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2024Al composite
Materials Science and Engineering A 497 (2008) 44–50
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Experimental and simulation investigations of tensile response of (Al2 O3f /Al)w /2024Al composite Wenlong Zhang a,b,∗ , Mu Zhang a , Shojiro Ochiai b , Mingyuan Gu a a b
State Key Lab of MMCs, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, China International Innovation Center, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
a r t i c l e
i n f o
Article history: Received 28 March 2008 Received in revised form 4 June 2008 Accepted 14 July 2008 Keywords: Fiber reinforced metal matrix composites Tensile response FEM simulation Fracture mode
a b s t r a c t Tensile response of a 2024Al alloy reinforced with Al2 O3f /Al composite wires [(Al2 O3f /Al)w /2024Al composite] has been investigated by both performing tensile tests and using the finite element method (FEM). Microstructure and tensile fracture surfaces of (Al2 O3f /Al)w /2024Al composites were examined using optical microscope (OM) and scanning electron microscope (SEM), respectively. It is found that the longitudinal tensile response of (Al2 O3f /Al)w /2024Al composite is nearly elastic up to failure. The longitudinal tensile strength of (Al2 O3f /Al)w /2024Al composite reaches its fiber bundle strength and the longitudinal elastic modulus is consistent with that predicted using the rule of mixture. The contribution of matrix is negligible to the longitudinal tensile strength but not to the modulus of elasticity. Coating composite wires (CWs) with Ni can improve the interfacial bonding between the CWs and 2024Al alloy. Diffusion annealing can further improve the interfacial bonding. The transverse tensile strength is dependent strongly on the interfacial bonding between the CWs and 2024Al alloy. The stronger the interfacial bonding, the higher the transverse tensile strength. Following the diffusion annealing, ageing treatment can improve the transverse tensile strength of (Al2 O3f /Al)w /2024Al composite to a higher level due to the increased strength of matrix 2024Al alloy. To reveal the fracture mode of (Al2 O3f /Al)w /2024Al composite under transverse loading, the transverse tensile response of a model material, (Al2 O3f /matrix1)w /matrix2 composite, whose microstructure is similar to that of the (Al2 O3f /Al)w /2024Al composite, was analyzed using FEM. FEM results indicate that, under transverse loading, the stress distribution in the (Al2 O3f /matrix1)w /matrix2 composite depends on the strength of matrix2. When matrix1 and matrix2 are both pure Al, failure initiates in matrix1. When matrix1 is pure Al and matrix2 is aged 2024Al alloy, however, failure initiates in matrix2 (2024Al). Therefore, for (Al2 O3f /Al)w /2024Al composites with weak interfacial bonding between CWs and 2024Al matrix, cracks initiate at interfaces between the CWs and 2024Al matrix and propagate along the interfaces and then through 2024Al matrix, resulting in the global fracture. For (Al2 O3f /Al)w /2024Al composite with strong interfacial bonding between CWs and 2024Al matrix, cracks initiate in either pure Al or 2024Al alloy and then propagate in both Al and 2024Al, resulting in the global fracture. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Continuous alumina fiber reinforced aluminum matrix composites (CF-AMCs) have significant potential for structural applications due to their outstanding combination of high stiffness, high strength, low density as well as relatively high electrical conductivity [1]. In recent years, many researchers have focused their concerns on this kind of materials [2–6]. One of the main applications of CF-AMCs is to reinforce aluminum alloys with CF-AMCs
∗ Corresponding author at: State Key Lab of MMCs, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, China. Tel.: +86 21 54742476; fax: +86 21 34202749. E-mail address: [email protected] (W. Zhang). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.07.048
shapes such as I-beams, C-sections, wires etc. [1,2]. Rather than producing an aluminum matrix composite that is completely reinforced, the composite shapes may be strategically placed in areas of Al alloys which locally experience high stresses. Two major difficulties encountered when direct fiber reinforcement of cast parts is attempted, the required high infiltration pressure and the tendency of the reinforcing fibers to float away from their intended place, are eliminated when the fibers are introduced into the casting in the form of prefabricated MMC wire. The use of selective reinforcement enables Al alloys to be used in more demanding loading environments and hence the application of aluminium alloys in aerospace industry can be greatly widened. To get better reinforcing effects, the mechanical response of aluminum alloys reinforced with CF-AMCs shapes should be investigated.
W. Zhang et al. / Materials Science and Engineering A 497 (2008) 44–50
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Fig. 2. Longitudinal section of (Al2 O3f /Al)w /2024Al composite.
Fig. 1. Typical microstructure of the wire.
In this study, an 2024Al alloy reinforced with CF-AMCs composite wires [(Al2 O3f /Al)w /2024Al composite] was fabricated using a vacuum pressure infiltration method and the tensile response in both longitudinal and transverse directions was investigated by both performing tensile tests and using the finite element method (FEM). In some cases, composite wires (CWs) were coated with Ni to improve the interfacial bonding between the CWs and 2024Al alloy matrix. 2. Experimental procedure The composite wire was produced by 3M (St. Paul, MN, USA) and supplied on a reel. The fiber volume fraction in the wire is about 45%. The diameter of the wire was about 2.0 mm. A typical microstructure of the wire is shown in Fig. 1. Fiber properties are listed in Table 1. An 2024Al alloy reinforced with Al2 O3f /Al composite wires [(Al2 O3f /Al)w /2024Al composite] was fabricated using a vacuum pressure infiltration method, in which the composite wires were closely packed together as a preform and then the preform was pressure infiltrated with molten 2024Al alloy. The morphology of the as-fabricated (Al2 O3f /Al)w /2024Al composite is shown in Fig. 2. The fiber volume fraction in (Al2 O3f /Al)w /2024Al composite is measured with LEICA MEF4Me image analyzer to be about 30%. In some cases, before infiltration, CWs were coated with Ni to improve the interfacial bonding between the CWs and 2024Al alloy matrix. The morphology of a Ni-coated composite wire was shown in Fig. 3. The thickness of Ni-coating is about 30 m. To further increase the interfacial bonding, Ni-coated (Al2 O3f /Al)w /2024Al composites [(Al2 O3f /Al)w(Ni) /2024Al] were diffusion annealed at 490 ◦ C for 12 h. To improve the transverse strength of (Al2 O3f /Al)w /2024Al composites, some specimens were solution treated at 490 ◦ C followed by Table 1 Properties of the Nextel 610TM alumina fiber [1] Composition
>99% Al2 O3
Mean tensile strength (for 2.54 cm length) Weibull modulus Modulus of elastisity Density Diameter Average coefficient of thermal expansion (20–500 ◦ C) Maximum temperature up to 900 ◦ C
2.8–3.5 GPa 9–12 380–400 GPa 3.90–3.95 g/cm3 10–12 m 7 × 10−6 ◦ C−1 90% strength retention
Fig. 3. The morphology of Ni-coated composite wire.
water quenching and then aged at 190 ◦ C for 8 h. Tensile response of (Al2 O3f /Al)w /2024Al composites was investigated by both performing tensile tests and using the finite element method. Tensile tests were performed on MTS 810 servohydraulic testing machine with standard flat dog-bone specimens. Microstructures and tensile fracture surfaces of (Al2 O3f /Al)w /2024Al composites were examined using optical microscope (OM) and scanning electron microscope (SEM), respectively. 3. Results 3.1. Longitudinal tensile response 3.1.1. (Al2 O3f /Al)w /2024Al composite A typical longitudinal tensile stress–strain curve of (Al2 O3f /Al)w /2024Al composite is shown in Fig. 4. It can be seen that the stress–strain response is nearly linear up to failure. The measured mean tensile strength and mean modulus of elasticity are 845 MPa and 170 GPa, respectively. According to data listed in Table 1, the fiber bundle strength of the (Al2 O3f /Al)w /2024Al composite can be calculated using Vf f to be 840–1050 MPa. The modulus of elasticity can be assessed using the rule of mixture (Vf Ef + (1 − Vf )Em ) and data in Table 1 as well as EAl = 70 GPa (the fiber volume fraction in (Al2 O3f /Al)w /2024Al is taken as 30%). The calculated modulus of elasticity is in the range of 163–169 GPa. Compared with the calculated ones, it can be found that the tensile strength of (Al2 O3f /Al)w /2024Al reaches its fiber bundle strength and the modulus of elasticity of (Al2 O3f /Al)w /2024Al is consistent with the prediction by the rule of mixture. 3.1.2. (Al2 O3f /Al)w(Ni) /2024Al The longitudinal tensile stress–strain response of (Al2 O3f / Al)w(Ni) /2024Al is similar in shape to that for (Al2 O3f /Al)w /2024Al.
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W. Zhang et al. / Materials Science and Engineering A 497 (2008) 44–50
The mean tensile strength and mean modulus of elasticity of (Al2 O3f /Al)w(Ni) /2024Al are 844 MPa and 171 GPa, respectively, indicating that the tensile strength of (Al2 O3f /Al)w(Ni) /2024Al also reaches its fiber bundle strength and the modulus of elasticity is consistent with the prediction by the rule of mixture.
Fig. 4. Typical longitudinal tensile stress–strain curve of (Al2 O3f /Al)w /2024Al composite.
3.1.3. Longitudinal tensile fracture surfaces The longitudinal tensile fracture surfaces of (Al2 O3f /Al)w /2024Al are shown in Fig. 5. From Fig. 5a, it can be seen that the fracture surface of (Al2 O3f /Al)w /2024Al includes two parts, one is the fracture surface of CWs and the other is the fracture surface of matrix 2024 alloy. The fracture morphology of matrix 2024 alloy consists of a number of dimples, indicating that a large amount of plastic deformation has occurred before fracture. The magnified dimple morphology is shown in Fig. 5b. The bonding between CWs and 2024Al matrix alloy is weak as debonding can be clearly seen from Fig. 5a. The fracture surfaces of round CWs in (Al2 O3f /Al)w /2024Al look like a terrace and the detailed terrace-like fracture surface is shown in Fig. 5c. From Fig. 5c, no debonding can be found between Al2 O3f and Al, indi-
Fig. 5. Longitudinal tensile fracture surfaces: (a) fracture surface of (Al2 O3f /Al)w /2024Al; (b) dimples in 2024Al matrix; (c) terrace-like fracture surface of (Al2 O3f /Al)w in (a) and (d) fracture surface of (Al2 O3f /Al)w(Ni) /2024Al.
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Fig. 6. Transverse tensile stress–strain curves of (Al2 O3f /Al)w /2024Al Composites.
cating a strong interfacial bonding in CWs (Fig. 5c). The fracture surface of (Al2 O3f /Al)w(Ni) /2024Al is shown in Fig. 5d. It can be seen that the fracture surface of (Al2 O3f /Al)w(Ni) /2024Al is quite similar to that of (Al2 O3f /Al)w /2024Al. Comparing Fig. 5d with Fig. 5a it can be found that the extent of debonding between CWs and 2024Al matrix is smaller for (Al2 O3f /Al)w(Ni) /2024Al than for (Al2 O3f /Al)w /2024Al, indicating that the bonding between CWs and 2024Al matrix appears stronger for (Al2 O3f /Al)w(Ni) /2024Al than for (Al2 O3f /Al)w /2024Al. Therefore, Ni-coating can improve the interfacial bonding between CWs and 2024Al matrix. 3.2. Transverse tension 3.2.1. Transverse stress–strain response Fig. 6 illustrates the transverse tensile stress–strain curves of (Al2 O3f /Al)w /2024Al composites. The yield strength, ultimate strength and elongation of (Al2 O3f /Al)w /2024Al composites are listed in Table 2. It can be seen from Table 2 that yield strength, ultimate strength and elongation are higher for (Al2 O3f /Al)w /2024Al composite with Ni-coating than without Ni-coating. After annealing, yield strength, ultimate strength and elongation of Ni-coated (Al2 O3f /Al)w /2024Al composite are further increased. Among the four types of composites, the aged one possesses the highest yield strength, ultimate strength and elongation. 3.2.2. Transverse fracture surfaces A transverse tensile fracture surface of (Al2 O3f /Al)w /2024Al composite is shown in Fig. 7a. From Fig. 7a, debonding between CW and 2024Al matrix can be clearly seen. This indicates that the interfacial bonding between CWs and 2024Al is weak and cracks propagate along interfaces between CWs and 2024Al matrix, resulting in the wave-like fracture surface as shown in Fig. 7a. The transverse tensile fracture surface of (Al2 O3f /Al)w(Ni) /2024Al is shown in Fig. 7b. From Fig. 7b it can be found that the transverse fracture of (Al2 O3f /Al)w(Ni) /2024Al takes place through CWs rather than along interfaces between CWs and 2024Al. Meanwhile,
Fig. 7. Transverse tensile fracture surfaces: (a) (Al2 O3f /Al)w /2024Al; (b) Ni-coated and annealed (Al2 O3f /Al)w /2024Al and (c) magnified fracture surface of Ni-coated (Al2 O3f /Al)w /2024Al.
no debonding can be found between CWs and 2024Al, indicating a strong interfacial bonding between coated CWs and 2024Al. A local magnification of the fracture surface in Fig. 7b is shown in Fig. 7c. From this figure it is clear that almost all the fibers on the surface are clad by a layer of Al, indicating a strong bonding between alumina fiber and Al.
Table 2 Mechanical properties of (Al2 O3f /Al)w /2024Al composites
0.2% Yield strength (MPa) UTS (MPa) Elongation (%)
Without Ni-coating
Ni-coated
Ni-coated+annealed
Ni-coated + annealed + aged
98.42 117.20 0.50
115.05 147.81 0.657
134.61 167.22 0.67
152.70 186.77 0.683
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W. Zhang et al. / Materials Science and Engineering A 497 (2008) 44–50 Table 3 Materials parameters for finite-element calculations Parameters
Al
2024Al
Modulus (GPa) Poisson ratio Strain-hardening exponent Yield stress (MPa) Tensile strength (MPa)
68.9 0.33 0.1 60 120
72.4 0.33 0.1 274 412
Fig. 8. Schematic drawing of the microstructure of (Al2 O3f /Al)w /2024Al.
3.2.3. FEM simulation According to Fig. 2, the composite wires in (Al2 O3f /Al)w /2024Al composite distribute nearly hexagonally in the 2024Al matrix. So, the microstructure of (Al2 O3f /Al)w /2024Al can be schematically shown in Fig. 8. For regularly hexagonal arrangement, according to [7–9], based on Fig. 8, a representative volume element (RVE) can be given, as shown in Fig. 9. In the RVE, the fiber volume fraction is 30%, and the fibers and Al as well as CW and 2024Al are assumed to be bonded perfectly. The boundary condition is as follows: on the left and the bottom of REV there are symmetry boundary conditions, and the right is loaded with a horizontal displacement load. The upper sideline of the REV remains straight during the displacement loading, namely, on the upper there is a straight boundary condition. The fibers in the RVE are assumed to be elastic and their elastic modulus and Poisson’s ratio are 390 GPa and 0.23, respectively. According to [10,11], the uniaxial stress–strain curve of a typical work-hardening material can be described as follows:
=
Eε, 0.2
ε n ε0.2
ε ≤ ε0.2 , ε ≥ ε0.2
(1)
where 0.2 is the yield strength obtained at ε0.2 and n = 1/m is the strain-hardening exponent. The materials properties of Al and 2024Al alloy can be determined according to Eq. (1). The materials parameters for finite-element calculations are listed in Table 3. A finite element method, using the MARC code, was employed to carry out the stress–strain calculation. In this analysis, the 4 noded plane strain elements with full integration was used for all elements. The values of equivalent Von Mises stress for each phase
Fig. 10. Calculated transverse tensile stress–strain curves.
(fiber, Al and 2024Al) were specified during the calculation. As the displacement load increases, the failure of an element takes place when its equivalent Von Mises stress reaches the equivalent Von Mises stress corresponding to the local ultimate tensile strength (the values in Table 1). The global ultimate tensile strength was calculated when the initial failure took place. 3.2.3.1. Transverse tensile response. The calculated stress–strain curves are shown in Fig. 10. It can be seen from this figure that the flow stress of (Al2 O3f /Al)w /2024Al composite is higher than that of (Al2 O3f /Al)w /Al composite. This is a result that the strength of 2024Al alloy is higher than that of pure Al. The calculated tensile properties are listed in Table 4. It can be seen that the calculated yield stress, ultimate tensile stress and elongation are higher for (Al2 O3f /Al)w /2024Al composite than for (Al2 O3f /Al)w /Al composite. Compared with tensile properties experimentally obtained (in Table 1), the calculated properties of (Al2 O3f /Al)w /2024Al composite are closer to those of aged (Al2 O3f /Al)w(Ni) /2024Al composite, indicating that the interfacial bonding in aged (Al2 O3f /Al)w(Ni) /2024Al composite is nearly perfect. 3.2.3.2. Stress distribution. The calculated total equivalent plastic strain distributions at the initiation of plastic flow are shown in Fig. 11. It can be found that plastic flow always occurs first in Al (matrix1) in spite of matrix2, indicating that the yielding of (Al2 O3f /matrix1)w /matrix2 composites is determined by matrix1. The calculated equivalent Von Mises stress distributions are shown Table 4 Calculated mechanical properties of (Al2 O3f /Al)w /2024Al and (Al2 O3f /Al)w /Al composites
Fig. 9. Representative volume element of the microstructure of (Al2 O3f /Al)w /2024Al: X-axis is the horizontal direction and Y-axis the vertical direction.
0.2% Yield stress (MPa) Ultimate tensile stress (MPa) Elongation (%)
(Al2 O3f /Al)w /Al
(Al2 O3f /Al)w /2024Al
90.4 99.89 0.65
150.18 183.93 0. 71
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in Fig. 12. For (Al2 O3f /Al)w /Al, the maximum stress concentration is located in pure Al matrix in the CWs (Fig. 12a) and fracture initiates at the maximum stress concentration locations (Fig. 12c, as shown by the arrows). For (Al2 O3f /Al)w /2024Al, however, the maximum stress concentration is located in 2024Al matrix (Fig. 12b) and fracture initiates at the maximum stress concentration locations (Fig. 12d, as shown by the arrows). This is a result that 2024Al can carry more stress than pure Al due to its higher strength. Therefore, for (Al2 O3f /matrix1)w /matrix2 composites, increasing the strength of matrix2 can change the fracture initiation location from matrix1 to matrix2. 4. Discussion 4.1. Longitudinal tensile properties Due to the low fracture strain of the composite (about 0.5%), the contribution from the matrix strength to the strength of composite should be very small. The fact that the strength of (Al2 O3f /Al)w /2024Al composite equals its fiber bundle strength indicates that the contribution from matrix to strength of (Al2 O3f /Al)w /2024Al composite is negligible. If we take the modulus of elasticity of matrix as 70 GPa, and take the modulus of elasticity of fiber as 380 GPa, the modulus of elasticity of 30 vol.% (Al2 O3f /Al)w /2024Al composites is calculated to be 163 GPa, in which the contribution of matrix (including Al and 2024Al) is 49 GPa, accounting for about 30% of the total. Therefore, although the contribution of matrix to the tensile strength is negligible but the contribution of matrix to modulus of elasticity of the (Al2 O3f /Al)w /2024Al composite is significant. Fig. 11. Calculated total equivalent plastic strain at the initiation of plastic flow for (a) (Al2 O3f /Al)w /Al and (b) (Al2 O3f /Al)w /2024Al.
4.2. Transverse tensile properties For (Al2 O3f /Al)w /2024Al composite, the interfacial bonding between CWs and 2024Al is weak and debonding between CWs and 2024Al occurs under transverse tensile loading, resulting in
Fig. 12. Calculated equivalent Von Mises stress distributions: (a) (Al2 O3f /Al)w /Al, before failure initiation; (b) (Al2 O3f /Al)w /2024Al, before failure initiation; (c) (Al2 O3f /Al)w /Al, after fracture initiation and (d) (Al2 O3f /Al)w /2024Al, after fracture initiation.
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that the transverse yield stress, tensile strength and elongation of (Al2 O3f /Al)w /2024Al composite are low. Coating CWs with Ni can improve the bonding between CWs and matrix 2024Al alloy. After diffusion annealing, the bonding between CWs and matrix 2024Al alloy is further improved, so that the transverse yield stress, tensile strength and elongation are higher for annealed (Al2 O3f /Al)w(Ni) /2024Al than for the (Al2 O3f /Al)w(Ni) /2024Al. After ageing treatment, the fact that the tensile strength reaches the highest value is a result that the matrix 2024 alloy is further strengthened by the ageing treatment. This is consistent with the result from the FEM calculation that increasing the strength of matrix2 can improve the global strength of the (Al2 O3f /matrix1)w /matrix2 composite (Fig. 10). From the FEM calculation, the increase in strength of matrix2 can reduce the stress concentration in CWs. When the strength of matrix2 reaches the strength level of aged 2024Al, the location of crack initiation can even change from matrix1 to matrix2. Due to the change of crack initiation location from matrix1 to matrix2, crack initiation may be greatly postponed and hence the elongation of the (Al2 O3f /Al)w(Ni) /2024Al composite annealed and aged is the highest (Table 2). FEM calculation indicates the yielding of bulk composites always occurs in matrix1 in spite of matrix2. With increasing the strength of matrix2, the yielding of matrix1 can be delayed. As a result, the yield stress of (Al2 O3f /Al)w /2024Al composite is higher than that of (Al2 O3f /Al)w /Al composite (Table 4). At the same time, with increasing the strength of matrix2, the tensile strength of (Al2 O3f /matrix1)w(Ni) /matrix2 composites can be improved due to both the delayed yielding in matrix1 and the increase in strength of matrix2. 4.3. Fracture mode under transverse tension For (Al2 O3f /Al)w /2024Al composites, the interfacial bonding between composite wires and 2024Al is weak, hence fracture initiates at interfaces between CWs and 2024Al, resulting the wave-like shape fracture surface (Fig. 7a). For annealed (Al2 O3f /Al)w(Ni) /2024Al composites, the interfacial bonding between CWs and 2024Al is greatly improved due to the coating and annealing treatment, so interfacial debonding between composite wires and 2024Al cannot occur. As a result, fracture occurs through CWs not along interfaces between CWs and 2024Al (Fig. 7b). According to the FEM result shown in Fig. 11, when the strength of matrix2 is low (say pure Al alloy), failure initiates in matrix1. When the tensile strength of matrix2 reaches that of aged 2024Al (about 410 MPa), failure initiates in matrix2. Therefore, for the (Al2 O3f /Al)w(Ni) /2024Al composite annealed, the fracture mode can be described as follows: cracks initiate in pure Al (matrix1) and then propagate through Al and 2024Al, finally, a number of cracks meet together, resulting in the global fracture. For the (Al2 O3f /Al)w(Ni) /2024Al composites annealed and aged: cracks initiate in 2024Al (matrix2) and then propagate through 2024Al and Al, finally, a number of cracks link together, resulting in the global fracture. Therefore, for (Al2 O3f /Al)w /2024Al composite with strong interfacial bonding, under transverse loading, cracks ini-
tiate either in pure Al or in 2024Al and then propagate through CWs and 2024Al alloy, resulting in the fracture surface as shown in Fig. 7b. 5. Conclusions (1) The longitudinal tensile stress–strain response of (Al2 O3f / Al)w /2024Al composite is nearly elastic up to failure. The strength of (Al2 O3f /Al)w /2024Al composite reaches its fiber bundle strength and the elastic modulus is consistent with that predicted using the rule of mixture. Although the contribution of matrix to the longitudinal tensile strength is negligible but the contribution of matrix to the modulus of elasticity of (Al2 O3f /Al)w /2024Al composite is significant. (2) Coating CW with Ni can improve the bonding between CW and 2024Al alloy matrix with a further improvement by diffusion annealing, so that the tensile strength is higher for (Al2 O3f /Al)w(Ni) /2024Al composites with than without diffusion annealing. Ageing treatment further improves the strength of 2024Al alloy and hence improves the transverse tensile strength of (Al2 O3f /Al)w(Ni) /2024Al composite. Therefore, the aged (Al2 O3f /Al)w(Ni) /2024Al composite has the highest transverse tensile strength. (3) Under transverse loading, the change in strength of matrix2 can change the stress distribution in (Al2 O3f /matrix1)w /matrix2 composite. When matrix1 and matrix2 are both pure Al, failure initiates in matrix1 and when matrix1 is pure Al and matrix2 is aged 2024Al alloy, failure initiates in matrix2 (2024Al). For (Al2 O3f /Al)w /2024Al composites, the fracture mode is: cracks initiate at interfaces between CWs and 2024Al, and then propagate along the interfaces and through 2024Al matrix, resulting in the global fracture. For annealed (Al2 O3f /Al)w(Ni) /2024Al composite without or with ageing, the fracture mode is: cracks initiate either in pure Al (matrix1) or in 2024Al alloy (matrix2) and then propagate through Al and 2024Al, finally, a number of cracks meet together, resulting in the global fracture. Acknowledgment The authors are grateful for the supply of the composite wires by 3M (St. Paul, MN, USA). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
H.E. Deve, C. McCullough, JOM 47 (7) (1995) 33–37. J.T. Blucher, J. Dobranszky, U. Narusawa, Mater. Sci. Eng. A 387 (2004) 867–872. W. Zhang, M. Gu, J. Chen, et al., Mater. Sci. Eng. A 341 (2003) 9–17. U. Ramamurty, F.W. Zok, F.A. Leckie, H.E. Deve, Acta Mater. 45 (11) (1997) 4603–4613. H.E. Deve, Acta Mater. 45 (12) (1999) 5041–5046. M.D. Kriese, J.W. Hoehn, H.E. Deve, et al., Process. Fabric. Adv. Mater. IV 42 (1996) 595–607. D.B. Zahl, S. Schmauder, R.M. Mcmmeeking, Acta Metall. Mater. 42 (1994) 2983–2997. D.B. Zahl, S. Schmauder, Comp. Mater. Sci. 3 (1994) 293–299. S. Schmauder, Annu. Rev. Mater. Res. 32 (2002) 437–465. W.A. Curtin, Composites 24 (2) (1993) 98–102. R.Q. Ye, B.Q. Han, E.J. Lavernia, Metall. Mater. Trans 36A (2005) 1833.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Experimental and simulation investigations of tensile response of (Al2 O3f /Al)w /2024Al composite Wenlong Zhang a,b,∗ , Mu Zhang a , Shojiro Ochiai b , Mingyuan Gu a a b
State Key Lab of MMCs, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, China International Innovation Center, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
a r t i c l e
i n f o
Article history: Received 28 March 2008 Received in revised form 4 June 2008 Accepted 14 July 2008 Keywords: Fiber reinforced metal matrix composites Tensile response FEM simulation Fracture mode
a b s t r a c t Tensile response of a 2024Al alloy reinforced with Al2 O3f /Al composite wires [(Al2 O3f /Al)w /2024Al composite] has been investigated by both performing tensile tests and using the finite element method (FEM). Microstructure and tensile fracture surfaces of (Al2 O3f /Al)w /2024Al composites were examined using optical microscope (OM) and scanning electron microscope (SEM), respectively. It is found that the longitudinal tensile response of (Al2 O3f /Al)w /2024Al composite is nearly elastic up to failure. The longitudinal tensile strength of (Al2 O3f /Al)w /2024Al composite reaches its fiber bundle strength and the longitudinal elastic modulus is consistent with that predicted using the rule of mixture. The contribution of matrix is negligible to the longitudinal tensile strength but not to the modulus of elasticity. Coating composite wires (CWs) with Ni can improve the interfacial bonding between the CWs and 2024Al alloy. Diffusion annealing can further improve the interfacial bonding. The transverse tensile strength is dependent strongly on the interfacial bonding between the CWs and 2024Al alloy. The stronger the interfacial bonding, the higher the transverse tensile strength. Following the diffusion annealing, ageing treatment can improve the transverse tensile strength of (Al2 O3f /Al)w /2024Al composite to a higher level due to the increased strength of matrix 2024Al alloy. To reveal the fracture mode of (Al2 O3f /Al)w /2024Al composite under transverse loading, the transverse tensile response of a model material, (Al2 O3f /matrix1)w /matrix2 composite, whose microstructure is similar to that of the (Al2 O3f /Al)w /2024Al composite, was analyzed using FEM. FEM results indicate that, under transverse loading, the stress distribution in the (Al2 O3f /matrix1)w /matrix2 composite depends on the strength of matrix2. When matrix1 and matrix2 are both pure Al, failure initiates in matrix1. When matrix1 is pure Al and matrix2 is aged 2024Al alloy, however, failure initiates in matrix2 (2024Al). Therefore, for (Al2 O3f /Al)w /2024Al composites with weak interfacial bonding between CWs and 2024Al matrix, cracks initiate at interfaces between the CWs and 2024Al matrix and propagate along the interfaces and then through 2024Al matrix, resulting in the global fracture. For (Al2 O3f /Al)w /2024Al composite with strong interfacial bonding between CWs and 2024Al matrix, cracks initiate in either pure Al or 2024Al alloy and then propagate in both Al and 2024Al, resulting in the global fracture. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Continuous alumina fiber reinforced aluminum matrix composites (CF-AMCs) have significant potential for structural applications due to their outstanding combination of high stiffness, high strength, low density as well as relatively high electrical conductivity [1]. In recent years, many researchers have focused their concerns on this kind of materials [2–6]. One of the main applications of CF-AMCs is to reinforce aluminum alloys with CF-AMCs
∗ Corresponding author at: State Key Lab of MMCs, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, China. Tel.: +86 21 54742476; fax: +86 21 34202749. E-mail address: [email protected] (W. Zhang). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.07.048
shapes such as I-beams, C-sections, wires etc. [1,2]. Rather than producing an aluminum matrix composite that is completely reinforced, the composite shapes may be strategically placed in areas of Al alloys which locally experience high stresses. Two major difficulties encountered when direct fiber reinforcement of cast parts is attempted, the required high infiltration pressure and the tendency of the reinforcing fibers to float away from their intended place, are eliminated when the fibers are introduced into the casting in the form of prefabricated MMC wire. The use of selective reinforcement enables Al alloys to be used in more demanding loading environments and hence the application of aluminium alloys in aerospace industry can be greatly widened. To get better reinforcing effects, the mechanical response of aluminum alloys reinforced with CF-AMCs shapes should be investigated.
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Fig. 2. Longitudinal section of (Al2 O3f /Al)w /2024Al composite.
Fig. 1. Typical microstructure of the wire.
In this study, an 2024Al alloy reinforced with CF-AMCs composite wires [(Al2 O3f /Al)w /2024Al composite] was fabricated using a vacuum pressure infiltration method and the tensile response in both longitudinal and transverse directions was investigated by both performing tensile tests and using the finite element method (FEM). In some cases, composite wires (CWs) were coated with Ni to improve the interfacial bonding between the CWs and 2024Al alloy matrix. 2. Experimental procedure The composite wire was produced by 3M (St. Paul, MN, USA) and supplied on a reel. The fiber volume fraction in the wire is about 45%. The diameter of the wire was about 2.0 mm. A typical microstructure of the wire is shown in Fig. 1. Fiber properties are listed in Table 1. An 2024Al alloy reinforced with Al2 O3f /Al composite wires [(Al2 O3f /Al)w /2024Al composite] was fabricated using a vacuum pressure infiltration method, in which the composite wires were closely packed together as a preform and then the preform was pressure infiltrated with molten 2024Al alloy. The morphology of the as-fabricated (Al2 O3f /Al)w /2024Al composite is shown in Fig. 2. The fiber volume fraction in (Al2 O3f /Al)w /2024Al composite is measured with LEICA MEF4Me image analyzer to be about 30%. In some cases, before infiltration, CWs were coated with Ni to improve the interfacial bonding between the CWs and 2024Al alloy matrix. The morphology of a Ni-coated composite wire was shown in Fig. 3. The thickness of Ni-coating is about 30 m. To further increase the interfacial bonding, Ni-coated (Al2 O3f /Al)w /2024Al composites [(Al2 O3f /Al)w(Ni) /2024Al] were diffusion annealed at 490 ◦ C for 12 h. To improve the transverse strength of (Al2 O3f /Al)w /2024Al composites, some specimens were solution treated at 490 ◦ C followed by Table 1 Properties of the Nextel 610TM alumina fiber [1] Composition
>99% Al2 O3
Mean tensile strength (for 2.54 cm length) Weibull modulus Modulus of elastisity Density Diameter Average coefficient of thermal expansion (20–500 ◦ C) Maximum temperature up to 900 ◦ C
2.8–3.5 GPa 9–12 380–400 GPa 3.90–3.95 g/cm3 10–12 m 7 × 10−6 ◦ C−1 90% strength retention
Fig. 3. The morphology of Ni-coated composite wire.
water quenching and then aged at 190 ◦ C for 8 h. Tensile response of (Al2 O3f /Al)w /2024Al composites was investigated by both performing tensile tests and using the finite element method. Tensile tests were performed on MTS 810 servohydraulic testing machine with standard flat dog-bone specimens. Microstructures and tensile fracture surfaces of (Al2 O3f /Al)w /2024Al composites were examined using optical microscope (OM) and scanning electron microscope (SEM), respectively. 3. Results 3.1. Longitudinal tensile response 3.1.1. (Al2 O3f /Al)w /2024Al composite A typical longitudinal tensile stress–strain curve of (Al2 O3f /Al)w /2024Al composite is shown in Fig. 4. It can be seen that the stress–strain response is nearly linear up to failure. The measured mean tensile strength and mean modulus of elasticity are 845 MPa and 170 GPa, respectively. According to data listed in Table 1, the fiber bundle strength of the (Al2 O3f /Al)w /2024Al composite can be calculated using Vf f to be 840–1050 MPa. The modulus of elasticity can be assessed using the rule of mixture (Vf Ef + (1 − Vf )Em ) and data in Table 1 as well as EAl = 70 GPa (the fiber volume fraction in (Al2 O3f /Al)w /2024Al is taken as 30%). The calculated modulus of elasticity is in the range of 163–169 GPa. Compared with the calculated ones, it can be found that the tensile strength of (Al2 O3f /Al)w /2024Al reaches its fiber bundle strength and the modulus of elasticity of (Al2 O3f /Al)w /2024Al is consistent with the prediction by the rule of mixture. 3.1.2. (Al2 O3f /Al)w(Ni) /2024Al The longitudinal tensile stress–strain response of (Al2 O3f / Al)w(Ni) /2024Al is similar in shape to that for (Al2 O3f /Al)w /2024Al.
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The mean tensile strength and mean modulus of elasticity of (Al2 O3f /Al)w(Ni) /2024Al are 844 MPa and 171 GPa, respectively, indicating that the tensile strength of (Al2 O3f /Al)w(Ni) /2024Al also reaches its fiber bundle strength and the modulus of elasticity is consistent with the prediction by the rule of mixture.
Fig. 4. Typical longitudinal tensile stress–strain curve of (Al2 O3f /Al)w /2024Al composite.
3.1.3. Longitudinal tensile fracture surfaces The longitudinal tensile fracture surfaces of (Al2 O3f /Al)w /2024Al are shown in Fig. 5. From Fig. 5a, it can be seen that the fracture surface of (Al2 O3f /Al)w /2024Al includes two parts, one is the fracture surface of CWs and the other is the fracture surface of matrix 2024 alloy. The fracture morphology of matrix 2024 alloy consists of a number of dimples, indicating that a large amount of plastic deformation has occurred before fracture. The magnified dimple morphology is shown in Fig. 5b. The bonding between CWs and 2024Al matrix alloy is weak as debonding can be clearly seen from Fig. 5a. The fracture surfaces of round CWs in (Al2 O3f /Al)w /2024Al look like a terrace and the detailed terrace-like fracture surface is shown in Fig. 5c. From Fig. 5c, no debonding can be found between Al2 O3f and Al, indi-
Fig. 5. Longitudinal tensile fracture surfaces: (a) fracture surface of (Al2 O3f /Al)w /2024Al; (b) dimples in 2024Al matrix; (c) terrace-like fracture surface of (Al2 O3f /Al)w in (a) and (d) fracture surface of (Al2 O3f /Al)w(Ni) /2024Al.
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Fig. 6. Transverse tensile stress–strain curves of (Al2 O3f /Al)w /2024Al Composites.
cating a strong interfacial bonding in CWs (Fig. 5c). The fracture surface of (Al2 O3f /Al)w(Ni) /2024Al is shown in Fig. 5d. It can be seen that the fracture surface of (Al2 O3f /Al)w(Ni) /2024Al is quite similar to that of (Al2 O3f /Al)w /2024Al. Comparing Fig. 5d with Fig. 5a it can be found that the extent of debonding between CWs and 2024Al matrix is smaller for (Al2 O3f /Al)w(Ni) /2024Al than for (Al2 O3f /Al)w /2024Al, indicating that the bonding between CWs and 2024Al matrix appears stronger for (Al2 O3f /Al)w(Ni) /2024Al than for (Al2 O3f /Al)w /2024Al. Therefore, Ni-coating can improve the interfacial bonding between CWs and 2024Al matrix. 3.2. Transverse tension 3.2.1. Transverse stress–strain response Fig. 6 illustrates the transverse tensile stress–strain curves of (Al2 O3f /Al)w /2024Al composites. The yield strength, ultimate strength and elongation of (Al2 O3f /Al)w /2024Al composites are listed in Table 2. It can be seen from Table 2 that yield strength, ultimate strength and elongation are higher for (Al2 O3f /Al)w /2024Al composite with Ni-coating than without Ni-coating. After annealing, yield strength, ultimate strength and elongation of Ni-coated (Al2 O3f /Al)w /2024Al composite are further increased. Among the four types of composites, the aged one possesses the highest yield strength, ultimate strength and elongation. 3.2.2. Transverse fracture surfaces A transverse tensile fracture surface of (Al2 O3f /Al)w /2024Al composite is shown in Fig. 7a. From Fig. 7a, debonding between CW and 2024Al matrix can be clearly seen. This indicates that the interfacial bonding between CWs and 2024Al is weak and cracks propagate along interfaces between CWs and 2024Al matrix, resulting in the wave-like fracture surface as shown in Fig. 7a. The transverse tensile fracture surface of (Al2 O3f /Al)w(Ni) /2024Al is shown in Fig. 7b. From Fig. 7b it can be found that the transverse fracture of (Al2 O3f /Al)w(Ni) /2024Al takes place through CWs rather than along interfaces between CWs and 2024Al. Meanwhile,
Fig. 7. Transverse tensile fracture surfaces: (a) (Al2 O3f /Al)w /2024Al; (b) Ni-coated and annealed (Al2 O3f /Al)w /2024Al and (c) magnified fracture surface of Ni-coated (Al2 O3f /Al)w /2024Al.
no debonding can be found between CWs and 2024Al, indicating a strong interfacial bonding between coated CWs and 2024Al. A local magnification of the fracture surface in Fig. 7b is shown in Fig. 7c. From this figure it is clear that almost all the fibers on the surface are clad by a layer of Al, indicating a strong bonding between alumina fiber and Al.
Table 2 Mechanical properties of (Al2 O3f /Al)w /2024Al composites
0.2% Yield strength (MPa) UTS (MPa) Elongation (%)
Without Ni-coating
Ni-coated
Ni-coated+annealed
Ni-coated + annealed + aged
98.42 117.20 0.50
115.05 147.81 0.657
134.61 167.22 0.67
152.70 186.77 0.683
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W. Zhang et al. / Materials Science and Engineering A 497 (2008) 44–50 Table 3 Materials parameters for finite-element calculations Parameters
Al
2024Al
Modulus (GPa) Poisson ratio Strain-hardening exponent Yield stress (MPa) Tensile strength (MPa)
68.9 0.33 0.1 60 120
72.4 0.33 0.1 274 412
Fig. 8. Schematic drawing of the microstructure of (Al2 O3f /Al)w /2024Al.
3.2.3. FEM simulation According to Fig. 2, the composite wires in (Al2 O3f /Al)w /2024Al composite distribute nearly hexagonally in the 2024Al matrix. So, the microstructure of (Al2 O3f /Al)w /2024Al can be schematically shown in Fig. 8. For regularly hexagonal arrangement, according to [7–9], based on Fig. 8, a representative volume element (RVE) can be given, as shown in Fig. 9. In the RVE, the fiber volume fraction is 30%, and the fibers and Al as well as CW and 2024Al are assumed to be bonded perfectly. The boundary condition is as follows: on the left and the bottom of REV there are symmetry boundary conditions, and the right is loaded with a horizontal displacement load. The upper sideline of the REV remains straight during the displacement loading, namely, on the upper there is a straight boundary condition. The fibers in the RVE are assumed to be elastic and their elastic modulus and Poisson’s ratio are 390 GPa and 0.23, respectively. According to [10,11], the uniaxial stress–strain curve of a typical work-hardening material can be described as follows:
=
Eε, 0.2
ε n ε0.2
ε ≤ ε0.2 , ε ≥ ε0.2
(1)
where 0.2 is the yield strength obtained at ε0.2 and n = 1/m is the strain-hardening exponent. The materials properties of Al and 2024Al alloy can be determined according to Eq. (1). The materials parameters for finite-element calculations are listed in Table 3. A finite element method, using the MARC code, was employed to carry out the stress–strain calculation. In this analysis, the 4 noded plane strain elements with full integration was used for all elements. The values of equivalent Von Mises stress for each phase
Fig. 10. Calculated transverse tensile stress–strain curves.
(fiber, Al and 2024Al) were specified during the calculation. As the displacement load increases, the failure of an element takes place when its equivalent Von Mises stress reaches the equivalent Von Mises stress corresponding to the local ultimate tensile strength (the values in Table 1). The global ultimate tensile strength was calculated when the initial failure took place. 3.2.3.1. Transverse tensile response. The calculated stress–strain curves are shown in Fig. 10. It can be seen from this figure that the flow stress of (Al2 O3f /Al)w /2024Al composite is higher than that of (Al2 O3f /Al)w /Al composite. This is a result that the strength of 2024Al alloy is higher than that of pure Al. The calculated tensile properties are listed in Table 4. It can be seen that the calculated yield stress, ultimate tensile stress and elongation are higher for (Al2 O3f /Al)w /2024Al composite than for (Al2 O3f /Al)w /Al composite. Compared with tensile properties experimentally obtained (in Table 1), the calculated properties of (Al2 O3f /Al)w /2024Al composite are closer to those of aged (Al2 O3f /Al)w(Ni) /2024Al composite, indicating that the interfacial bonding in aged (Al2 O3f /Al)w(Ni) /2024Al composite is nearly perfect. 3.2.3.2. Stress distribution. The calculated total equivalent plastic strain distributions at the initiation of plastic flow are shown in Fig. 11. It can be found that plastic flow always occurs first in Al (matrix1) in spite of matrix2, indicating that the yielding of (Al2 O3f /matrix1)w /matrix2 composites is determined by matrix1. The calculated equivalent Von Mises stress distributions are shown Table 4 Calculated mechanical properties of (Al2 O3f /Al)w /2024Al and (Al2 O3f /Al)w /Al composites
Fig. 9. Representative volume element of the microstructure of (Al2 O3f /Al)w /2024Al: X-axis is the horizontal direction and Y-axis the vertical direction.
0.2% Yield stress (MPa) Ultimate tensile stress (MPa) Elongation (%)
(Al2 O3f /Al)w /Al
(Al2 O3f /Al)w /2024Al
90.4 99.89 0.65
150.18 183.93 0. 71
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in Fig. 12. For (Al2 O3f /Al)w /Al, the maximum stress concentration is located in pure Al matrix in the CWs (Fig. 12a) and fracture initiates at the maximum stress concentration locations (Fig. 12c, as shown by the arrows). For (Al2 O3f /Al)w /2024Al, however, the maximum stress concentration is located in 2024Al matrix (Fig. 12b) and fracture initiates at the maximum stress concentration locations (Fig. 12d, as shown by the arrows). This is a result that 2024Al can carry more stress than pure Al due to its higher strength. Therefore, for (Al2 O3f /matrix1)w /matrix2 composites, increasing the strength of matrix2 can change the fracture initiation location from matrix1 to matrix2. 4. Discussion 4.1. Longitudinal tensile properties Due to the low fracture strain of the composite (about 0.5%), the contribution from the matrix strength to the strength of composite should be very small. The fact that the strength of (Al2 O3f /Al)w /2024Al composite equals its fiber bundle strength indicates that the contribution from matrix to strength of (Al2 O3f /Al)w /2024Al composite is negligible. If we take the modulus of elasticity of matrix as 70 GPa, and take the modulus of elasticity of fiber as 380 GPa, the modulus of elasticity of 30 vol.% (Al2 O3f /Al)w /2024Al composites is calculated to be 163 GPa, in which the contribution of matrix (including Al and 2024Al) is 49 GPa, accounting for about 30% of the total. Therefore, although the contribution of matrix to the tensile strength is negligible but the contribution of matrix to modulus of elasticity of the (Al2 O3f /Al)w /2024Al composite is significant. Fig. 11. Calculated total equivalent plastic strain at the initiation of plastic flow for (a) (Al2 O3f /Al)w /Al and (b) (Al2 O3f /Al)w /2024Al.
4.2. Transverse tensile properties For (Al2 O3f /Al)w /2024Al composite, the interfacial bonding between CWs and 2024Al is weak and debonding between CWs and 2024Al occurs under transverse tensile loading, resulting in
Fig. 12. Calculated equivalent Von Mises stress distributions: (a) (Al2 O3f /Al)w /Al, before failure initiation; (b) (Al2 O3f /Al)w /2024Al, before failure initiation; (c) (Al2 O3f /Al)w /Al, after fracture initiation and (d) (Al2 O3f /Al)w /2024Al, after fracture initiation.
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that the transverse yield stress, tensile strength and elongation of (Al2 O3f /Al)w /2024Al composite are low. Coating CWs with Ni can improve the bonding between CWs and matrix 2024Al alloy. After diffusion annealing, the bonding between CWs and matrix 2024Al alloy is further improved, so that the transverse yield stress, tensile strength and elongation are higher for annealed (Al2 O3f /Al)w(Ni) /2024Al than for the (Al2 O3f /Al)w(Ni) /2024Al. After ageing treatment, the fact that the tensile strength reaches the highest value is a result that the matrix 2024 alloy is further strengthened by the ageing treatment. This is consistent with the result from the FEM calculation that increasing the strength of matrix2 can improve the global strength of the (Al2 O3f /matrix1)w /matrix2 composite (Fig. 10). From the FEM calculation, the increase in strength of matrix2 can reduce the stress concentration in CWs. When the strength of matrix2 reaches the strength level of aged 2024Al, the location of crack initiation can even change from matrix1 to matrix2. Due to the change of crack initiation location from matrix1 to matrix2, crack initiation may be greatly postponed and hence the elongation of the (Al2 O3f /Al)w(Ni) /2024Al composite annealed and aged is the highest (Table 2). FEM calculation indicates the yielding of bulk composites always occurs in matrix1 in spite of matrix2. With increasing the strength of matrix2, the yielding of matrix1 can be delayed. As a result, the yield stress of (Al2 O3f /Al)w /2024Al composite is higher than that of (Al2 O3f /Al)w /Al composite (Table 4). At the same time, with increasing the strength of matrix2, the tensile strength of (Al2 O3f /matrix1)w(Ni) /matrix2 composites can be improved due to both the delayed yielding in matrix1 and the increase in strength of matrix2. 4.3. Fracture mode under transverse tension For (Al2 O3f /Al)w /2024Al composites, the interfacial bonding between composite wires and 2024Al is weak, hence fracture initiates at interfaces between CWs and 2024Al, resulting the wave-like shape fracture surface (Fig. 7a). For annealed (Al2 O3f /Al)w(Ni) /2024Al composites, the interfacial bonding between CWs and 2024Al is greatly improved due to the coating and annealing treatment, so interfacial debonding between composite wires and 2024Al cannot occur. As a result, fracture occurs through CWs not along interfaces between CWs and 2024Al (Fig. 7b). According to the FEM result shown in Fig. 11, when the strength of matrix2 is low (say pure Al alloy), failure initiates in matrix1. When the tensile strength of matrix2 reaches that of aged 2024Al (about 410 MPa), failure initiates in matrix2. Therefore, for the (Al2 O3f /Al)w(Ni) /2024Al composite annealed, the fracture mode can be described as follows: cracks initiate in pure Al (matrix1) and then propagate through Al and 2024Al, finally, a number of cracks meet together, resulting in the global fracture. For the (Al2 O3f /Al)w(Ni) /2024Al composites annealed and aged: cracks initiate in 2024Al (matrix2) and then propagate through 2024Al and Al, finally, a number of cracks link together, resulting in the global fracture. Therefore, for (Al2 O3f /Al)w /2024Al composite with strong interfacial bonding, under transverse loading, cracks ini-
tiate either in pure Al or in 2024Al and then propagate through CWs and 2024Al alloy, resulting in the fracture surface as shown in Fig. 7b. 5. Conclusions (1) The longitudinal tensile stress–strain response of (Al2 O3f / Al)w /2024Al composite is nearly elastic up to failure. The strength of (Al2 O3f /Al)w /2024Al composite reaches its fiber bundle strength and the elastic modulus is consistent with that predicted using the rule of mixture. Although the contribution of matrix to the longitudinal tensile strength is negligible but the contribution of matrix to the modulus of elasticity of (Al2 O3f /Al)w /2024Al composite is significant. (2) Coating CW with Ni can improve the bonding between CW and 2024Al alloy matrix with a further improvement by diffusion annealing, so that the tensile strength is higher for (Al2 O3f /Al)w(Ni) /2024Al composites with than without diffusion annealing. Ageing treatment further improves the strength of 2024Al alloy and hence improves the transverse tensile strength of (Al2 O3f /Al)w(Ni) /2024Al composite. Therefore, the aged (Al2 O3f /Al)w(Ni) /2024Al composite has the highest transverse tensile strength. (3) Under transverse loading, the change in strength of matrix2 can change the stress distribution in (Al2 O3f /matrix1)w /matrix2 composite. When matrix1 and matrix2 are both pure Al, failure initiates in matrix1 and when matrix1 is pure Al and matrix2 is aged 2024Al alloy, failure initiates in matrix2 (2024Al). For (Al2 O3f /Al)w /2024Al composites, the fracture mode is: cracks initiate at interfaces between CWs and 2024Al, and then propagate along the interfaces and through 2024Al matrix, resulting in the global fracture. For annealed (Al2 O3f /Al)w(Ni) /2024Al composite without or with ageing, the fracture mode is: cracks initiate either in pure Al (matrix1) or in 2024Al alloy (matrix2) and then propagate through Al and 2024Al, finally, a number of cracks meet together, resulting in the global fracture. Acknowledgment The authors are grateful for the supply of the composite wires by 3M (St. Paul, MN, USA). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
H.E. Deve, C. McCullough, JOM 47 (7) (1995) 33–37. J.T. Blucher, J. Dobranszky, U. Narusawa, Mater. Sci. Eng. A 387 (2004) 867–872. W. Zhang, M. Gu, J. Chen, et al., Mater. Sci. Eng. A 341 (2003) 9–17. U. Ramamurty, F.W. Zok, F.A. Leckie, H.E. Deve, Acta Mater. 45 (11) (1997) 4603–4613. H.E. Deve, Acta Mater. 45 (12) (1999) 5041–5046. M.D. Kriese, J.W. Hoehn, H.E. Deve, et al., Process. Fabric. Adv. Mater. IV 42 (1996) 595–607. D.B. Zahl, S. Schmauder, R.M. Mcmmeeking, Acta Metall. Mater. 42 (1994) 2983–2997. D.B. Zahl, S. Schmauder, Comp. Mater. Sci. 3 (1994) 293–299. S. Schmauder, Annu. Rev. Mater. Res. 32 (2002) 437–465. W.A. Curtin, Composites 24 (2) (1993) 98–102. R.Q. Ye, B.Q. Han, E.J. Lavernia, Metall. Mater. Trans 36A (2005) 1833.