Materials Science and Engineering A276 (2000) 243–249

Effect of interfacial properties on tensile strength inSiC/Ti–15–3 composites

Akira Fukushima a,*, Chikara Fujiwara a, Yutaka Kagawa b, Chitoshi Masuda c

a Nagoya Aerospace Systems, Mitsubishi Hea6y Industries, Ltd., 10, Oye-cho, Minato-ku, Nagoya 455-8515, Japanb Institute of Industrial Science, Uni6ersity of Tokyo, 7-22-1, Roppongi, Minato-ku, Tokyo 106-0032, Japan

c National Research Institute for Metal, 1-2-1, Segen, Tsukuba, Ibaraki 305-0047, Japan

Received 7 December 1998; accepted 16 February 1999

Abstract

Effect of interfacial properties on tensile strength in unidirectional SiC/Ti–15–3 composites was discussed with varyingconsolidation time of the composites. The strength of composites and extracted fibers decreases with increasing consolidation timeof the composites, while the strength of matrix is independent of it. Furthermore, the interfacial shear strength increases withincreasing consolidation time of the composites, while the interfacial sliding stress is independent of it. Based on theseexperimental results, it is clear that the rule of mixtures modified with the shear transfer mechanism at the interface, characterizedby the interfacial shear strength, could estimate the tensile strength of titanium matrix composites more accurately than theconventional rule of mixtures. © 2000 Elsevier Science S.A. All rights reserved.

Keywords: Titanium matrix composite; SiC; Ti–15–3; Interface; Tensile strength

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1. Introduction

It is believed that titanium matrix composites could bepromising materials for aerospace structural applicationsbecause of their high specific strength and stiffness atintermediate temperatures. The properties of compositesvary because of the interfacial reaction between fiber andmatrix during the consolidation process and the sec-ondary fabrication process and in service at high temper-atures. It is shown that the mechanical properties of thefiber/matrix interface play an important role in themechanical properties of the composite [1–7]. But therelationship between interfacial properties and mechani-cal properties of composites has not been establishedquantitatively yet. The objectives of this study are toreveal the relationship between tensile properties of thecomposites and interfacial properties of the fiber/matrixinterface and to assess the rule of mixtures modified withthe interfacial shear strength.

2. Experimental procedure

2.1. Composite materials

The material used in this study was unidirectionalaligned continuous silicon carbide fiber-reinforcedmetastable b titanium alloys (Ti–15V–3Cr–3Al–3Sn:hereafter denotes Ti–15–3). Detailed chemical analysisof the matrix indicated that it was composed of 15.22wt% V; 3.26 wt% Cr; 3.12 wt% Al; 2.94 wt% Sn; 0.23 wt%Fe; 0.101 wt% O; 0.005 wt% N; 0.009 wt% C; 0.0060 wt%H; and the remainder Ti. The SiC fiber (SCS-6, producedby Textron Systems, Lowell, MA, USA) has a diameterof 140 mm, with a 3-mm thick double carbon coatingsurface layer. The matrix alloy foil, whose thickness was130 mm, was supplied by Kobe Steel Ltd.(Kobe, Japan).

Four fiber layers composite panels were fabricatedthrough foil/fiber/foil consolidation process. The stackedmaterials were hot isostatic pressed (HIPed) at 1153 Kunder a hydrostatic pressure of 100 MPa. In order tostudy the effect of the interfacial properties and theconstituent properties on the composite behavior, theconsolidation times were varied: 5.4, 21.6, 86.4 and 360ks.

* Corresponding author. Tel.: +81-52-6118013; fax: +81-52-6119353.

E-mail address: akira–fukushima@mx.nasw.mhi.co.jp (A.Fukushima)

0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.

PII: S0921 -5093 (99 )00151 -3

A. Fukushima et al. / Materials Science and Engineering A276 (2000) 243–249244

Fig. 1. Configurations of tensile test specimen: (a) composite and (b)matrix.

Fig. 3. Reaction zone growth rate at several temperatures.

Fig. 4. Arrhenius plot of SiC/Ti–15–3.

Before the consolidation, the stacked materials wereput into steel capsules and were evacuated at 873 K toa vacuum of 10−3 Pa for 7.2 ks and then sealed. Afterconsolidation, the panels in HIPed capsules were solu-tion treated at 1053 K for 1.8 ks and then aged at 755K for 60 ks. Following the heat treatments, the panels(80 mm in length, 50 mm in width, and 2 mm inthickness) were decapsuled and finished. Matrix foillaminated panels were also fabricated and heat treatedat the same conditions of the composite for evaluationof tensile properties.

2.2. Reaction kinetics

Additional heat treatment was performed for somecomposites, which had been consolidated at 1153 K for

5.4 ks, in a vacuum of 10−3 Pa at 1203 and 1253 K.These heat-treatment temperatures, including the con-solidation temperature, are above the b transus temper-ature range 1023–1043 K [8]. The heat-treatedspecimens were cut and polished perpendicular to thefiber axis, etched and observed with both optical micro-scope and scanning electron microscope (SEM). Etch-

Fig. 2. Interfacial reaction zones: (a) consolidated at 1153 K for 5.4 ks and (b) consolidated at 1153 K for 5.4 ks and heat treated at 1253 K for5.4 ks.

A. Fukushima et al. / Materials Science and Engineering A276 (2000) 243–249 245

ing was done by 1% HF+3% HNO3 acid. Meanreaction zone thickness were measured from randomlyselected portions in the SEM micrographs.

2.3. Tensile properties of extracted fibers

Fibers were carefully extracted from the compositepanels using 10% HF acid at 353 K. Thereafter, theextracted fibers were rinsed in water and dried in air at373 K. A total of 20 fibers were subjected to tensile testfor each consolidation condition. Tensile tests werecarried out using an Instron type universal testingmachine at room temperature (296 K). All the testswere conducted using a 17 mm gauge length with acrosshead speed of 0.04 mm min−1. Pristine fibers werealso evaluated for comparison. Test condition was thesame as that of the extracted fibers

2.4. Tensile properties of composite and matrix

The configurations of tensile test specimens of com-posite and matrix were rectangular type and dog-bonedtype, respectively, as shown in Fig. 1. diamond saw andthen both surfaces were mechanically polished with1-mm diamond paste finish. The thickness of specimenswas about 0.5 mm.

Interfacial mechanical properties were examined atroom temperature by fiber pushout test. Details ofexperimental apparatus is shown elsewhere [9]. Theapplied load was measured by a load cell which wasmounted on the specially designed load cell holder.Indenter displacement was measured by laser reflectivetensile tests were done using an Instron type universaltesting machine at room temperature. An extensometerwith a 15-mm gauge length was attached to the edge ofthe specimen to measure the elongation. In addition,two 1-mm strain gauges were fixed to both surfaces tomeasure the 0.2% proof stress and Young’s modulus.All the tests were conducted at the crosshead speed of0.1 mm min−1. The mean fiber volume fraction ofcomposite tensile test specimens was 25%.

2.5. Interfacial mechanical properties

The mounted composites were cut perpendicular tothe fiber axis with a low speed e displacement meter. Inaddition, acoustic emission (AE) event count rates wasalso monitored. The cylindrical indenter was a round-bottomed sintered tungsten carbide punch with a di-ameter of 100 mm. The specimen was placed onto thetop of the bronze support plate and the center of thefiber was located at the center of a 500-mm diameterhole. Then the indenter was aligned over the 140-mmdiameter fiber visually, by use of a high magnificationvideo microscope. All tests were conducted in air at 296K with a constant indenter speed of 1 mm s−1. All thedata were stored with a digital memory scope at asampling time of 10 ms and analyzed withmicrocomputer.

Fig. 5. Variation of the fiber strength as a function of the consolida-tion time at 1153 K

Fig. 6. Variation of the fiber strength as a function of the reactionzone thickness.

Fig. 7. Weibull plots of extracted fibers from SiC/Ti–15–3.

A. Fukushima et al. / Materials Science and Engineering A276 (2000) 243–249246

Fig. 8. Fiber surfaces extracted from composites: (a) pristine fiber and (b) consolidated at 1153 K for 86.4 ks.

3. Results and discussions

3.1. Reaction kinetics

Fig. 2 shows a typical SEM photographs at SiC/Ti–15–3 interfaces. The measured reaction zone thick-nesses are plotted in Fig. 3 as a function of the squareroot of heat treatment time for each of the three heattreatment conditions: consolidation (1153 K) and addi-tional heat treatment (1203 and 1253 K). The reactionzone thickness, x, increases linearly with the squareroot of the heat treatment time. This indicates that thereaction kinetics between fiber and matrix is diffusioncontrolled. Thus, the kinetics of reaction zone growthcan be described by a parabolic equation of the form:

x=kt1/2+b (1)

where x is the reaction zone thickness, k is the reactionconstant and t is the time at a specific temperature. Thereaction constant, k, is given by Arrhenius relation:

k=k0 exp�−Q

RT�

(2)

where k0 is the pre-exponential factor, Q is the activa-tion energy for the process, R is the gas constant and Tis the exposure temperature in degrees Kelvin. Rewrit-ing Eq. (2) yields:

ln k= ln k0 −Q

RT(3)

Plotting ln k versus 1/T yields a straight line with aslope of −Q/R (Fig. 4). A least squares line fitting ofthe relation gives the values of Q and k0, 143 KJ mol−1

and 8.7×10−4 m s1/2, respectively. This activationenergy of 143 KJ mol−1 is higher than 80.51 and 74.7KJ mol−1 measured by Gundel et al. [10] and Morel[11], respectively. But their values were measured in anextremely thick reaction zone up to 10 mm relative tothat of up to 4 mm in this study. The value in this studywould be more reliable for thin reaction zone thickness.

Hall et al. show that the reaction proceeds by initialformation of a layer of TiC followed by a layer ofmixed silicides [12]. The difference between reactionproducts in the thin and thick zones could result in thedifferent activation energy between this study and pre-vious studies.

Fig. 9. Typical stress-strain curves of composite and matrix.

Fig. 10. Tensile properties of composite and matrix as the function ofconsolidation time at 1153 K.

1 Gundel et al. calculated the activation energy of 161 KJ mol−1

using k=koexp(−Q/RT) in their paper [10]. Using k=ko exp(−Q/RT), as in this study, the result is 80.5 KJ mol−1.

A. Fukushima et al. / Materials Science and Engineering A276 (2000) 243–249 247

Fig. 11. Grain growth of Ti–15–3 as the function of consolidationtime at 1153 K.

3.2. Tensile properties of extracted fibers

Fig. 5 shows the effect of consolidation and heat-treatment time on the strength of the extracted fiber.The strength of the extracted fiber decreases with in-creasing consolidation time. The fiber strength dependson the fiber surface defects and the reaction products asa result of reaction at interface between fiber andmatrix. Then the strength of the extracted fiber isrearranged as a function of the reaction zone thicknessin Fig. 6. This result indicates that the critical reactionzone thickness of the extracted fibers, in which the fiberstrength decreases, is about 1 mm. This value of thecritical reaction zone thickness is in good agreementwith 0.93 mm measured by Yang et al. [2].

The statistical distribution in fiber strength is gener-ally characterized by the Weibull function as follows[4]:

Pf=1−exp��−L

L0

��sf

s0

�mn(4)

where Pf is the cumulative failure probability, sf is thetensile stress acting on the fiber, L is the effective fiberlength which equals to gage length, s0 and L0 arereference values of stress and length, and m is theWeibull modulus. Fig. 7 shows the Weibull plots of theextracted fiber strength. The graph indicates that theWeibull modulus of the fiber after fabrication reducedfrom that of pristine fiber. This reduction of Weibullmodulus is similar to that the drop from 18 to 5.3 asfound by Weber et al. [4]. The observation of extractedfiber surfaces indicates that the fiber surface roughnessincreases with increasing consolidation time of the com-posites (Fig. 8). This increase of fiber surface roughnessas a result of increasing reaction zone thickness couldplay a role in providing surface defects that are initia-tion sites for fracture cracks. In other words, the fiberstrength is depend on the fiber surface roughness thereaction zone thickness

3.3. Tensile properties of composite and matrix

Fig. 9 shows typical stress-strain curves of compositeand matrix. The composites consolidated for 4.8 ksexhibited linear elastic behavior up to fracture. How-ever the composites consolidated for 360 ks exhibitednon-linear elastic behavior occurred between a stresslevel of 600 and 700 MPa because of their weak fiberbreakage. Tensile strength of the composite and 0.2%proof stress, tensile strength and elongation of thematrix are summarized as a function of consolidationtime in Fig. 10. Tensile strength of composite decreaseswith increasing consolidation time, while that of ma-trices is independent of it. Elongation of matrix de-creases with increasing consolidation time, which seemsto be caused by the grain growth of the matrix as

Fig. 12. Young’s moduli of composite and matrix as the function ofconsolidation time at 1153 K.

Fig. 13. Typical load-displacement curve during fiber pushout testwith AE event count rates.

A. Fukushima et al. / Materials Science and Engineering A276 (2000) 243–249248

Fig. 14. Interfacial mechanical properties as a function of consolida-tion time at 1153 K.

Fig. 16. Experimental tensile strength and tensile strength derived bythe rule of mixtures and the modified rule of mixtures as a functionof reaction zone thickness.

shown in Fig. 11. (The average grain size of the matrixwas measured in accordance with ASTM E112.) Elonga-tion of composites was not more than 1% as was also thecase for elongation of extracted fibers.

The Young’s moduli of the composites and matricesare independent of consolidation time as shown in Fig.12. The Young’s modulus of the composites is consistentwith the one calculated by using the rule of mixtures.

Considering the low fiber volume fraction of 25%, thecomposites consolidated for 4.8 ks has the high tensilestrength of 1724 compared with those of 1400 MPa with34% fiber volume fraction and of 1728 MPa with 39%fiber volume fraction, as examined by other researchers[3,5,13]. This suggests that the fabrication process of thecomposites consolidated for 4.8 ks could be most appro-priate for SCS6/Ti–15–3 composites.

3.4. Interfacial mechanical properties

Fig. 13 shows a typical example of load-displacementcurve during the fiber pushout test with AE event countrates. The fiber pushout process is divided into threecharacteristic regions such as pre-debonding region,debonding region and sliding region. Each region isidentified with load and AE profiles. The interfacial

shear debonding stress, ti, and the interfacial shearsliding stress, ts, were calculated from the maximumloads in the debonding region and the load in the slidingregion with the following equations, respectively [14]:

ti=Fi

2pRfL(5)

ts=Fs

2pRf(L−d)(6)

where Fi is maximum load in the debonding region, Fs

is load in the sliding region, Rf is the fiber radius, L isspecimen thickness and d is sliding displacement. Fig. 14shows the relationship between consolidation time andthe interfacial shear mechanical properties. The interfa-cial shear debonding stress increases with increasingconsolidation time, while the interfacial shear slidingstress is independent of the consolidation time.

3.5. Fractography

The fracture surfaces of the composites were examinedusing SEM. Typical fracture surfaces of the composites

Fig. 15. Fracture surfaces of composites: (a) consolidated at 1153 K for 5.4 ks and (b) for 360 ks.

A. Fukushima et al. / Materials Science and Engineering A276 (2000) 243–249 249

are shown in Fig. 15. The pull-out length of the fibertended to decrease with increasing consolidation time.This would arise from a decrease in the fiber strengthand a increase in the interfacial shear strength.

3.6. Rule of mixtures

The conventional rule of mixtures (ROM) is writtenas:

sc=sfVf+sm* (1−Vf) (7)

where sc is tensile strength of the composite, sf istensile strength of the fiber, sm* is tensile stress of thematrix at composite breakage and Vf is fiber volumefraction. When the fiber strength is in ideal distributionand the fiber stress distribution is identical to that givenfor a single short fiber composite by simple slip theory,Kagawa et al. modified the rule of mixtures with theshear transfer mechanism at the interface, characterizedby interfacial shear strength and expressed as [7]:

sc=sfVf�

1−Rfsf

2tiLg

�+sm* (1−Vf) (8)

where Lg is gauge length of the specimen. Fig. 16 showsthe experimental tensile strength and the tensilestrength derived by the rule of mixtures (from Eq. (7))and the modified rule of mixtures (from Eq. (8)) as afunction of reaction zone thickness. In this experiment,the fiber strength is in relatively narrow distribution asshown in Fig. 8 and the fracture mechanism ideallydepends on the fiber strength and the interfacial shearstrength as shown in Fig. 15. Then Fig. 16 clearlyshows that the modified rule of mixtures could estimatethe tensile strength more accurately than the conven-tional rule of mixtures.

4. Conclusions

(1) The rule of mixtures modified with interfacialmechanical property could estimate the tensile strengthsof titanium matrix composites more accurately than theconventional rule of mixtures.

(2) The strengths of the composite and the extracted

fiber decrease with increasing consolidation time, whilethe strength of the matrix is independent of it.

(3) The interfacial shear strength increases with in-creasing consolidation time, while the interfacial slidingstress is independent of it.

(4) Interfacial reaction kinetics shows values of Qand k0 to be 143 KJ mol−1 and 8.7×10−4 m s1/2,respectively.

Acknowledgements

This study was performed through Special Coordina-tion Funds of the Science and Technology Agency ofthe Japanese Government. The authors would like torecord their appreciation for cooperation with Mr S.Terazawa and Mr T. Ito of Churyo Engineering Co.Ltd.

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