ceramic composites

January

1998

Materials Letters 33 (1998) 255-260

ELSEVIER

The mechanical behavior of layered brazed metal/ceramic composites Dov Sherman Department

of Materials

Engineering,

Technion-Israel

*

Institute

of Technology,

Haija 32000, Israel

Received 7 April 1997; accepted 11 April 1997

Abstract Brazing as a method of joining thin metal and ceramic plates to form a layered composite for structural applications is examined. The constituents are Ti-6Al-4V alloy sheets and alumina thin plates and the brazing alloy is 63 wt% Ag, 1.75 Ti and bal. Cu active braze alloy. The interfacial shear strength of the joint is relatively high, which makes it attractive for structural applications. A model Ti alloy/alumina bilayer and laminate joined by active brazing was evaluated for its basic mechanical behavior. The bilayer structure tested under bending exhibited improved properties when the alumina layer was under compression during loading. In that case, a combination of the high compressive strength of ceramics, the high toughness of metals and the high shear strength of the interface are causes of the improved mechanical properties when compared with those of the monolithic metal. Using composite beam theory, the properties of bilayers were evaluated for design purposes and, in particular, the interfacial shear strength. The laminated structure tested under bending showed reduced strength but improved undamaged stiffness when compared with the metal constituent. 0 1998 Elsevier Science B.V. Keywords:

Composite;

Bilayer;

Metal/ceramics;

Ti-6Al-4V/Al,O,;

1. Introduction Among the new structural materials, metal matrix composites [I] are attractive because they combine the properties of two or more materials. In such material systems, the role of the metal is to provide ductility and toughness as damage in the form of cracks appears in the ceramic, while the ceramic constituent is aimed at increasing the stiffness of the undamaged material. to compensate for the reduced strength and stiffness of metals at elevated temperatures and to provide wear resistance [2-41. It is also

* Fax: +972-48-321978.

Brazing;

Interfacial

shear strength; Compressive;

Toughness

expected that ceramics reduce creep deformation and increase fatigue threshold stresses and the fatigue life of the composite compared metals. In fiber architecture, ceramic fibers provide high strength. However, the high cost of fibrous composites, especially in the form of metal matrix composites, limits their use. Laminated systems have attracted the attention of the research community for decades [5,6] and lately more intensively [7-l 11. The layered configuration is being adopted as an alternative metal/ceramic composite. The system is constructed by alternating commercially available thin metal and ceramic monolithic layers. The joining method is active brazing [12-141. The main advantages of such a material

00167-577X/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII SO167-577X(97)00116-X

256

D. Shrrmun / Materials Letters 3.3 (I YYXi 255-260

system are the in-plane isotropy of both strength and modulus, relatively high specific strength and modulus, high toughness and a relatively low cost due to a simpler processing route. However, all the above properties depend on the architecture of the composite. The interface in composites plays an important role in controlling the mechanical behavior [2,15- 171. A strong interface results in a moderate improvement of the longitudinal strength in fiber reinforced MMC, with relatively good transverse properties, while a weak interface is responsible for better longitudinal properties but poor transverse ones. In a layered structure, however, the interface should be strong. The present study was aimed at characterizing the basic mechanical behavior of layered metal/ceramic composites and defining the role of the interface. A model for a bilayered structure, based on composite beam theory, predicts the behavior of the bilayer and is suitable for design purposes. The model enables determination of a lower bound to the interfacial shear strength of the interface between dissimilar materials.

2. Experimental

program

2. I. Materials The materials in this investigation were Coors thin Al,O, ADS96R plates as the ceramics and commercially available Ti-6Al-4V plates as the metal. The properties of both materials are given in Table 1. They were chosen for the low mismatch of thermal expansion coefficients and the relatively simple joining route, which does not affect the metal’s properties. For use as the bilayer specimens the titanium alloy and the alumina were in the form of 3.4 mm and 1 mm thick layers, respectively. For use in the multilayered structure, the titanium alloy Table 1 The constituents

Alumina Ti-6AI-4V

and their properties

and the alumina were 1 mm and 0.63 mm thick, respectively. Multilayers were constructed from a total of 7 layers and the alumina layers always formed the outer surfaces. 2.2. Processing

routes

Bilayers and multilayers were constructed by alternating 50 X 50 mm2 plates of alumina and titanium alloy, joined in active brazing by a 50 pm thick Wesgo Cusil active braze alloy (63 wt% Ag, 1.75 Ti, bal. Cu). The eutectic temperature of the brazing alloy is 840°C. Brazing was chosen for its simplicity (viz. relatively low temperature, pressure and time requirement). No special surface treatment was applied, aside from cleaning the materials in an ultrasonic acetone bath. Brazing was carried out under a vacuum of IO-’ Torr, with a maximum temperature of 850°C for 10 min (Fig. 1). The specimens were put under low pressure (0.15 MPa), applied by a solid fixture at room temperature. Brazing causes thermal residual stresses, resulting from thermal expansion coefficient mismatch of both constituents. The alumina layers are in compression the titanium alloy layers in tension. However, due to the low thermal expansion coefficient mismatch and the low processing temperature, the radius of curvature of the bilayer is long (approximately 2 m>. hence the thermal stresses are less than 10 MPa and ignored in this study. 2.3. Specimens Bending specimens were prepared, 50 mm long, 5 mm wide and 4.4 and 5.75 mm thick nominal for the bilayers and the multilayers, respectively. In the latter, the volume fractions of alumina, Ti alloy and brazing alloy were 43.5, 51.7 and 4.8%, respectively and 23, 75.9 and I .I% for the bilayer. The specimens were cut with a diamond wheel cutter and both

in this work

Young’s M., E (GPa)

Poison R., v

320 120

0.2 0.3

1

a Median strength and Weibull modulus, respectively h Yield stress and strength, respectively [19].

Strength,

g (MPa)

280, 16 a 1100,1500 h

[ 181.

TEC, n (IO-‘/“C)

Density, p (kg/cmj)

x.3 8.7

3.95 4.1

D. Sherman /Materials

257

Letters 33 f 1998) 255-260

pression, i.e. h I h,, which is obtained when E * h * 2 2 1. The maximum tensile stress at the bilayer (in the lower part of the metallic constituent) normalized by that of a monolithic beam of the same dimensions: =;E*(l+h*)$ (TnlO”O

V

I -G

>

10-3 hIPa At

Fig.

I. Scheme of the processing route.

sides were polished after cutting. Equivalent monolithic Ti-6AI-4V beam specimens were prepared for comparison. 2.4. Experimental

procedure

Tests were carried out under three (3PB) and four (4PB) point bending, with a span of 40 mm and an aspect ratio of 2 for the 4PB configuration. Tests were carried out with a computerized elecro-mechanical Instron 8.562 machine under displacement controlled condition, with a displacement rate of 0.3 mm/min. The specimens were loaded in a fully articulate bending bridge to assure unidirectional bending of the specimens.

(1)

where ~=E’(~3+h3)+(1+&3-~3 and a= 0.5(E*h*2 - l)/(l + E*h*). The bending rigidity of the bilayer normalized by that of a monolithic beam made of constituent 2 is obtained by:

4

---=E1bilayer

c

(2)

E’ (l+h*)3

Km,

Plots of the normalized stress and the normalized bending rigidity as a function of E * and of h * are shown in Fig. 3a and b, respectively. A reduction in

0.6

t’

I

3. Elastic bilayer behavior The bilayer composite was analyzed using Euler-Bernoulli composite beam theory. Schematic presentation of the composite beam is shown in Fig. 2. For E * = E,/EJ and h* = h/h, the neutral axis is located at h = hh, from the lower surface of the beam, Fig. 2, where h is expressed by: XT

1 +2h*

+E*h**

2(1 +E*h*) In order to benefit from the high compressive strength of ceramics, the ceramic layer should be under com-

(b)

Fig. 2. The bilayered structure ceramic and 2 for the metal).

and its constituents

(1 for the

Fig. 3. The normalized maximum tensile stress at 2 (a) and the normalized bending rigidity (b) of the bilayer as a function of the normalized thickness and moduli of the constituents.

3 point bending

the maximum tensile stresses of up to 40% is expected for low E * bilayers, as well as significant stiffness enhancement. The normalized shear stresses at the interface between the two materials obey:

4JM

,

(3) where V is the shear force at a point and b the width. The above formulation was used to analyze the test results. I 4. Results and discussion The brazed multilayered specimens were tested under 3PB, since the 4PB configuration was inapplicable due to the low span to width ratio, which may cause failure in shear at one of the interfaces. Equivalent monolithic titanium alloy beams were tested under the same conditions, for comparison. The load versus load point deflection of the laminate and of the monolithic beam are shown in Fig. 4a, together with the maximum tensile stresses in the monolithic beam. The deflections are the LVDT read-out, in mm. At an equivalent stress of 470 MPa in the monolithic beam, damage in the form of transverse cracks initiated in the alumina layers. The yield stress of the titanium alloy is 1100 MPa, which constitutes a significant reduction in strength. Cracking is associated with a minor load drop. The cracks formed progressively (from the far alumina layer to that close to the load point) in the alumina layers in the vicinity of the central loading point (Fig. 4b). Some of the cracks propagated towards the braze alloy. but arrested at the titanium alloy/brazed alloy interface (Fig. 4~). After initiation, cracking occurred throughout the loading history. The large load drop (Fig. 5a) indicates the failure of the upper alumina layer due to high stress concentration beneath the loading point. The brazed bilayers were tested in two separate configurations: in the first. the alumina layer was under tension (designated BT), while it experienced compressive stresses in the second (designated BC). Experiments were carried out in a 4PB configuration. Load versus load point deflection curves of the two

(a)

Deflection

15

7

2.5

[mm)

Fig. 4. Normalized load versus load point deflection of the laminate and the monolithic beam under 3PB (a). the damaged laminate (b) and crack deflection from the alumina layer to the brazed alloy cc).

configurations and of the monolithic titanium alloy beam are shown in Fig. 5, together, again, with the maximum tensile stresses in the monolithic beam. The deflection is again the LVDT read-out, in mm.

D. Sherman/Materials

0

0

1,.&LILL 0.5

I

Deflection.

1.5

i 1..LLl-_~ 0 2 2.5

[mm]

Fig. 5. Normalized load versus load point deflection bilayer configurations and of the monolithic titanium under 4PB.

of the two alloy beam

The effective Young’s modulus of the bilayers is discussed first. For this purpose, the machine compliance was calculated on the basis of the deflection of the monolithic beam and subtracted from the actual compliance obtained in the tests. The effective Young’s modulus of both bilayers (the modulus before cracking of the BT specimen and that of the BC specimens) were found to be 195.3 GPa which reflects an improvement of 63% over that of the titanium alloy, for only 23% of the volume fraction of alumina. The prediction of the model for the effective modulus is 181.7 GPa, which is 7% lower than that measured. Damage in the form of transverse cracks in the BT specimens (Fig. 6a) initiated in the monolithic beam at an equivalent stress of 190 MPa, as a result of the poor tensile strength of ceramics. The model (Eq. (1)) predicts that the maximum tensile stress in the alumina layer is 263 MPa at the onset of crack-

(a)

Fig. 6. The fractured bilayer specimens when the alumina is under tension (a) and under compression (b).

Letters

33

f I9981

259

255-260

ing, which is within the probable failure stresses of the alumina in tension. The crack saturation state was achieved at equivalent 290 MPa. No additional cracking was observed beyond that stress. Yielding of the BC specimens was delayed compared to the yielding of the monolithic metallic beam. Plastic flow in the metal layer initiated at equivalent applied maximum stresses of 1380 MPa, a 25% improvement over the yield stress of monolithic Ti-6Al-4V (see Table l), which is in good agreement with 24.7%, the predicted value of the model. Failure of the BC specimen occurred instantaneously and the alumina layer was shattered into small pieces. The fractured interface exhibited zones of adhered alumina and others with a fully dismantled alumina layer (Fig. 6b1, presumably due to the complex propagation of the crack at the interface. When applying the maximum load in the linear regime, i.e. before yielding occurs, the model (Eq. (3)) predicts that the shear stress before yielding and hence the interfacial shear strength at this point, is 135 MPa. This calculated value sets the lower bound to the inter-facial shear strength. The calculated interfacial shear strength is in good agreement with that obtained experimentally for the same combination [131.

5. Concluding

remarks

The behavior of a brazed metal/ceramic system in the forms of bilayered and multilayered was examined. It was shown that in the laminated system the apparent stress for first damage to occur is appreciably lower than the yield stress of the metal. This is due to the low tensile strength of the ceramic layer that constitutes the outer surfaces. Hence, a laminated metavceramic composite is a reduced strength material. Two bilayered systems were also tested. In one, the ceramic layer was under tension, and behaved like the multilayered system. In the other, the ceramic layer was under compression during the loading history and exhibited improved properties of strength and modulus. This system takes advantage of the best properties of the constituents: the high modulus and compressive strength of ceramics, the high toughness and tensile strength of the

260

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/Materials

metal and the high inter-facial shear strength of the brazing. It is an example of the effectiveness of appropriate material architecture. Active brazing was found suitable for joining metal and ceramic layers for structural application, in which it provides high interfacial shear strength of the interface between the metal and the ceramic. The experimental program and the model enable definition of the lower bound to the inter-facial shear strength of dissimilar media with simple means.

Acknowledgements This research was supported by the Fund for the Promotion of Research at the Technion.

References [I] M. Taya, R.J. Arsenault,

Metal Matrix Composites, Pergamon Press, 1989. [2] A.G. Evans, Mater. Sci. Eng. A 143 (1991) 63. [3] Y.N. Bahei-El-Din, G-l. Dvorak, J. Appl. Mech. 49 (1982) 327.

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Bl Y.N. Bahei-El-Din,

G.J. Dvorak, in: W.W. Johnson (Ed.), Metal Matrix Composites: Testing. Analysis and Failure Modes. ASTM STP 1032, Philadelphia, 1989. p. 262. [51 O.B. Boggild, K. Dansk, Vidensk. Selskabs Skrifter. 9 (I 930) 23s. [61 J.D. Currey, Proc. R. Sot. B I96 (1977) 443. [71 W.J. Clegg. K. Kendall, N.M. Alford. D. Birchall. T.W. Button, Nature 347 (1990) 455. [El W.J. Clegg, Acta. Metal]. Mater. 40 (I I) (1992) 3085. [91 Z. Chen, J.J. Mecholski Jr., J. Am. Ceram. Sot. 76 (1993) 1258. [lOI D. Sherman. J. Lemaitre F.A, F.A. Leckie, Acta Metall. Mater. 43 (1995) 3261. [I II D. Sherman, J. Lemaitre, F.A. Leckie, Acta M&all. Mater. 43 (1995) 44x3. [I21 C.H. Cho, J. Yu. Ser. Met. 26 (1990) 1737. [I31 M. Naka, K. Sahpath, I. Okamoto, Y. Arata, Mater. Sci. Eng. 98 (1988) 307. [I41 M. Naka, K. Kim. I. Okamoto, Trans. JWRI I3 (1984) 157. [I51 A.G. Evans, A. Bartlett, J.B. Davis, B.D. Flinn, M. Turner, I.E. Reimanis, Ser. Met. Mater. 25 (1991) 1003. [I61 I.E. Reimanis, B.J. Dalgleish. M. Brahy, M. Ruhle. A.G. Evans, Acta Metall. Mater. 38 (1990) 2645. [I71 M.Y. He, A.G. Evans. Acta. Metal]. Mater. 39 (1991) 1587. [I81 D. Sherman. unpublished results. [I91 Metals Handbook. 8th ed.. vol. I, American Society for Metals. 1975.