aluminum alloy;Transmission electron microscopy;Phase transformation;Segregation;Q0-phase1.IntroductionAddition of copper to Al–Mg–Si alloys en-hances hardness and refines microstructure[1]. Numerous papers report precipitates of Q-,Q0-and B0-phases in Cu-containing6000series alu-minum alloys,and many discuss which precipitate contributes to mechanical properties of these al-loys.The crystal structure of the Q0-phase in Al–Mg–Si–Cu alloys has been discussed in detail[2,3].A previous paper by the authors reports the exis-tence of the Q0-phase in Cu-containing Al–Mg–Si alloy and concludes that the Q0-phase is a qua-ternary metastable phase distinct from the ternary Type-C precipitate found in the excess-Si Al–Mg–Si alloys.Nonetheless,the two phases have iden-tical crystal lattice[4].Thus,Cu is considered crucial for formation of the Q0-phase.Recently, Abe et al.have pointed out the possibility of in-terfacial Cu-segregation between the precipitate and the a-Al matrix,as evidenced by electrical resistivity measurement[5].The present study focuses on the distribution of Cu around the Q0-phase in Cu-containing Al–Mg–Si alloy as ex-amined by energy-filtered transmission electron microscopy(EFTEM).2.Experimental procedureIngots of an Al–1.0mass%Mg2Si–0.5mass% Cu alloy(0.5Cu alloy)and an Al–1.0mass% Mg2Si–0.4mass%Si alloy(excess Si alloy)wereScripta Materialia47(2002)833–837*Corresponding author.Tel./fax:+81-76-445-6841.E-mail address:matsuda@eng.toyama-u.ac.jp(K.Mat-suda).1359-6462/02/$-see front matterÓ2002Acta Materialia Inc.Published by Elsevier Science Ltd.All rights reserved.PII:S1359-6462(02)00325-1prepared.These were homogenized at723K for four days,and then hot-and cold-rolled to sheets of0.2mm thickness.The sheets were solution heat-treated at848K for3.6ks and quenched in chilled water at277K,followed by aging at523K. Transmission electron microscopy(TEM)samples were prepared by electro-polishing.The TEM (EM-002B,Topcon Co.Ltd.,Tokyo,Japan)and EFTEM(JEOL-4010T,JEOL Co.Ltd.,Tokyo, Japan)were operated at200and400kV,respec-tively.Elemental maps(eV)by EFTEM were ob-tained by means of the three window method[6,7]. Table1shows pre-edge1,pre-edge2and post-edge of Al–K,Si–K,Mg–K and Cu–K in the present work.3.Results and discussionFig.1shows a brightfield image of the0.5Cu alloy aged at523K for2.4ks.Rod-shaped pre-cipitates are oriented along the[100]and[010] directions of the matrix.In addition,the transverse cross-sections(hereafter referred to as‘‘the lath-section’’)of lath-shaped precipitates are elongated in the[001]direction of the matrix.This lath-section has an elongated form,which is a typical characteristic of the Q0-phase[2–4].Fig.2shows the EFTEM images of the lath-section shown in Fig.1.A typical hexagonal network is observed in the zero-loss image of Fig.2(a).The network has a spacing of 1.04nm,which corresponds to the a-axis lattice constant of the Q0-phase reported in our recent paper[4].Interfacial copper segregation between the Q0-phase and the matrix(hereafter referred to as‘‘Q0/a-Al interface’’)can be observed in the Cu map shown in Fig.2(b),which is pro-vided for comparison with the zero-loss image of Fig.2(a).Mg and Si-maps shown in Fig.2(c)and (d)exhibit a homogeneous distribution through-out the lath-section.Fig.2(e)and(f)show EF-TEM images of the Type-C precipitate in the Al–1.0mass%Mg2Si–0.4mass%Si alloy that had been aged at473K for ks.This precipitate has the same crystal lattice as the Q0-phase,but a different composition.The Q0-phase is the quaternary Al–Mg–Si–Cu,whereas Type-C is the Cu-free ternary Al–Mg–Si[4,8].A Cu-map could not be obtained for the Type-C precipitate,suggesting that the Type-C precipitate does not contain Cu and that no segregation occurs around its periphery.EF-TEM images are known to sometimes include diffraction contrasts[9].To avoid this artifact,in the present study higher loss energy electrons and the three windows method were used to create eV. Had the Cu-map in Fig.2(b)contained diffraction contrast contribution,a Cu-map would have ap-peared in the case of the Type-C precipitate in Fig. 2(f)as well.The absence of a Cu-map in Fig.2(f) confirms the absence of the effect of strain contrast in our Cu-maps.Cu distribution was confirmed by the nano-probe EDS analysis shown in Fig.3(d), in which the analysis points are numbered1 through5.Cu content is higher at the Q0/a-Al in-terface than at other points;11–12at.%higherTable1Parameters used for eVSlit width EnergyoffsetPre-edge1Pre-edge2Post-edgeCu–L50931801856931 Mg–K501305119012451305 Si–K501839170517791839Fig.1.Zero-loss image of the0.5Cu alloy aged at523K for2.4ks.834K.Matsuda et al./Scripta Materialia47(2002)833–837K.Matsuda et al./Scripta Materialia47(2002)833–837835Fig.2.EFTEM images of the Q0-phase in the0.5Cu alloy aged at523K for2.4ks and the type-C precipitate in the excess Si alloy aged at523K for2.4ks.(a)Zero-loss image,(b)Cu–L map,(c)Mg–K map,(d)Si–K map.EFTEM images of the Type-C precipitate,(e) zero-loss image and(f)Cu–L map.than in the matrix.This is in close agreement with the CCD intensity profile (Fig.3(b))obtained from a line in Fig.3(a).EDS results also shows that the Cu content is low at the center of the Q 0-phase.The ratio of Si/Mg at the center of the Q 0-phase is about 1.2,which is similar to that in the case of the Type-C precipitate [8].The average ratio of Mg:Al:Si:Cu for five analyzed points in Fig.3(a)is 6:4:7:1,which is nearly equal to that in our recent report [4].In order to account for Cu segregation,the lattice misfit,d ,at the Q 0/a -Al interface was cal-culated by the following equation:Misfit :d ð%Þ¼100Âd ðhkil ÞQ 0hÀn Âd ðhkl ÞAl i =nÂÂd ðhkl ÞAl Ã;ð1Þwhere n is the integer obtained by truncating the quotient ½d ðhkil ÞQ 0=d ðhkl ÞAl .The misfits d of f 200g Al with respect to f 10 10g Q 0,and that with respect to f 11 20g Q 0are about 11.0%and 2.46%,respectively.Cu atoms,the smallest of the four elements Al,Mg,Si and Cu,would be expected tobe attracted towards the f 200g Al =f 1010g Q 0in-terface expanded by 11.0%.Abe et al.[5]have reported the result of electrical resistivity mea-surements in Al–Mg–Si–Cu alloys,and suggest that Cu is dissolved in the matrix in the early stage of aging because of lower diffusivity and high solubility in Al.However,as aging progresses,Cu most likely segregates towards the precipitate/matrix interface by a process similar to Ag-segre-gation of the X -phase/matrix interface in Al–Cu–Mg–Ag alloy [10].Meanwhile,Murayama et al.have reported AP-FIM results,which indicate that Cu is not associated with GP zones formation,but rather promotes b 00-phase formation during artifi-cial aging at 448K and the b 00-phase evolves to the Q phase after long-term aging [11].Caylon and Buffat [2]investigated the crystal structures of metastable phases in Cu-containing Al–Mg–Si and Si-containing Al–Cu–Mg alloys.They concluded that,in the presence of Cu the b 0-phase forms,rather than the Q 0-phase.Thus,the quaternary metastable phase successively transforms from the QP-or QC-phase,which are precursors of the Q-phase,to Q-phases.Our most recent paper builds on their paper [2],showing that the b 0-phasecanFig.3.(a)Distribution of Cu atoms around the Q 0-phase in Fig.2(a).(b)CCD intensity profile obtained from a line in (a),and (d)the result of EDS analysis obtained from points 1–5marked in (c),which is the same zero-loss image of Fig.2(a).836K.Matsuda et al./Scripta Materialia 47(2002)833–837contain Si or Cu beyond the stoichiometry of Mg2Si and that it transforms to the Type-C pre-cipitate in the presence of excess Si,or to the Q0-phase in the presence of Cu[4].In view of these reports,Cu is believed to form solid-solution in the matrix during the early stage of aging,and to contribute to the nucleation of the metastable b00-, b0-and Q0-phases.As aging proceeds,Cu atoms are attracted to the region of lattice mismatch at the Q0/a-Al interface.Cu segregation probably limits the diffusional growth of Q0-phase and hence producesfiner microstructure than that found in an alloy without Cu.As shown in Fig.2(a),the Q0-phase retains its high coherency at the Q0/a-Al interface for a long time;Cu-segregation should continue until that coherency is mostly lost.Ac-tually,a large lath-shaped phase in the sample aged at higher temperature(573K)shows a uni-form Cu distribution in its Cu-map,although its figure is omitted in this report.From these results in the present work,we suggest that Al–Mg–Si–Cu alloy contains the Type-C precipitate but no Q0-phase.Cu segregation at the Type-C/a-Al interface is consumed in the transformation from the Type-C precipitate to the Q-phase.4.Concluding remarksCu segregation was detected by EFTEM at the Q0/a-Al interface in an Al–1.0mass%Mg2Si–0.5 mass%Cu alloy.No segregation of Cu was de-tected at the periphery of Type-C precipitates in a Cu-free Al–1.0mass%Mg2Si–0.4mass%Si alloy. Thus,the phenomenon is unique to the Q0-phase in Cu-containing Al–Mg–Si alloys.The ratio of Si/ Mg at the central area of the Q0-phase was found to be nearly equal to that of the Type-C pre-cipitate.Cu-segregation at the Q0/a-Al interface probably limits the diffusional growth of the Q0-phase and producesfiner microstructure. AcknowledgementsThis work was supported by the Nanotechnol-ogy Metal Project(NEDO,Japan).The authors would like to express their deep gratitude to the Hokuriku Fabrication Center of Shin-Nikkei Co. 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