Ag对Cu-Fe原位复合材料显微组织和性能的影响(英文原文+中文翻译).pdf

Eff ect of Ag on the microstructure and properties of Cu Fe in situ composites Haiyan Gao Jun Wang Da Shu Baode Sun The State Key Laboratory of Metal Matrix Composites Shanghai Jiao Tong University Shanghai 200030 PR China Received 11 July 2005 received in revised form 24 July 2005 accepted 26 July 2005 Available online 19 August 2005 Abstract Microstructure and properties of three Cu Fe Ag in situ composites were investigated Distribution of Fe and Ag in the matrix was analyzed by energy dispersive X ray spectroscopy The presence of Ag reduced the solution of Fe in the matrix at high tempe rature and the strength and conductivity of Cu Fe Ag composites increased at the same time 2005 Acta Materialia Inc Published by Elsevier Ltd All rights reserved Keywords Cu Fe Ag In situ composite Microstructure Mechanical properties Electrical properties 1 Introduction Binary copper base metal matrix composites contain ing a body centered cubic bcc transition metal such as Nb Fe Cr prepared by mechanical processing have been the subject of extensive research in the past two decades 1 6 Upon solidifi cation of these alloys bcc dendrites form in the copper matrix and subsequent mechanical deformation reduces the dendrites to aligned fi laments with a ribbon like cross section 3 4 7 The limited solubility of these bcc metals in copper allows the matrix to retain a high conductivity making this in situ composite interesting for applications where high conductivity and high strength are required Investiga tions following these lines were also conducted on binary Cu Ag composites 8 11 The Cu Fe system is of particular interest because of the low cost of iron compared to other possible metals Pure Nb and Ag are costly metals which impedes the large scale application of such new materials in devices other than high fi eld magnets 12 However the Cu Fe systems have a lower conductivity than other copper base in situ composites The reason lies in 1 the relatively higher solubility of iron in copper at high temperatures coupled with slow kinetics of iron precip itation at lower temperatures and 2 the particularly harmful eff ect on the conductivity of iron atoms in solid solution namely 9 2 lX cm wt Fe 13 Therefore it is important to remove as much iron from solid solution in the copper as possible Hansen and Anderko 14 reported the following solution correlation x 2540 exp 8800 T which predicts an equilibrium solubility of less than 1 ppm Fe at 253 C If the predicted solubil ity at 253 C could be achieved the conductivity of the copper matrix would be reduced by only 1 IACS InternationalAnnealedCopperStandard 17 241 nX m is defi ned as 100 IACS using the above resisti vity decrement Thermal mechanical treatments have been employed to optimize the strength and conducti vity 13 In the recent years eff orts have been made to widen the spectrum of in situ composites towards ternary copper base alloys 15 18 The development of ternary 1359 6462 see front matter 2005 Acta Materialia Inc Published by Elsevier Ltd All rights reserved doi 10 1016 j scriptamat 2005 07 028 Corresponding author Tel 86 21 62932914 fax 86 21 62932870 E mail addresses gaohaiyan H Gao bdsun sjtu B Sun www actamat Scripta Materialia 53 2005 1105 1109 Cu Fe composites usually aims at further improving the strength and conductivity of conventional binary alloys and reducing the costs as well Adding a third element allows the exploitation of a larger variety of possible kinetic paths for attaining a certain fl ow stress conduc tivity profi le For example Cu 15 wt Fe 0 1 wt Mg has a tensile strength of 1080 MPa with a conductivity of 56 IACS 13 Song et al 19 reported that Cu 9 wt Fe 1 2 wt Aghasastrength conductivity combination of 939 MPa 56 2 IACS through thermal mechanical treatment In this study the eff ects of Ag on the microstructure and properties in the cast and drawn Cu Fe Ag in situ composites were examined 2 Experimental details Three compositions were chosen for investigation namely Cu 12 wt Fe 1 wt Ag denotedCu 12 Fe 1Ag Cu 14 wt Fe 3 wt Ag denoted Cu 14 Fe 3Ag andCu 11 wt Fe 6 wt Ag denoted Cu 11Fe 6Ag Semi sphere ingots of about 20 mm in diameter were separately prepared from electrolytic Cu commercial Fe and Ag with at least 99 9 wt purity using tungsten arc melting with electromagnetic stirring The ingots were hot forged in the open air and then fac ing machined to a sectional dimension of 14 14 mm to remove the oxidation layer and the surface defects Composite wires were produced through cold rolling and subsequent drawing through various successive drawing dies During rolling the sectional dimension of the samples was reduced to 5 5 mm in a series of steps During drawing the sample diameter was further reduced to a minimum diameter of 0 4 mm For com parison purposes a binary Cu 12 wt Fe composite was also prepared along with Cu Fe Ag composites using the same process The cold work strain was defi ned by g ln A0 Af where A0is the initial sectional area obtained after hot forging and Af is the fi nal sectional area The evolution of the microstructure at diff erent strains was investigated using a Sirion 200 fi eld emission scanning electron microscopy FESEM The distribu tion of Fe and Ag in the copper matrix were analyzed by an energy dispersive spectrometer EDS in conjunc tion with the FESEM Tensile tests on deformed wire specimens at room temperature were conducted using a Zwick Roell machine equipped with an extensometer for accurate strain measurements The deformation rate was fi xed at 2 5 10 4s 1at the elastic stage and 5 0 10 4s 1at the plastic stage The ultimate tensile stress UTS was taken as a measure of the strength for comparison purposes because it was very reproduc ible and well defi ned for similar specimens Specimens were tested without any reduction in the gauge diameter due to their relatively small diameter At least three specimens were tested at each wire size and the reproducibility was within 2 Electrical resistivities of specimens were measured using a ZY9858 digital micro ohmmeter at room temperature 3 Results and discussion The four alloys exhibit similar cast microstructure i e the Fe dendrites are embedded in the copper matrix and randomly oriented with respect to the ingot axis The dendrite arms of Cu Fe Ag composites are about 2 4 lm in diameter smaller than that of binary Cu Fe composite 4 6 lm which is in agreement with the observations of Song 15 In addition the greater the amount of Ag added the smaller the Fe dendrites High er magnifi cation observation revealed that few precipi tates could be observed in the matrix due to chill cooling employed in the experiment However besides Fe dendrites some Cu Ag eutectic could also be seen distributing along the copper boundary in Cu 11Fe 6Ag as shown in Fig 1 The equilibrium phase diagram of Cu and Fe reveals that Cu and Fe have small mutual solubility that decays to near zero at the room temperature The maximum solubility of Fe in Cu is 4 1 wt at peritectic tempera ture 1096 C and that of Cu in a Fe is 2 1 wt at eutec toid temperature 850 C Without Ag addition the amount of Fe in the matrix which in this paper is used to refer to the microstructure excluding the Fe den drites should be the same for the four alloys because the total amounts of Fe in the four alloys are all far beyond the maximum solubility of Fe in Cu Cu and Ag have limitedmutualsolubility theirequilibriumphase diagram reveals the maximum solubility of Ag in Cu is 7 9 wt at the eutectic temperature 780 C and down to zero at room temperature During non equilibrium solidifi cation as Ag content is approaching 7 9 wt Fig 1 Microstructure of as cast Cu 11Fe 6Ag alloy Cu Ag eutectic could be seen clearly distributed along the copper boundaries the upper left corner of the picture is a higher magnifi cation image 25 000 of the eutectic 1106H Gao et al Scripta Materialia 53 2005 1105 1109 e g the weight ratio of Ag Cu is 6 7 wt in Cu 11Fe 6Ag the eutectic reaction is likely to take place during chill cooling EDS analysis was used for matrix composition measurements To improve measurement precision selected areas of 10 lm 5 lm in the matrix were analyzedathigher magnifi cationobservationsof 4000 and eight measurements were recorded for each specimen the average results are shown in Table 1 Most of the Fe forms into dendrites during solidifi cation and the rest dissolves in the matrix in the form of solid solution However Ag exists in the matrix either in the form of solid solution or Cu Ag eutectic It should be pointed out that the small deviation of Ag from the nominal composition of the Cu Fe Ag alloys is attri butable to the micro non homogeneity of the material For Cu 11Fe 6Ag in particular the amount of Ag in the matrix is not a constant which may have relevance to the formation of Cu Ag eutectic in the matrix Furthermore the amount of Fe in matrix decreases obviously with an increasing amount of Ag as shown in the fourth line in Table 1 which indicates that the presence of Ag promotes the precipitation of c Fe from liquid Cu i e reduces the maximum solution of Fe in Cu at high temperature This can be explained by selec tive solution of copper Compared with Fe Ag atoms have priority to dissolve in copper because of its similar atom radius electronic structure and crystal structure to copper During solidifi cation as the melt temperature drops near the liquidus curve e g 20 C above the liq uidus a quasi solid phase has formed in the melts due to component fl uctuation At this moment once super saturated Ag and Fe exist in copper melts at the same time Ag atoms with the help of their preferential solu tion properties will occupy many advantageous posi tions and impede further solution of Fe in Cu as well As a result greater amounts of Fe atoms are forced to precipitate in the form of primary phase Microstructures of deformed Cu Fe Ag composites are similar to those reported in previous studies In the longitudinal direction initially randomly distributed dendrites transformed into aligned fi bers gradually while the fi laments take on ribbon like morphology other than circular in the transverse direction In many previous studies 3 4 7 20 it had been reported that the irregular shape of the iron phase resulting from the h110i fi ber textures formed during deformation which promotes plane strain deformation rather than axially symmetric fl ow However the copper matrix does de form in an axially symmetric manner during the wire drawing and the iron fi laments are constrained and forced to fold or twist about the wire axis to maintain compatibility with the matrix resulting in the irregular cross sectional shapes as shown in Fig 2 Further wire drawing results in homogeneity and refi nement of fi la ments It is noted that the fi ner fi laments with a grey contrast with darker Fe ribbons observed in Fig 2 should be Ag fi laments which were deformed from the as cast Cu Ag eutectics As discussed later these fi ner Ag fi laments also contribute to the strength of the composites Fig 3 compares the eff ects of the drawing strain prior to the tensile test on the ultimate stress for Cu 14Fe 3Ag Cu 14Fe 1Ag Cu 11Fe 6AgandCu 12Fe Stress strain curves show exponential dependence for each composite in general which is in accordance with previous research 3 4 21 Because non machined wires were used for the tensile tests failure breaking always appears near the grip holder because of stress concentra tion and the measurements were somewhat lower than the actual tensile stresses At a higher draw ratio while the size of specimens became smaller than 0 5 mm in diameter the stress concentration eff ect became stronger and sometimes small decrements in measurements were Table 1 Distribution of Fe and Ag of composites Cu 12FeCu 14Fe 1AgCu 14Fe 3AgCu 11Fe 6Ag Nominal compositionFe wt 12 014 314 110 6 Ag wt 1 12 85 6 MatrixFe wt 4 54 13 82 5 Ag wt 1 12 82 8 8 3a Fe in form of dendrites wt 7 510 210 38 1 a Amount of Ag measured is given in a range Fig 2 The transverse section of deformed Cu 11Fe 6Ag composite at g 6 9 H Gao et al Scripta Materialia 53 2005 1105 11091107 observed In the range 2 g 5 the strength of Cu 14Fe 3Ag increases fas ter and at a draw ratio of 6 0 the ultimate tensile stress of Cu 14Fe 1Ag Cu 14Fe 3Ag and Cu 11Fe 6Ag composites reach 1119 1578 and 1357 MPa respec tively while that of Cu 12Fe is only 978 MPa Previous studies revealed that during the initial stage of deformation initially randomly oriented dendrites transformed into aligned fi bers and the strength of copper base in situ composite in this stage obeys the rule of mixtures and Hall Petch correlations thereafter Hence the strengths of Cu 14Fe 3Ag and Cu 11Fe 6Ag are close in the range 2 g 5 due to the higher Fe fi ber content Ternary Cu Fe Ag alloys have additional Ag dissolved in the copper matrix leading to solution strengthening of the matrix As a result even with similar iron fi ber content the strength of Cu 14Fe 3Ag is always higher than that of Cu 14Fe 1Ag and so does between Cu 11Fe 6Ag and Cu 12Fe Hong and Hill suggested 9 that the strengthening component due to alloying in Cu 8 wt Ag was 300 MPa If the strengthening eff ect due to Ag precipitation is propor tional to C1 2 where C is the silver content the friction stress due to Ag precipitation in Cu 14Fe 1Ag Cu 14Fe 3Ag and Cu 11Fe 6Ag will be 106 184 and 260 MPa respectively without consideration of the for mation of Cu Ag eutectic which is in good agreement with the observed UTS diff erences between the alloys In addition for Cu 11Fe 6Ag a small amount of Cu Ag eutectic may also contribute to the strength especially as Cu Ag eutectic can deform to small fi bers duringthesubsequentwiredrawingandfurther strengthen the composite Table 2 gives the measured electrical resistivity of the specimens at a draw ratio of 2 5 The resistivities of the composites decrease obviously with Ag addition into the composites i e the presence of Ag increases the conductivity of binary Cu Fe composite which may be due to the lower amount of dissolved Fe in Cu Resis tivities of Cu Fe composites can be evaluated using a parallel circuit model 22 1 qC 1 qCu fCu 1 qFe fFe where fCuand fFeare volume fractions of Cu and Fe respectively Because of minor diff erences in the total amount of fi laments plus the higher resistivity of Fe the diff erence in composite resistivity resulting from the amount of iron fi lament is negligible Therefore the main diff erence lies in the resistivity of copper matrix which can be partitioned into the contribution of four main scattering mechanisms 22 qC qpho qdis qint qimp where qphois the resistivity contribu tion from the phonon scattering qdisthe dislocation resistivity qintthe interface scattering and qimpthe impurity scattering Composites with the same draw ratio have similar qphoand qdis it is apparent that qimp and qintare the resistivity control items Because of the relatively small draw ratio the interface between matrix and iron fi bers is similar and the diff erence in qint between composites is also small The main diff erences in the measured resistivities come from the amount of Fe and Ag dissolved in Cu Many previous investiga tions 7 13 19 reported that the conductivity diff erences between Cu Fe and Cu Nb composites with the same volume fraction of Fe or Nb were also caused by the dis solved Fe in the copper matrix As mentioned in the introduction section every 1 wt Fe dissolved will in crease the resistivity of Cu by 9 2 nX m while 1 wt Ag leads to an increase of only 1 nX m Resistivity diff er ences resulting from diff erent dissolved concentrations of Fe and Ag between Cu Fe Ag and Cu Fe could be calculated based upon the above resistivity decrement the results are listed in Table 2 considering all Ag added Table 2 Resistivities of composites Cu 12FeCu 14Fe 1AgCu 14Fe 3AgCu 11Fe 6Ag Resistivity measured nX m41 238 437 331 5 Resistivity diff erences calculated nX mDue to Fe in matrix 3 7 5 7 18 4 Due to Ag in matrix 1 1 2 8 5 6 Total 2 6 2 9 12 8 Fig 3 Eff ect of draw ratio g on the ultimate tensile strength of the four composites 1108H Gao et al Scripta Materialia 53 2005 1105 1109 in the form of solid solution It is found that the calcu lated diff erences agree well with the measurement diff er ences for three Cu Fe Ag composites 4 Conclusion Ternary Cu Fe Ag in situ composites were investi gated The eff ects of Ag in Cu Fe Ag composites can be summarized as follows 1 refi ning the primary Fe dendrites 2 reducing the solubility of Fe in Cu at high temperature and improving the conductivity of binary Cu Fe composites and 3 strengthening composites in the form of solid solution or Cu Ag eutectic and its fi bers during subsequent deformation So the addition of Ag to Cu Fe gives both better strength and conduc tivity of the in situ composites If proper heat treatments are used during deformation the strength and conduc tivity of the composites will be further improved Acknowledgments The authors acknowledge M Y Xia and B Chen of Shanghai Nonferrous Institute for materials prepa rations References 1 Bevk J Harbison JP Bell JL J Appl Phys 1978 49 6031 2 Funkenbusch PD Courtney TH Kubisch DG Scripta Metall 1984 18 1099 3 Funkenbusch PD Courtney TH Scripta Metall 1981 15 1349 4 Spitzig WA Pelron AR Laars FC Acta Metall 1985 35 2427 5 SnoeckE LecouturierF ThillyL et al ScriptaMater 1998 38 1643 6 Raabe D Hangen U Comput Mater Sci 1996 5 195 7 BiselliI C Morris DG Acta Metall Mater 1994 42 163 8 Benghalem A Morris DG Acta Metall Mater 1997 45 397 9 Hong SI Hill MA Acta Mater 1998 46 4111 10 Hong SI Hill MA Mater Sci Eng 1999 A246 151 11 Han K Vasquez AA Xin Y Kalu PN Acta Mater 2003 51 767 12 Raabe D Ge J Scripta Mater 2004 51 915 13 Verhoeven JD Chuen SC Gibson ED J Mater Sci 1989 24 1748 14 Starke MR Holt DL Acta Metall 1972 20 569 15 Song JS Hong SI Kim HS J Mater Proc Technol 2001 113 610 16 Song JS Kim HS Lee CT Hong SI J M