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Yanchun Zhou, Wanli Gu: Chemical reaction and stability of Ti3SiC2 in Cu during high temperature

Yanchun Zhou, Wanli GuHigh-Performance Ceramic Division, Shenyang National Laboratory for Materials Science, Institute of Metal Research,Chinese Academy of Sciences, P. R. China

1. Introduction

Ti3SiC2 is a novel structure/function ceramic that hasgained interest due to its unique combination of the meritsof both metals and ceramics. Recent works have disclosedthat it has low density, high modulus and strength, highelectrical conductivity, good thermal conductivity, lowcoefficient of friction, good thermal stability, and excellenthigh-temperature oxidation resistance 11-51. The abovesignificant properties are traced to the layered crystal struc-ture and the characteristics of atomic bonding in Ti3SiC2,for which a mixture of metallic, covalent and ionic bondswas shown by electronic structure calculations 16,71. Ta-ble 1 lists the typical properties of Ti3SiC2. The uniquecombination of these properties, especially the high modu-lus, low coefficient of friction and metallic conductivity,makes Ti3SiC2 a promising reinforcement for copper. Pre-vious works demonstrated that Ti3SiC2 dispersion andTi3SiC2 particulate reinforced copper showed superiorproperties [8, 9]. The advantage of Ti3SiC2 reinforced cop-per over other ceramic reinforced one is that high strengthand modulus as well as low coefficient of friction will be

Chemical reaction and stability 0f Ti3SiC2 in Cuduring high-temperature processing 0fCdTi3SiC2 composites

Chemical reactions and stability of Ti3SiC2 in Cu duringprocessing of Cu/Ti3SiC2 composites in the temperaturerange of 900-1070oC were investigated using X-ray dif-fraction and scanning electron microscopy. The results indi-cated that Cu reacted with Ti3SiC2 above 900 oC, and thereaction products were closely related to the reaction tem-perature and the relative ratio of Cu and Ti3SiC2. At lowTi3SiC2 content or temperatures below 1000'C, Cu(Si)solid solution and TiC" were formed, whereas at high reac-tion temperature and high Ti3SiC2 content, Cu*Si interme-tallic compounds like Cu5Si, Cul5Sia and (Cu, Si) 4'as wellas TiC" were observed. The transporting process for the re-action was investigated and described. It was found thatde-intercalation of Si from Ti3SiC2 dominated the reaction,which dissolved in copper to form Cu(Si) solid solution orCu-Si intermetallic compounds such as Cu5Si, Cul5Siaand (Cu, Si) 4'. TiC" was always present as the decomposi-tion product of Ti3SiC2.

Keywords: Ti:SiCz; Cu; Composite; Chemical reaction;Processing; Microstructure

Table 1. Typical properties of Ti:SiCz.

Space group

Lattice parameter

Density

Young's modulus

Vickers hardness

Compressive strength

Tensile strength

Fracture toughness

Brittle{o-ductile transitiontemperature

Electrical conductivity

Thermal conductivity

Thermal expansion coefficient

Oxidation resistance

P63/mmc

a = 3 . 0 6 7 L ; c = 1 7 . 6 a 5 A

4.52 glcm3

326 GPa

4 GPa

900 MPa (RT)500 MPa (1150 'C)

200 MPa (RT)

8 . 5 M P a . m l / 2

1050'c

9.6 x 106 C) rm 1

4 3 W m . K

8.6 x 10-6 /K (a-axes), 9.7 x1}-6/K(c-axes)

2 x 10-8 kgz . * -+ r - t

(at 1000'C)

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Yanchun Zhou, Wanli Gu: Chemical reaction and stability of Ti:SiCz in Cu during high temperature

achieved without loss of electrical conductivity. In prepar-ing Cu/Ti3SiC2 composites, however, interfacial reactionsbetween copper matrix and Ti3SiC2 is one of the key factorsin controlling the microstructure and properties of the com-posites. The stability of Ti3SiC2 in copper matrix is also im-portant in selecting the preparation method and processingparameters. When higher temperature or casting process isselected in the processing of Cu/Ti:SiCz composites, inter-facial reactions and stability of the reinforcements are ex-tremely important. A good understanding and the abilityto control the interfacial reactions between Cu matrix andTi3SiC2 enable the mechanical and electrical properties tobe optimized. Up till now, to the authors' knowledge, no in-formation on the reaction between copper and titanium sili-con carbide is available. The purpose of this paper is to re-port, for the first time, on the reactions between Ti3SiC2and Cu in the temperature range of 900- 1070 "C, which isthe typical temperature range used in the processing of Cumatrix composites especially when the powder metallurgymethod is selected. The reaction products and transport pro-cess that control the reaction are also described. This workis useful in both selecting parameters for the processing ofCu/Ti3SiC2 composites and understanding the factors thataffect the stability of Ti3SiC2.

2. Experimental procedure

2.1. Material preparation

The starting materials used in this work were Cu powders(from Institute of Non-Ferrous Metals, Beijing, China) witha mean particle size of 40 pm and Ti3SiC2 particles (pre-pared at the authors' lab) with an average particle size of10 pm. The Ti:SiCz particles were synthesized by the so-called fluctuation method utilizing Ti, Si, and graphite pow-ders as initial materials. Details for the synthesis and char-acteristics of Ti:SiCz particles were described elsewhere

[10]. Briefly, the Ti3SiC2 particles were prepared by heating

the mixture of Ti, Si and graphite powders to 1300'C in Aratmosphere and held for 10 minutes. Then, the temperaturewas decreased to 1250oC and held for 120 minutes. Theas-prepared powders contained TiC by X-ray diffractionanalyses, which were described in the previous works [10,I 1l . A very small amount of SiC also existed as an impurityphase which could be distinguished from TiC in TEM dueto the high density of stacking faults in SiC [12]. To investi-gate the interfacial reaction between Ti3SiC2 and Cu, Cu/Ti3SiC2 composites with 20 to 7O vol.Vo Ti3SiC2 were pre-pared by hot pressing the mixture of Cu and Ti3SiC2 pow-ders at 900- 1070 oC under 40 MPa for 30 minutes, respec-tively. As no obvious reaction between Ti3SiC2 and Cu wasobserved in the samples made at temperatures below900'C, no attention was paid to the microstructure of thesamples prepared in this temperature region.

2.2. Investigation of the reactions between Ti3SiC2 and Cu

To identify the reaction products, the Cu/Ti3SiC2 compo-sites prepared at different temperatures and with differentTi3SiC2 contents were characterized by X-ray diffraction(XRD) and scanning electron microscopy (SEM). Powdersdrilled from the bulk samples prepared by the hot-pressingmethod were used for XRD analysis. X-ray diffraction datawere collected by a step-scanning diffractometer (RigakuD/max-2500PC, Japan) using Cu Ko radiation. The pol-ished surfaces of the composites were observed in an5-360 scanning electron microscope (Cambridge Instru-ment Ltd., UK) in secondary and backscattered modes. En-ergy dispersive X-ray analysis (EDX) was used to identifyand calculate the approximate compositions of the reactionproducts. To understand the reaction mechanism, bulkTi3SiC2 and bulk Cu were stacked together and werebonded at 900 "C and 1000 oC under 40 MPa for 1 and 2 h,respectively. The composition concentration profile acrossthe bonding interface was monitored and the high tempera-ture reaction route was purposed.

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40Fig. l. X-ray diffraction patterns of (a) as-mixed powders and hot-pressed samples ofCu-50 vol. 7o TlSiC2 prepared under 40 MPaat (b) 900'C, (c) 950'C, (d) 1000'C and (e)1070'C for 30 minutes, respectively.

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Yanchun Zhou, Wanli Gu: Chemical reaction and stability of Ti:SiCz in Cu during high temperature

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Fig. 2. X-ray diffraction patterns of Cu-20 vol.% Ti3SiC2 reacted at (a) 900 "C,(b) 950 "C, (c) 1000'C, and (d) 1070 "C for 30 minutes, respectively.

3. Results

3.1. Reaction between Cu and TirSiCz at differenttemperatures and compositions

The Ti:SiCz particles were uniformly distributed in the as-prepared samples. X-ray diffraction analysis revealed thatat temperatures below 900'C no apparent reaction betweenTi3SiC2 and Cu was observed. Thus, attention was onlypaid to the samples prepared at temperatures above900'C. Fig. 1 shows the X-ray diffraction patterns of theas-mixed Ct-50vol.Vo Ti3SiC2 powders (Fig. 1a) and hot-pressed samples prepared under 40 MPa at 900 "C(Fig. lb), 950'C (Fig. 1c), 1000'C (Fig. ld) and 1070'C(Fig. le) for 30 minutes, respectively. It is seen from the fig-

ure that besides Cu and Ti3SiC2 a little amount of TiC,exists in the as-mixed powders (because the carboncontent in TiC changed with the extent of reaction, xwas used in this work, and cubic phase was used forthe identification of TiC" [13]). After hot pressing at900"C, the intensity of (111) and (200) reflectionsfrom TiC" increased. However, no diffraction peaksfrom new phases could be detected. For the sampleprepared at 950 oC, the intensity of the diffractionpeaks of TiC, became even stronger, indicating thatthe amount of TiC" increased with the preparationtemperature. In the sample prepared at 1000oC, be-sides Ti3SiC2, Cu and TiC", reflections from newlyformed Cu5Si and Cul5Sia were identified from theX-ray diffraction pattern (Fig. 1d). In the sample pre-pared at 1070'C, the intensity of reflections from Cualmost disappeared. Meanwhile, the intensity of re-flections from TiC" changed synchronously with theincrease of the intensity of reflections from Cu5Siand Cu15Sia. The figure also shows that with the in-crease in the preparation temperature, the peak posi-

tion of Cu shifted to small angle, which will be discussedin later sections.

The above results demonstrate that Cu reacted withTi3SiC2 at high temperatures above 900 "C and the reactionproducts were temperature dependent. We also investigatedthe effect of Ti3SiC2 content on the extent of reaction. Toachieve this, a composition of Cu-20 vol.7o Ti3SiC2 wasused first. Fig. 2 shows the X-ray diffraction patterns ofCu-20 vol.Vo Ti3SiC2 composite prepared at 900'C(Fig.2a) , 950'C (Fig.2b) , 1000"C (Fig.2c) and 1070"C(Fig. 2d) for 30 minutes, respectively. The figure shows thatwith the increase of temperature, the intensity of TiC" in-creases but no new phases like Cu5Si and Cul5Sia are iden-tified. Thus, we can conclude at this point that at lowTi3SiC2 content the reaction only resulted in the increaseof TiC" without formation of compounds like Cu5Si andCu15Sia. We have known that [14] Si can be dissolved inCu to form solid solution, we will show in the later section

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Fig. 3. X-ray diffraction patterns ofCu-30,40,60 and 70 vol.% Ti3SiC2 prepared at 1000'Cfor 30 minutes.

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Yanchun Zhou, Wanli Gu: Chemical reaction and stability of TijSiC2 in Cu during high temperature

that the excess of Si produced due to the reaction betweenCu and Ti3SiC2 goes into Cu to form Cu(Si) solid solution.To further investigate the effect of Ti3SiC2 content on thereaction products, Cu-30 vo7.7o Ti3SiC2, Cl-40 vol.%Ti3SiC2, Cu-60 vol.7o Ti3SiC2, and Cu-70 vol.% Ti3SiC2composites were prepared at 1000 oC for 30 minutes, andthe X-ray diffraction patterns of these composites areshown in Fig. 3. For Cu-30 vol.Vo Ti3SiC2 (Fig. 3a) andCu-40 vol.Vo Ti3SiC2 fig. 3b) composites, no new phaseslike Cu5Si and Cul5Sia were identified in the X-ray diffrac-tion patterns as was the case for the Cu-20 vol.Vo Ti3SiC2composite. For the Cu-60 vol.Vo Ti3SiC2 composite(Fig. 3c), Cu5Si and Cul5Sia formed as reaction products.For the Ct-J} vol.Va TilSiC2 composite (Fig. 3d), theX-ray diffraction pattern was more complicated. The inten-sity of diffraction peaks from Cul5Sia was stronger and that

of diffraction peaks from Cu5Si was weaker than those forthe Cu-60 vol.% Ti3SiC2 composite. Generally, it is diffi-cult to distinguish Cu15Si4 from Cu5Si in X-ray diffractionpatterns due to overlap of many peaks between the twocompounds, but the characteristic peak position of Cul5Sia(321) at 20 = 34.6', which is not overlapped with the peakof Cu5Si, can be used to distinguish it from Cu5Si. In sucha way we conclude that the intensity of Cul5Sia in the Cu-'70

vol.Vo Ti3SiC2 composite became stronger. Meanwhile,the intensity of reflections from elemental Cu further de-creased such that some reflections almost disappeared. Be-sides Cu5Si and Cu15Sia, a new phase (Cu, Si) ry'was identi-fied in the Cu-'70 vol.Vo Ti3SiC2 composite. We have knownfrom the literature (Cu-Si phase diagram) that the contentof Si in (Cu, Si) ri' is from 22.3 at.Vo to 23.4 at.Vo ll4l,which is higher than that in Cu15Sia. The formation of high

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Yanchun Zhou, Wanli Gu: Chemical reaction and stability of Ti3SiC2 in Cu during high temperature

Fig. 5. Back-scattered electron images of Cu-50 vol.% Ti3SiC2 pre-pared at 1000'C for 30 min.

Si content Cu-Si compounds like (Cu, Si) 4' in the Cu-1O vol.Vo Ti3SiC2 composite indicates that higher contentof Ti3SiC2 in the Cu-Ti3SiC2 composite is favorable forthe formation of Cu-Si compounds with higher siliconcontent. From the investigation of the effect of Ti3SiC2 con-tent on the extent of high temperature reaction, we haveknown that the relative amount of Cu and Ti3SiC2 is a domi-nant factor in affecting the reaction products. At lowTi3SiC2 content, Cu reacted with Ti3SiC2 to form Cu(Si) so-lid solution and TiC,. At high Ti3SiC2 content, beside theformation of Cu(Si) solid solution and TiC*, Cu reactedwith Ti3SiC2 to form Cu-Si intermetallic compounds likeCu5Si, Cul5Sia and (Cu, Si) ry'.

3.2. Reaction path

In the above section, we have shown that during the high-temperature processing of Cu/Ti3SiC2 composite, Cu re-acted with Ti3SiC2, and the reaction is dependent on bothTi3SiC2 content and temperature. To reveal the reactionprocess, a polished pure bulk Cu and a bulk Ti3SiC2 arestacked together and hot pressed at 900"C and 1000'C

with a pressure of 40MPa for 60 and 120 minutes, respec-tively. The bonded samples were cut across the interfaceand were then polished without etching and were examinedin a scanning electron microscope. The back-scattered elec-tron images and the EDX line scan across the interface ofthe bonded Cu/Ti3SiC2 at 1000oC are shown in Fig.4.Two features are obvious in the figure. The first feature thatcan be seen from the EDX line scan (Fig.4c and Fig.4d)along the line shown in Figs. 4a and b is that inter-diffusionof Si and Cu occurred during the high-temperature bonding.Cu diffused into the grain boundary of Ti3SiC2 first andthen reacted with the Si in Ti3SiC2 to form Cu(Si) solid so-lution (bright line at grain boundary of Ti3SiC2 in bothFig. 4a and Fig. 4b), which resulted in the formation of TiC"(dark part marked T in Fig. 4a) near the bright Cu diffusedregion. The above conclusion was made based on thesemi-quantitative analysis of the composition difference ofdifferent regions across the grain boundary by EDX. Thediffusion of Cu into the grain boundary of Ti3SiC2 is alsoobvious in Fig. 4c wherein the Cu content in the positionof bright line is higher, as marked Gl , G2, G3 in the figure.The second feature is that interfacial reaction layers wereformed, in which layer A was composed mainly of Cu andSi as well as Ti, whereas layer B was mainly composed ofTi and Si, based on the semi-quantitative composition ana-lysis by SEM/EDX. The concentration of C was small andnot accurate due to resolution limit. EDX semi-quantitativeanalysis revealed that the Cu content in the reaction-formedlayer B is less than that in layer A. The concentration gradi-ent of Cu near the reaction-formed interfacial layers impliesthat the reaction-formed interfacial layer A consists ofCu5Si, Cul5Sia and TiC,, while interfacial layer B consistsof TiC", Cu(Si) solid solution and unreacted Ti3SiC2. Com-parison of Fig. 4a and Fig. 4b shows that the thickness ofthe interfacial layers increased with the reaction time. Themicrostructure characteristic of the Cu/Ti3SiC2 interfacebonded at 900'C was quite similar to that of Fig. 4a. Theonly difference is that the thickness of the reaction layerwas not as large as that bonded at 1000 oC. For brevity,these interfacial microstructures are not shown here.

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Fig.6. Slow-scanned X-ray diffraction pat-terns of pure copper and Ct-20 vol.Va Ti3SiC2composites prepared at 900'C, 950 "C,1000"C, and 1070"C for 30 minutes, respec-tively. Note, Si was used as an internal labelfor calibration of peak positions.

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Yanchun Zhou, Wanli Gu; Chemical reaction and stability of Ti3SiC2 in Cu during high temperature

To further understand the progress of the reaction, thehot-pressed samples were also investigated in the scanningelectron microscope. Fig. 5 shows a back-scattered electronimage of the polished surface of a Cu-50 vol.To TlSiC2composite prepared at 1000'C for 30 minutes. The atomicpercent ages from EDX analysis in the bright, gray andvery dark region are Cu: T i : S i = 78 :8:14, Cu: T i : S i =4l :51: 8 and Cu: T i : S i = 4 :94:2, respect ive ly . Based onthe above result, the bright region is assumed as Cu(Si) so-lid solution, the gray region is Ti3SiC2, Cu(Si) and TiC",the dark region is TiC". Again, trace of the diffusion of Cuinto Ti3SiC2 is seen in the figure as marked A in Fig. 5,where in the Ti:SiCz particle there is Cu (bright contrastwithin the particle). It is also interesting to note that in eachdark TiC" particle (confirmed by EDX) there is a brightlayer of Cu-Si. These microstructure characteristics inFig. 5 confirm that the reaction between Ti3SiC2 and Cu isclosely related to the inter-diffusion of silicon and copper,which resulted in the formation of Cu(Si) solid solutionand TiC" due to the decomposition of Ti3SiC2. Cu-Si inter-metallic compounds formed with the further reaction.

4. Discussion

Analysis of the X-ray diffraction patterns for the Cu-5O vol.Vo Ti3SiC2 composites prepared at different tem-peratures indicates that Cu reacts with Ti3SiC2 at hightemperatures above 900 oC. In the samples prepared at900'C and 950 oC, no new phase was identified. The in-crease in the intensity of TiC" was assumed to be due tothe decomposition of Ti3SiC2 by forming a solid solutionof Cu(Si) and TiC". The decomposition of Ti3SiC2 inducedby de-intercalation of Si (forming a Cu(Si) solid solutionin this case) was previously described [11, 15]. Zhou etal. demonstrated in their early works [7 , l5l that the struc-ture of Ti3SiC2 could be described as a layer of Si interca-lated into the { 111} twin boundary of TiC and the chemi-cal bonding between Si and Ti was relatively weakcompared to the strong Ti-C bonding. The intercalationof Si into the twin boundary of TiC leads to the formationof Ti3SiC2 and the de-intercalation of Si from Ti3SiC2 re-sults in the decomposition of Ti3SiC2 [15]. Because of theabove structure relations, TiC always exists as the mainimpurity phase in Ti3SiC2. On the contrary, addition of Sito TiC produces Ti3SiC2. Based on this theoretical work,if Si was de-intercalated from Ti3SiC2 in a certain en-vironment, such as in a C-rich atmosphere at high tem-

Table2. Lattice spacing and atomic percent of silicon in copper.

peratures [11], Ti3SiC2 would be decomposed (and SiCwould form [11]). Since the de-intercalation of Si fromTi3SiC2 resulted in the decomposition of Ti3SiC2 and theformation of TiC" [7, 11, 15], in this work the increased in-tensity of TiC" with temperature in Ct-50 vol.Vo Ti3SiC2composite was attributed to the reaction of Cu withTi3SiC2 to form Cu(Si) solid solution and TiC,. The as-sumption that Cu and Si form a solid solution was sup-ported by the shift of reflections for Cu to the low angle,which resulted from the change of lattice parameters. An-dersen had studied the change of lattice constant of copperinduced by solute silicon atom [16]; his experimental re-sults were fitted to a straight line (by the present authors)as Equation (1)

a=0.36150 + 0.000065"r (1)

where a is the tested lattice constant of copper (nm) and r iscontent of Si(atVo). The solid solubility of silicon in coppercould be determined by measuring the lattice constant ofcopper exactly [16]. Fig. 6 shows X-ray diffraction patternsobtained with a slow scanning step of 0.02 degree and aspeed of 4 seconds per step for pure copper and the Cu-20 vol.Vo Ti3SiC2 composite prepared at 900'C, 950'C,1000 "C, and 1070 oC for 30 minutes, respectively. To deter-mine the lattice constants, Si was added to the compositepowders as an internal label for calibration of peak posi-tions. The position of the detected (220) peak position ofcopper was corrected by a least-square method withSi(400) and Si(331) peak positions. The calculated proce-dure of pure Cu is shown as follows: the detected peak posi-tions of Si1+o0), Si1331; and Cuezo)are69.067',76.332" and74.063', whilst the corresponding theoretical values forSi1+oo; and Si1331; are 69.100' and'16.348', respectively.The error in this range is considered to be linear with angle,and the detected peak position of Si can be calibrated by thefollowing equations:

69.061a+ f =69.100 (2)

76 .332a+ f=16 .348 (3 )

Solving the above equations simultaneously, we obtaina=0.99766 and B=0.19462, so the peak position ofCrtrzzo,, can be calibrated as:

Peak6. =14.063a + f =14.0885 (4)

According to the Bragg equation and the corrected peak po-sition of Cupzo), the lattice spacing of copper can be calcu-lated easily. To reduce the systematic error, the difference

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Yanchun Zhou, Wanli Gu: Chemical reaction and stability of Ti3SiC2 in Cu during high temperature

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of lattice spacing between pure Cu and composite samplesprepared at different temperatures was used to comparewith the results of Andersen, which are listed in Table 2.Itis seen from the table that the solid solubility of silicon incopper increases with temperature although the initial con-tent of Ti3SiC2 is the same in all samples. The change of lat-tice parameters with temperature gives a direct evidence forthe formation of solid solution of Cu(Si), which induced thedecomposition of Ti3SiC2. Based on the above results, thereaction between Ti3SiC2 and Cu at low Ti3SiC2 content orhigh Ti3SiC2 content but below 1000 "C can be described as:

Ti3SiC2 + Cu -+ 3 TiC6 67 + Cu(Si) (5)

where Cu(Si) denotes the solid solution of Cu and Si, andTiC6.67 instead of TiC, was used to simplify the equation.In the Cu-50 vol.% Ti3SiC2 composites prepared at 1000 "Cand 1070 oC, beside the increase of the intensity of TiC",new phases of Cu5Si and Cul5Sia formed, indicating thatthe reaction between Cu and Ti3SiC2 became more serious.We attributed the formation of new phases such as Cu5Siand Cul5Sia to the fact that the limited solubility of Si inCu was only 8 at.Vo ll4l so that the excess Si, producedfrom the decomposition of Ti3SiC2, that could not dissolvein Cu would react with it to form new compounds.

The reaction between Ti3SiC2 and Cu at high Ti3SiC2content and above 1000'C can then be described as:

Ti3SiC2 + Cu

-+ 3 TiC6.67 + Cu(Si) + Cu5Si + Cul5Sia + (Cu, Si) 4' (6)

The first bracket denotes that a portion of Si is dissolved inCu, which has been confirmed by the shifted reflections inFig. 2 and Fig. 6 as well as the calculated change of latticeparameters in Table 2. The second bracket denotes a Cu-Si compound, which has a complicated structure and usual-ly is named rf inthe Cu-Si phase diagram [14].

We have also known from the microstructure observationon the bonded interface and the hot-pressed bulk compo-sites that the reaction procedure between Cu and Ti3SiC2can be described as follows. The diffusion of Cu into the in-terfacial region and the subsequent formation of Cu(Si) so-lid solution caused the decomposition of Ti3SiC2 into TiC,.When the Ti3SiC2 content is high such that the decomposi-tion-produced Si content exceeds the solubility of Si in Cu,intermetallic compounds such as Cu5Si and Cul5Sia formbeside TiC, and Cu(Si) solid solution.Our work demonstrated that Cu reacted with Ti3SiC2 athigh temperatures and that the reaction depended on tem-perature and Ti3SiC2 content. The results of this work areinstructive in selecting the preparation method and process-ing parameters of Cu/Ti3SiC2 composite. If Ti3SiC2decomposed into TiC* during processing of Cu/Ti3SiC2composite, the electrical conductivity of the Cu matrixcomposite would be lost (although the strength and modu-lus would be dramatically improved) because the electricalconductivity of reaction-formed TiC, is much lower thanthat of Ti3SiC2. Due to the reaction between Cu matrix andthe TijSiC2 reinforcement at temperatures above 900'C,high-temperature processing such as the casting method isnot recommended. Powder metallurgy or other methodswith the processing temperatures below 900 oC is desirableto make Ti3SiC2 reinforced Cu. Our previous works demon-strated that the Ti3SiC2 particle reinforced Cu [8] andTi3SiC2 dispersion reinforced Cu [9] prepared at 800 and

56

850 "C using powder metallurgy method had superior me-chanical and electrical properties. But significant loss ofconductivity was observed in the composites prepared athigher temperatures. This work is also helpful in under-standing the stability of Ti3SiC2, i. e., if Si was de-interca-lated from Ti3SiC2 it will decompose to TiC,.

5. Conclusions

The reaction between Cu and Ti3SiC2 occurred in the tem-perature range of 900- 1070 oC, and the extent of reactionwas closely related to the temperature and Ti3SiC2 content.At low Ti3SiC2 content or temperatures below 1000 "C,Cu(Si) solid solution and TiC, were formed, whereas athigh reaction temperature and high Ti3SiC2 content newphases like CusSi, Cul5Sia and (Cu, Si) ry' were observed.De-intercalation of Si from Ti3SiC2 dominated the reaction,which dissolved in copper to form solid solution or formedCu-Si intermetallic compounds like Cu5Si, Cul5Sia and(Cu, Si) 11' . TiC, always existed as the decomposition pro-duct of Ti3SiC2. Due to the reaction between Cu matrixand the Ti3SiC2 reinforcement at temperatures above900 "C, it is desirable to make TijSiC2 reinforced Cu below900'c.

This work was supported by the National Outstanding YoungScientist Foundation for Y. C. Zhou under Grant No.59925208, Nat-ural Sciences Foundation of China under Grant No.50232040,No.50072034, '863' project, and High-tech Bureau of the ChineseAcademy of Sciences.

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(Received June 20, 2003; accepted October 1,2003)

Correspondence address

Dr. Yanchun ZhouProfessor and Director of High-Performance Ceramic DivisionShenyang National Laboratory for Materials ScienceInstitute of Metal Research, Chinese Academy of Sciences72 Wenhua Road, Shenyang I 10016, P. R. ChinaTel.: +86242397 1765Fax: +86242389 1320E-mail: yczhou@ imr.ac.cn

Z. Metallkd. 95 (20041