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Correlation of microstructure and compressive properties of amorphous matrix composites reinforced with tungsten continuous fibers or porous foams
Materials Science and Engineering A 527 (2010) 4028–4034
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Correlation of microstructure and compressive properties of amorphous matrix composites reinforced with tungsten continuous fibers or porous foams Chang-Young Son a , Sang-Bok Lee b , Sang-Kwan Lee b , Choongnyun Paul Kim a , Sunghak Lee a,∗ a b
Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea Composite Materials Laboratory, Korea Institute of Materials Science, Changwon 641-010, Republic of Korea
a r t i c l e
i n f o
Article history: Received 28 August 2009 Received in revised form 27 February 2010 Accepted 8 March 2010
Keywords: Composite Amorphous alloy Liquid process Continuous fiber Porous foam
a b s t r a c t Zr-based amorphous alloy matrix composites reinforced with tungsten continuous fibers or porous foams were fabricated without pores or defects by liquid pressing process, and their microstructures and compressive properties were investigated. About 65–70 vol.% of tungsten reinforcements were homogeneously distributed inside the amorphous matrix. The compressive test results indicated that the tungsten-reinforced composites showed considerable plastic strain as the compressive load was sustained by fibers or foams. Particularly in the tungsten porous foam-reinforced composite, the compressive stress continued to increase according to the work hardening after the yielding, thereby leading to the maximum strength of 2764 MPa and the plastic strain of 39.4%. This dramatic increase in strength and ductility was attributed to the simultaneous and homogeneous deformation at tungsten foams and amorphous matrix since tungsten foams did not show anisotropy and tungsten/matrix interfaces were excellent. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Amorphous alloys have excellent properties such as strength, stiffness, hardness, and corrosion resistance because of their peculiar liquid-like structures [1–5], and thus have been accepted as new advanced materials after amorphous alloys having high glass forming ability have been developed by conventional casting methods. For wider applications of amorphous alloys, however, there remain problems to be solved, typical one of which is brittle fracture. This brittle fracture seriously limits applications to high-performance structural components such as electronic parts and sports goods, and works as an obstacle to long life and good reliability. Active studies on developing composites in which secondary phases or reinforcements are dispersed in an amorphous alloy matrix have been conducted. Fabrication processes of amorphous matrix composites include partial crystallization of amorphous alloys to disperse nanocrystallines [6–8], formation of dendritic crystalline phases from the amorphous melt [9], addition of crystalline particles to the amorphous melt [10,11], casting of both reinforcements and amorphous alloys [12,13]. When fabricating cast amorphous matrix composites, it is important to control reactions of reinforcements with the amorphous melt. In order to effectively fabricate amorphous matrix composites, thus, it is nec-
∗ Corresponding author. Tel.: +82 54 279 2140; fax: +82 54 279 2399. E-mail address: [email protected] (S. Lee). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.03.025
essary to introduce new-concept fabrication technologies, one of which is a liquid pressing process [14–16] using a low pressure near to the theoretically required minimum loading pressure. This process might be considered as a reliable fabrication method because the crystallization of the amorphous matrix can be prevented or minimized by rapid cooling of the amorphous melt. In this study, amorphous matrix composites, whose matrix was a Zr-based amorphous alloy and reinforcements were tungsten continuous fibers or porous foams, were fabricated by the liquid pressing process. During the process, chemical reactions between tungsten reinforcements and amorphous matrix was prevented or minimized as tungsten reinforcements are thermally stable, and the thermal stress due to the difference of the thermal expansion at the tungsten/matrix interface was considerably reduced [17]. Microstructures of the fabricated composites were analyzed, and their compressive properties were evaluated. Deformation mechanisms were analyzed by observing fracture surfaces and deformed areas of the fractured composite specimens. Based on the test results, the feasibility of the liquid pressing process was verified, and mechanisms of the property improvement in the composites were investigated.
2. Experimental An ‘LM1’ alloy, which is a commercial brand name of the Liquidmetal Technologies, Lake Forest, CA, USA, was used for the composite matrix, and its chemical composition is
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Table 1 Physical and mechanical properties of tungsten and Zr-based LM1 amorphous alloy. Material Tungsten LM1 alloy
Density (g/cm3 ) 19.3 6.1
Melting temperature (◦ C) 3370 720
Zr41.2 Ti13.8 Cu12.5 Ni10.0 Be22.5 (at.%), and it has very high amorphous forming ability, hardness, strength, and corrosion resistance [1,18]. Tungsten continuous fibers or porous foams having high melting temperature, strength, and thermal stability were used as reinforcements. Scanning electron micrographs of tungsten fibers, tungsten porous foams and LM1 alloy are shown in Fig. 1(a)–(c). The diameter of tungsten fibers is about 100 m (Fig. 1(a)), and their surface is somewhat rough. Their ultimate tensile strength is about 2400 MPa, and the elongation is less than 1% because they are made by heavy deformation of drawing [19]. Tungsten porous foams were fabricated by injection molding of tungsten powders of 5 m
Thermal expansion coefficient (10−6 /◦ C) 4.6 8.5
Elastic modulus (GPa) 411 96
in diameter in Spectra-Mat. Inc., Watsonville, CA, USA (Fig. 1(b)). Open pores are sufficiently formed inside tungsten foams for the metal melt to be well infiltrated into the foams, and the porosity is measured to be about 30 vol.%. In the LM1 alloy, fine polygonal crystalline particles sized by 2–3 m are distributed in the amorphous matrix (Fig. 1(c)). These particles are identified to be fcc phases (lattice parameter; 1.185 nm) [20], and their volume fraction is 1–2%. Representative physical properties of the LM1 alloy and tungsten are summarized in Table 1. Fig. 2 shows a schematic diagram of the liquid pressing process used to fabricate the amorphous matrix composites. The inner size of the mold is 60 mm × 60 mm × 6 mm. A pre-form of tungsten fibers or tungsten porous foams, together with LM1 alloy plates, were inserted into the mold, degassed, and vacuumed. Since the melting point of the LM1 alloy was 664–720 ◦ C, the mold, inside which LM1 alloy plates were placed, was heated to 870 ◦ C and held for 5 min in order to sufficiently melt the interior LM1 alloy plates, and then pressed under a pressure of about 10 MPa. Pressing was accompanied with water cooling so that the solidified matrix could readily form amorphous phases. For convenience, the composite specimens reinforced with tungsten fibers and porous foams are referred to as ‘A’ and ‘B’, respectively. The composites were sectioned, polished, and etched in a solution of 70 ml H2 O, 25 g CrO3 , 20 ml HNO3 , and 2 ml HF for scanning electron microscope (SEM) observation. The volume fractions of tungsten fibers, tungsten porous foams, and polygonal particles in the matrix were measured by an image analyzer. The overall bulk hardness was measured by a Vickers hardness tester under a 300 g load, and the microhardness of the amorphous matrix, tungsten fibers, and tungsten porous foams were measured by an ultra-micro-Vickers hardness tester under a 2 g load. The composites were machined into cylindrical specimens of 3 mm in diameter and 6 mm in height, and room-temperature compression tests were conducted on these specimens at a strain rate of 5.6 × 10−4 s−1 by a universal testing machine (Model; 5567, Instron, USA) with capacity of 3000 kg. Fracture surfaces and deformed areas beneath the fracture surface were observed by a SEM after the test. 3. Results 3.1. Microstructure Fig. 3(a)–(d) are SEM micrographs of the amorphous matrix composites reinforced with tungsten fibers or tungsten porous
Fig. 1. (a)–(c) SEM micrographs of tungsten fibers, tungsten porous foams and the Zr-based amorphous (LM1) alloy.
Fig. 2. Schematic diagram of the liquid pressing process.
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Fig. 3. (a)–(d) SEM micrographs of the amorphous matrix composites reinforced with tungsten continuous fibers or porous foams.
foams. In the A specimen, tungsten fibers are homogeneously distributed in the amorphous matrix, and their volume fraction is 64% (Fig. 3(a)). Fine polygonal particles are observed in the matrix (Fig. 3(b)), and their fraction is measured to be about 2%, which is similar to that in the LM1 alloy (Fig. 1(c)). In the B specimen, tungsten foams are continuously connected inside the matrix, which implies that the amorphous metal melt was sufficiently infiltrated into open pores of tungsten foams (Fig. 3(c) and (d)). Pores or defects formed by misinfiltration or reaction products formed by interfacial reaction at tungsten/matrix interfaces are hardly found. This indicates a successful fabrication of the composites by the liquid pressing process. 3.2. Hardness The hardness test results are shown in Table 2. The matrix hardness of the A specimen is similar to that of the B specimen within the range of 550–600 VHN, but the hardness of tungsten fibers in
Table 2 Vickers hardness of the amorphous matrix, tungsten fiber or porous foam, and tungsten-foam-reinforced amorphous matrix composite. Material
A specimen B specimen
Vickers hardness (VHN) Amorphous matrix
Tungsten fiber or porous foam
546 ± 17 598 ± 37
680 ± 34 349 ± 25
Overall bulk – 472 ± 6
the A specimen is about twice higher than that of tungsten foams in the B specimen. This is because tungsten fibers were made by the severe deformation such as extrusion, whereas tungsten foams were made by the powder injection molding without the deformation process. The overall hardness of the B specimen is 472 VHN, and roughly satisfies with the rule of mixtures of amorphous matrix and tungsten foams. 3.3. Compressive properties Fig. 4 shows compressive stress–strain curves of the LM1 alloy and composites, and their compressive yield strength, maximum compressive strength, and plastic strain are summarized in Table 3. The yield and maximum compressive strengths of the LM1 alloy are 1920 and 1943 MPa, respectively, and the plastic strain is almost nil. The maximum strength of the A specimen is 2258 MPa, which is about 30% higher than that of the LM1 alloy. The A specimen shows the yielding phenomenon, and its plastic strain is about 1.5%. Fracture does not take place at one time after reaching the maxi-
Table 3 Compressive test results of the LM1 amorphous alloy and amorphous matrix composites reinforced with tungsten continuous fibers or tungsten porous foams. Material LM1 alloy A specimen B specimen
Yield strength (MPa) 1920 2258 1540
Ultimate strength (MPa) 1943 2477 2764
Plastic strain (%) ∼0 1.5 39.4
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Fig. 4. Compressive stress–strain curves of the LM1 amorphous alloy and amorphous matrix composites reinforced with tungsten continuous fibers or porous foams.
mum strength, but proceeds as the compressive load is sustained by fibers. The yield strength of the B specimen is 1540 MPa, which is lower than that of the LM1 alloy or the A specimen, but the maximum compressive strength is the highest (2764 MPa) among the three specimens. The B specimen shows the strain hardening after the yielding, reaches the fracture without showing the necking, and shows the plastic strain of 39.4%. Fig. 5(a) and (b) shows SEM micrograph of the side region and fractograph, respectively, of the compressively fractured specimen of the A specimen. The fracture proceeds at the maximum shear stress direction (about 45◦ to the compressive loading direction) as shown in Fig. 5(a). Longitudinal cracks along the compressive loading direction are observed inside fibers (marked by arrows), and some fibers are buckled while sustaining the applied load. Fibers work to withstand the applied load even after reaching the maximum strength, thereby showing a considerable amount of ductility. Cracks initiate along the maximum shear stress direction in the matrix, but the crack propagation is interrupted by fibers. Here, the tungsten/matrix interfacial separation is hardly found. The fracture surface contains vein patterns in the matrix region and cleavage facets of tungsten fibers (Fig. 5(b)). Fig. 6(a)–(c) are SEM micrographs of the side view, crosssectional area beneath the fracture surface, and fracture surface of the compressively fractured specimen of the B specimen. The fracture occurs at about 45◦ from the loading direction (maximum shear stress direction), as in the A specimen (Fig. 6(a)). According to the SEM micrograph (Fig. 6(b)) of the cross-sectional area beneath the fracture surface, the tungsten foam and amorphous matrix are relatively homogeneously deformed along the shear direction. As the tungsten foam and amorphous matrix are simultaneously deformed without forming tungsten/matrix interfacial separations, cracks, or voids, the B specimen shows very high ductility, while maintaining a certain level of strength (Fig. 4). On the fracture surface, prior-powder boundaries are exposed, and some boundaries seem to have been smeared by compressive loading (Fig. 6(c)). Vein patterns are observed in the matrix region, and the intergranular fracture mode is popular in the tungsten foam region. A few secondary cracks are also observed as indicated by arrows in Fig. 6(c). 4. Discussion In the present liquid pressing process, the hydrostatically applied pressure readily overrides the theoretical pressure required for infiltration. Thus, the amorphous melt sufficiently infiltrates
Fig. 5. (a) SEM micrograph of the side region and (b) SEM fractograph of the compressive test specimen of the amorphous matrix composite reinforced with tungsten continuous fibers.
into the pre-form of tungsten fibers or tungsten porous foams, and pores formed by solidification or contraction are eliminated. Other crystalline phases except polygonal particles are not found in the amorphous matrix (Fig. 3(b)), which indicates that the crystallization due to the diffusion from tungsten to the matrix or due to tungsten/matrix interfacial reaction did not occur during the liquid pressing process. This is because of the high thermal stability of tungsten as the melting temperature of tungsten fibers is 3370 ◦ C, which is much higher than the maximum processing temperature (870 ◦ C). According to the fractographic results of Figs. 5(b) and 6(c), fiber pull-out or tungsten/matrix interfacial separation is hardly found, which tells a good tungsten/matrix interfacial bonding. This also implies a successful fabrication of the amorphous matrix composites reinforced with tungsten fibers or tungsten porous foams by the liquid pressing process. The deformation in the LM1 alloy is concentrated on highly localized shear bands, and only a few shear bands work until reaching the final brittle fracture [21–25]. This is why the LM1 alloy shows a stress-shear curve observed in brittle materials such as ceramics (Fig. 4), and its plastic strain is not shown. On the other hand, the A composite shows the plastic strain of 1.5%. This is because tungsten fibers show some plastic deformation such as buckling and work to withstand a considerable amount of applied
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Fig. 6. SEM micrographs of the (a) side view, (b) cross-sectional area beneath the fracture surface, and (c) fracture surface of the compressive test specimen of the amorphous matrix composite reinforced with tungsten porous foams.
loads, while the amorphous matrix is fractured by shear cracking (Fig. 5(a)). As shown in the compressive stress–strain curve of Fig. 4, the compressive stress maintains 2200–2300 MPa after the maximum compressive stress point by the plastic deformation of fibers. This stress–strain behavior is associated with the quite strong tungsten/matrix interfaces. Inside tungsten fibers, some longitudinal cracks are formed along the compressive loading direction (Fig. 5(a)). These cracks lie on {1 0 0} planes of tungsten fibers, and play a role in interrupting the propagation of shear cracks initi-
ated at the amorphous matrix. During the compressive test of the A specimen, thus, fibers play an important role in improving the ductility by interrupting the radical propagation of shear cracks initiated at the amorphous matrix and by taking over a considerable amount of compressive loads, while fibers themselves are buckled. In addition, the strong tungsten/matrix bonding strength and the increased elongation improve the compressive strength of the A specimen over the LM1 alloy by about 30%. However, the plastic strain of about 1.5% of the A specimen cannot be sufficient, and this is attributed to the limitation of plastic deformation mechanisms of amorphous matrix and tungsten fibers. The improvement in plastic strain is limited when fibers are not sufficiently deformed, although the applied loads are homogeneously dispersed as the propagation of shear bands is interrupted by fibers. In order to further improve the plastic strain, thus, it is necessary to positively make use of plastic deformation mechanisms of tungsten reinforcements. In other words, it is necessary to change the major plastic deformation mechanism from the formation of multiple shear bands in the amorphous matrix to the activation of slips by promoting sufficient plastic strain in tungsten itself. The B specimen reinforced with tungsten foams shows a different deformation behavior from that of the LM1 alloy or A specimen. Shear bands are formed along the maximum shear stress direction, and the fracture proceeds along shear bands. Tungsten foams and matrix are homogeneously deformed together without forming tungsten/matrix interfacial separations, cracks, or voids. Consequently, the ductility dramatically increases, although the compressive yield strength decreases due to the mixing with tungsten foams which are softer than the LM1 alloy (Fig. 4). The B specimen also shows the work hardening which is rarely observed in amorphous alloys. This is associated with the homogeneous plastic deformation of both tungsten foams and amorphous matrix, thereby offering advantages for engineering designing of amorphous matrix composites. As shown in Fig. 4, the LM1 alloy does not show the yielding phenomenon, but the B specimen shows the yielding at about 1540 MPa. This is attributed to the yielding of tungsten foams. When tungsten foams and matrix do not show the anisotropy and tungsten/matrix interfaces are excellent, as in the B specimen, tungsten foams and matrix show the homogeneous deformation. Here, the yielding is not localized at one place, but occurs homogeneously throughout tungsten foams according to the constraint of the adjacent amorphous matrix. In order to examine the deformation behavior of the B specimen, the side region of a compressive specimen (rectangular bar of 3 mm × 3 mm × 6 mm) was polished, and the microstructural change of the same region was observed at the (1)–(5) stress stages as marked in Fig. 4. The resultant SEM micrographs are shown in Fig. 7(a)–(h). At the (1)–(2) stages of the elastic deformation region, the microstructural variation is not observed (Fig. 7(a) and (b)). At the (3) stage where the yielding takes place, deformation bands are initiated at some tungsten foams as indicated by arrows in Fig. 7(c) and (d), while the amorphous matrix is hardly deformed. At the (4) stage where the work hardening occurs after the yielding, deformation bands are formed at most of tungsten foams, while shear bands are formed in the amorphous matrix (Fig. 7(e) and (f)). Deformation bands formed at tungsten foams and shear bands formed at the matrix are not connected, and tend to develop in different directions. Tungsten/matrix interfacial separations, cracks, or voids are not observed at all. At the (5) stage where a considerable amount of strain is shown before the fracture, deformation bands or shear bands are developed well at all tungsten foams and amorphous matrix, which shows the homogeneous deformation (Fig. 7(g) and (h)). At this stage, cracks are initiated as some tungsten foams are broken or some tungsten prior-powder boundaries are separated (arrow-marked in Fig. 7(h)). Under further loading, it is expected
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Fig. 7. (a)–(h) SEM micrographs of the side region of the compressive test specimen of the amorphous matrix composite reinforced with tungsten porous foams. Each micrograph corresponds to (1)–(5) stages marked in Fig. 4. (d), (f), and (h) are high-magnification micrographs of (c), (e), and (g), respectively.
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that these cracks propagate and coalesce to reach the final fracture. Thus, the fracture surface shows mostly the intergranular fracture mode in the tungsten foam region, and tungsten priorpowder boundaries are exposed, while vein patterns are observed in the matrix region (Fig. 6(c)). Abrupt fracture in the LM1 alloy occurs due to the absence of obstacles when the applied stress is concentrated at one or two shear bands to reach the fracture. On the contrary, in the B specimen, shear bands or deformation bands are formed almost simultaneously in the amorphous matrix and tungsten foams, respectively. The deformation is not localized because of excellent tungsten/matrix interfaces, but takes place homogeneously throughout the specimen as the applied stress can be effectively dispersed. The amorphous matrix and tungsten foams hardly show any plastic strains, but when they are combined, the composite shows the plastic strain of 39.4%, which is much higher ductility than that of either one alone. This distinctive compressive property of the B specimen is a synergy effect arising from the mixing of amorphous matrix and tungsten foams. The maximum compressive strength reaches 2764 MPa as the work hardening occurs after the yielding and the stress continues to rise until the plastic strain of 39.4%. The present study on the amorphous matrix composites reinforced with tungsten continuous fibers or porous foams not only provides better understanding of the fabrication process of the composites, but also confirms the possibility to simultaneously enhance the strength and ductility while maintaining merits of amorphous alloys. The composites show the strength above 2.4 GPa under compressive loading, while showing the considerable ductility. In particular, the B specimen shows the excellent maximum compressive strength of 2764 MPa as well as the high plastic strain of about 40%. Since the B specimen has outstanding properties of high strength and ductility, which have been unreported in previous studies on amorphous matrix composites, it presents new applications to structural materials requiring excellent properties. Particularly, it can be applied to penetrators, in which the self-sharpening should be well promoted while keeping high specific gravity, sufficient strength, and fracture toughness. Cracks can be initiated by the formation of shear bands in the amorphous matrix or by the separation of tungsten prior-powder boundaries, and thus can work for easy removal of edge parts of the penetrator, thereby leading to the well-promoted self-sharpening and the subsequent improvement of penetration performance. In order to further enhance properties of the B specimen, intensive studies to select or develop new reinforcements and matrix alloys by new alloy designing, to establish conditions for the liquid pressing process, and to clarify deformation mechanisms involved in volume fraction of porous foams should be continued. 5. Summary In the present study, the Zr-based amorphous matrix composites reinforced with tungsten fibers or tungsten porous foams were fabricated by the liquid pressing process, and their microstructures and compressive properties were investigated.
(1) The composites with strong tungsten/matrix interfaces were successfully fabricated without pores and misinfiltration by the liquid pressing process. About 60–70 vol.% of fibers or foams were homogeneously distributed in the amorphous matrix of the composites. (2) According to the compressive test results of the tungsten-fiberreinforced composite, the fracture did not take place at one time after the maximum strength point, but proceeded as the applied load was sustained by fibers, thereby leading to the maximum strength of 2477 MPa and the plastic strain of 1.5%. Fibers played an important role in improving the strength and ductility by interrupting the radical propagation of shear cracks initiated at the amorphous matrix. (3) Though the yielding took place at 1540 MPa in the tungstenfoam-reinforced composite, the compressive stress continued to increase according to the work hardening after the yielding, thereby leading to the maximum strength of 2764 MPa and the plastic strain of 39.4%. This dramatic increase in ductility was attributed to the simultaneous and homogeneous deformation at tungsten foams and amorphous matrix since tungsten foams did not show anisotropy and tungsten/matrix interfaces were excellent. Acknowledgements This work was supported by the Center for Advanced Materials Processing (CAMP) of the 21st Century Frontier R&D Program (No. F00030492007-311006000115) funded by Ministry of Knowledge Economy, Korea, and the National Research Laboratory Program (No. M10400000361-06J0000-36110) funded by the Korea Science and Engineering Foundation (KOSEF), Korea. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
A. Peker, W.L. Johnson, Appl. Phys. Lett. 63 (1993) 2342. A. Inoue, T. Zhang, T. Masumoto, Mater. Trans. Jim. 31 (1990) 425. A. Inoue, Acta Mater. 48 (2000) 279. K. Lee, S.-B. Lee, S.-K. Lee, S. Lee, Metall. Mater. Trans. A 40 (2009) 828. H.-J. Jun, K.S. Lee, C.P. Kim, Y.W. Chang, Met. Mater. Int. 14 (2008) 297. C. Fan, C. Li, A. Inoue, V. Haas, Phys. Rev. B 61 (2000) 3761. K.W. Park, M. Wakeda, Y. Shibutani, E. Fleury, J.C. Lee, Met. Mater. Int. 14 (2008) 159. Y.H. Jo, D.H. Yeon, B.C. Mohanty, Y.S. Cho, Met. Mater. Int. 14 (2008) 493. L.Q. Xing, Y. Li, K.T. Ramesh, J. Li, T.C. Hufnagel, Phy. Rev. B 64 (2001), 180201(R). H. Choi-Yim, W.L. Johnson, Appl. Phys. Lett. 71 (1997) 3808. R.B. Dandliker, R.D. Conner, W.L. Johnson, J. Mater. Res. 13 (1998) 2896. R.D. Conner, R.B. Dandliker, W.L. Johnson, Acta Mater. 46 (1998) 6089. P. Wadhwa, J. Heinrich, R. Busch, Scripta Mater. 56 (2007) 73. Y.H. Jang, S.S. Kim, Y.C. Jung, S.K. Lee, J. Kor. Inst. Met. Mater. 42 (2004) 425. Y.H. Jang, S.S. Kim, S.K. Lee, D.H. Kim, M.K. Um, Compos. Sci. Technol. 65 (2005) 781. S.B. Lee, K. Matsunaga, Y. Ikuhara, S.K. Lee, Mater. Sci. Eng. A 449 (2007) 778. S.-B. Lee, S.-K. Lee, S. Lee, N.J. Kim, Metall. Mater. Trans. A 39 (2008) 763. W.H. Wang, C. Dong, C.H. Shek, Mater. Sci. Eng. R 44 (2004) 45. K. Lee, S.-B. Lee, S.-K. Lee, S. Lee, Metall. Mater. Trans. A 39 (2008) 1319. K. Lee, K. Euh, D.H. Nam, S. Lee, N.J. Kim, Mater. Sci. Eng. A 449 (2007) 937. A.C. Lund, C.A. Schuh, Intermetallics 12 (2004) 1159. Z.F. Zhang, G. He, J. Eckert, L. Schultz, Phys. Rev. Lett. 91 (2003), Article Num. 045505. C. Gilbert, R.O. Ritchie, W.L. Johnson, Appl. Phys. Lett. 71 (1997) 476. K. Kawamura, H. Kato, A. Inoue, T. Masumoto, Appl. Phys. Lett. 67 (1995) 2008. S.J. Lee, B.G. Yoo, J.I. Jang, J.C. Lee, Met. Mater. Int. 14 (2008) 9.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Correlation of microstructure and compressive properties of amorphous matrix composites reinforced with tungsten continuous fibers or porous foams Chang-Young Son a , Sang-Bok Lee b , Sang-Kwan Lee b , Choongnyun Paul Kim a , Sunghak Lee a,∗ a b
Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea Composite Materials Laboratory, Korea Institute of Materials Science, Changwon 641-010, Republic of Korea
a r t i c l e
i n f o
Article history: Received 28 August 2009 Received in revised form 27 February 2010 Accepted 8 March 2010
Keywords: Composite Amorphous alloy Liquid process Continuous fiber Porous foam
a b s t r a c t Zr-based amorphous alloy matrix composites reinforced with tungsten continuous fibers or porous foams were fabricated without pores or defects by liquid pressing process, and their microstructures and compressive properties were investigated. About 65–70 vol.% of tungsten reinforcements were homogeneously distributed inside the amorphous matrix. The compressive test results indicated that the tungsten-reinforced composites showed considerable plastic strain as the compressive load was sustained by fibers or foams. Particularly in the tungsten porous foam-reinforced composite, the compressive stress continued to increase according to the work hardening after the yielding, thereby leading to the maximum strength of 2764 MPa and the plastic strain of 39.4%. This dramatic increase in strength and ductility was attributed to the simultaneous and homogeneous deformation at tungsten foams and amorphous matrix since tungsten foams did not show anisotropy and tungsten/matrix interfaces were excellent. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Amorphous alloys have excellent properties such as strength, stiffness, hardness, and corrosion resistance because of their peculiar liquid-like structures [1–5], and thus have been accepted as new advanced materials after amorphous alloys having high glass forming ability have been developed by conventional casting methods. For wider applications of amorphous alloys, however, there remain problems to be solved, typical one of which is brittle fracture. This brittle fracture seriously limits applications to high-performance structural components such as electronic parts and sports goods, and works as an obstacle to long life and good reliability. Active studies on developing composites in which secondary phases or reinforcements are dispersed in an amorphous alloy matrix have been conducted. Fabrication processes of amorphous matrix composites include partial crystallization of amorphous alloys to disperse nanocrystallines [6–8], formation of dendritic crystalline phases from the amorphous melt [9], addition of crystalline particles to the amorphous melt [10,11], casting of both reinforcements and amorphous alloys [12,13]. When fabricating cast amorphous matrix composites, it is important to control reactions of reinforcements with the amorphous melt. In order to effectively fabricate amorphous matrix composites, thus, it is nec-
∗ Corresponding author. Tel.: +82 54 279 2140; fax: +82 54 279 2399. E-mail address: [email protected] (S. Lee). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.03.025
essary to introduce new-concept fabrication technologies, one of which is a liquid pressing process [14–16] using a low pressure near to the theoretically required minimum loading pressure. This process might be considered as a reliable fabrication method because the crystallization of the amorphous matrix can be prevented or minimized by rapid cooling of the amorphous melt. In this study, amorphous matrix composites, whose matrix was a Zr-based amorphous alloy and reinforcements were tungsten continuous fibers or porous foams, were fabricated by the liquid pressing process. During the process, chemical reactions between tungsten reinforcements and amorphous matrix was prevented or minimized as tungsten reinforcements are thermally stable, and the thermal stress due to the difference of the thermal expansion at the tungsten/matrix interface was considerably reduced [17]. Microstructures of the fabricated composites were analyzed, and their compressive properties were evaluated. Deformation mechanisms were analyzed by observing fracture surfaces and deformed areas of the fractured composite specimens. Based on the test results, the feasibility of the liquid pressing process was verified, and mechanisms of the property improvement in the composites were investigated.
2. Experimental An ‘LM1’ alloy, which is a commercial brand name of the Liquidmetal Technologies, Lake Forest, CA, USA, was used for the composite matrix, and its chemical composition is
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Table 1 Physical and mechanical properties of tungsten and Zr-based LM1 amorphous alloy. Material Tungsten LM1 alloy
Density (g/cm3 ) 19.3 6.1
Melting temperature (◦ C) 3370 720
Zr41.2 Ti13.8 Cu12.5 Ni10.0 Be22.5 (at.%), and it has very high amorphous forming ability, hardness, strength, and corrosion resistance [1,18]. Tungsten continuous fibers or porous foams having high melting temperature, strength, and thermal stability were used as reinforcements. Scanning electron micrographs of tungsten fibers, tungsten porous foams and LM1 alloy are shown in Fig. 1(a)–(c). The diameter of tungsten fibers is about 100 m (Fig. 1(a)), and their surface is somewhat rough. Their ultimate tensile strength is about 2400 MPa, and the elongation is less than 1% because they are made by heavy deformation of drawing [19]. Tungsten porous foams were fabricated by injection molding of tungsten powders of 5 m
Thermal expansion coefficient (10−6 /◦ C) 4.6 8.5
Elastic modulus (GPa) 411 96
in diameter in Spectra-Mat. Inc., Watsonville, CA, USA (Fig. 1(b)). Open pores are sufficiently formed inside tungsten foams for the metal melt to be well infiltrated into the foams, and the porosity is measured to be about 30 vol.%. In the LM1 alloy, fine polygonal crystalline particles sized by 2–3 m are distributed in the amorphous matrix (Fig. 1(c)). These particles are identified to be fcc phases (lattice parameter; 1.185 nm) [20], and their volume fraction is 1–2%. Representative physical properties of the LM1 alloy and tungsten are summarized in Table 1. Fig. 2 shows a schematic diagram of the liquid pressing process used to fabricate the amorphous matrix composites. The inner size of the mold is 60 mm × 60 mm × 6 mm. A pre-form of tungsten fibers or tungsten porous foams, together with LM1 alloy plates, were inserted into the mold, degassed, and vacuumed. Since the melting point of the LM1 alloy was 664–720 ◦ C, the mold, inside which LM1 alloy plates were placed, was heated to 870 ◦ C and held for 5 min in order to sufficiently melt the interior LM1 alloy plates, and then pressed under a pressure of about 10 MPa. Pressing was accompanied with water cooling so that the solidified matrix could readily form amorphous phases. For convenience, the composite specimens reinforced with tungsten fibers and porous foams are referred to as ‘A’ and ‘B’, respectively. The composites were sectioned, polished, and etched in a solution of 70 ml H2 O, 25 g CrO3 , 20 ml HNO3 , and 2 ml HF for scanning electron microscope (SEM) observation. The volume fractions of tungsten fibers, tungsten porous foams, and polygonal particles in the matrix were measured by an image analyzer. The overall bulk hardness was measured by a Vickers hardness tester under a 300 g load, and the microhardness of the amorphous matrix, tungsten fibers, and tungsten porous foams were measured by an ultra-micro-Vickers hardness tester under a 2 g load. The composites were machined into cylindrical specimens of 3 mm in diameter and 6 mm in height, and room-temperature compression tests were conducted on these specimens at a strain rate of 5.6 × 10−4 s−1 by a universal testing machine (Model; 5567, Instron, USA) with capacity of 3000 kg. Fracture surfaces and deformed areas beneath the fracture surface were observed by a SEM after the test. 3. Results 3.1. Microstructure Fig. 3(a)–(d) are SEM micrographs of the amorphous matrix composites reinforced with tungsten fibers or tungsten porous
Fig. 1. (a)–(c) SEM micrographs of tungsten fibers, tungsten porous foams and the Zr-based amorphous (LM1) alloy.
Fig. 2. Schematic diagram of the liquid pressing process.
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Fig. 3. (a)–(d) SEM micrographs of the amorphous matrix composites reinforced with tungsten continuous fibers or porous foams.
foams. In the A specimen, tungsten fibers are homogeneously distributed in the amorphous matrix, and their volume fraction is 64% (Fig. 3(a)). Fine polygonal particles are observed in the matrix (Fig. 3(b)), and their fraction is measured to be about 2%, which is similar to that in the LM1 alloy (Fig. 1(c)). In the B specimen, tungsten foams are continuously connected inside the matrix, which implies that the amorphous metal melt was sufficiently infiltrated into open pores of tungsten foams (Fig. 3(c) and (d)). Pores or defects formed by misinfiltration or reaction products formed by interfacial reaction at tungsten/matrix interfaces are hardly found. This indicates a successful fabrication of the composites by the liquid pressing process. 3.2. Hardness The hardness test results are shown in Table 2. The matrix hardness of the A specimen is similar to that of the B specimen within the range of 550–600 VHN, but the hardness of tungsten fibers in
Table 2 Vickers hardness of the amorphous matrix, tungsten fiber or porous foam, and tungsten-foam-reinforced amorphous matrix composite. Material
A specimen B specimen
Vickers hardness (VHN) Amorphous matrix
Tungsten fiber or porous foam
546 ± 17 598 ± 37
680 ± 34 349 ± 25
Overall bulk – 472 ± 6
the A specimen is about twice higher than that of tungsten foams in the B specimen. This is because tungsten fibers were made by the severe deformation such as extrusion, whereas tungsten foams were made by the powder injection molding without the deformation process. The overall hardness of the B specimen is 472 VHN, and roughly satisfies with the rule of mixtures of amorphous matrix and tungsten foams. 3.3. Compressive properties Fig. 4 shows compressive stress–strain curves of the LM1 alloy and composites, and their compressive yield strength, maximum compressive strength, and plastic strain are summarized in Table 3. The yield and maximum compressive strengths of the LM1 alloy are 1920 and 1943 MPa, respectively, and the plastic strain is almost nil. The maximum strength of the A specimen is 2258 MPa, which is about 30% higher than that of the LM1 alloy. The A specimen shows the yielding phenomenon, and its plastic strain is about 1.5%. Fracture does not take place at one time after reaching the maxi-
Table 3 Compressive test results of the LM1 amorphous alloy and amorphous matrix composites reinforced with tungsten continuous fibers or tungsten porous foams. Material LM1 alloy A specimen B specimen
Yield strength (MPa) 1920 2258 1540
Ultimate strength (MPa) 1943 2477 2764
Plastic strain (%) ∼0 1.5 39.4
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Fig. 4. Compressive stress–strain curves of the LM1 amorphous alloy and amorphous matrix composites reinforced with tungsten continuous fibers or porous foams.
mum strength, but proceeds as the compressive load is sustained by fibers. The yield strength of the B specimen is 1540 MPa, which is lower than that of the LM1 alloy or the A specimen, but the maximum compressive strength is the highest (2764 MPa) among the three specimens. The B specimen shows the strain hardening after the yielding, reaches the fracture without showing the necking, and shows the plastic strain of 39.4%. Fig. 5(a) and (b) shows SEM micrograph of the side region and fractograph, respectively, of the compressively fractured specimen of the A specimen. The fracture proceeds at the maximum shear stress direction (about 45◦ to the compressive loading direction) as shown in Fig. 5(a). Longitudinal cracks along the compressive loading direction are observed inside fibers (marked by arrows), and some fibers are buckled while sustaining the applied load. Fibers work to withstand the applied load even after reaching the maximum strength, thereby showing a considerable amount of ductility. Cracks initiate along the maximum shear stress direction in the matrix, but the crack propagation is interrupted by fibers. Here, the tungsten/matrix interfacial separation is hardly found. The fracture surface contains vein patterns in the matrix region and cleavage facets of tungsten fibers (Fig. 5(b)). Fig. 6(a)–(c) are SEM micrographs of the side view, crosssectional area beneath the fracture surface, and fracture surface of the compressively fractured specimen of the B specimen. The fracture occurs at about 45◦ from the loading direction (maximum shear stress direction), as in the A specimen (Fig. 6(a)). According to the SEM micrograph (Fig. 6(b)) of the cross-sectional area beneath the fracture surface, the tungsten foam and amorphous matrix are relatively homogeneously deformed along the shear direction. As the tungsten foam and amorphous matrix are simultaneously deformed without forming tungsten/matrix interfacial separations, cracks, or voids, the B specimen shows very high ductility, while maintaining a certain level of strength (Fig. 4). On the fracture surface, prior-powder boundaries are exposed, and some boundaries seem to have been smeared by compressive loading (Fig. 6(c)). Vein patterns are observed in the matrix region, and the intergranular fracture mode is popular in the tungsten foam region. A few secondary cracks are also observed as indicated by arrows in Fig. 6(c). 4. Discussion In the present liquid pressing process, the hydrostatically applied pressure readily overrides the theoretical pressure required for infiltration. Thus, the amorphous melt sufficiently infiltrates
Fig. 5. (a) SEM micrograph of the side region and (b) SEM fractograph of the compressive test specimen of the amorphous matrix composite reinforced with tungsten continuous fibers.
into the pre-form of tungsten fibers or tungsten porous foams, and pores formed by solidification or contraction are eliminated. Other crystalline phases except polygonal particles are not found in the amorphous matrix (Fig. 3(b)), which indicates that the crystallization due to the diffusion from tungsten to the matrix or due to tungsten/matrix interfacial reaction did not occur during the liquid pressing process. This is because of the high thermal stability of tungsten as the melting temperature of tungsten fibers is 3370 ◦ C, which is much higher than the maximum processing temperature (870 ◦ C). According to the fractographic results of Figs. 5(b) and 6(c), fiber pull-out or tungsten/matrix interfacial separation is hardly found, which tells a good tungsten/matrix interfacial bonding. This also implies a successful fabrication of the amorphous matrix composites reinforced with tungsten fibers or tungsten porous foams by the liquid pressing process. The deformation in the LM1 alloy is concentrated on highly localized shear bands, and only a few shear bands work until reaching the final brittle fracture [21–25]. This is why the LM1 alloy shows a stress-shear curve observed in brittle materials such as ceramics (Fig. 4), and its plastic strain is not shown. On the other hand, the A composite shows the plastic strain of 1.5%. This is because tungsten fibers show some plastic deformation such as buckling and work to withstand a considerable amount of applied
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Fig. 6. SEM micrographs of the (a) side view, (b) cross-sectional area beneath the fracture surface, and (c) fracture surface of the compressive test specimen of the amorphous matrix composite reinforced with tungsten porous foams.
loads, while the amorphous matrix is fractured by shear cracking (Fig. 5(a)). As shown in the compressive stress–strain curve of Fig. 4, the compressive stress maintains 2200–2300 MPa after the maximum compressive stress point by the plastic deformation of fibers. This stress–strain behavior is associated with the quite strong tungsten/matrix interfaces. Inside tungsten fibers, some longitudinal cracks are formed along the compressive loading direction (Fig. 5(a)). These cracks lie on {1 0 0} planes of tungsten fibers, and play a role in interrupting the propagation of shear cracks initi-
ated at the amorphous matrix. During the compressive test of the A specimen, thus, fibers play an important role in improving the ductility by interrupting the radical propagation of shear cracks initiated at the amorphous matrix and by taking over a considerable amount of compressive loads, while fibers themselves are buckled. In addition, the strong tungsten/matrix bonding strength and the increased elongation improve the compressive strength of the A specimen over the LM1 alloy by about 30%. However, the plastic strain of about 1.5% of the A specimen cannot be sufficient, and this is attributed to the limitation of plastic deformation mechanisms of amorphous matrix and tungsten fibers. The improvement in plastic strain is limited when fibers are not sufficiently deformed, although the applied loads are homogeneously dispersed as the propagation of shear bands is interrupted by fibers. In order to further improve the plastic strain, thus, it is necessary to positively make use of plastic deformation mechanisms of tungsten reinforcements. In other words, it is necessary to change the major plastic deformation mechanism from the formation of multiple shear bands in the amorphous matrix to the activation of slips by promoting sufficient plastic strain in tungsten itself. The B specimen reinforced with tungsten foams shows a different deformation behavior from that of the LM1 alloy or A specimen. Shear bands are formed along the maximum shear stress direction, and the fracture proceeds along shear bands. Tungsten foams and matrix are homogeneously deformed together without forming tungsten/matrix interfacial separations, cracks, or voids. Consequently, the ductility dramatically increases, although the compressive yield strength decreases due to the mixing with tungsten foams which are softer than the LM1 alloy (Fig. 4). The B specimen also shows the work hardening which is rarely observed in amorphous alloys. This is associated with the homogeneous plastic deformation of both tungsten foams and amorphous matrix, thereby offering advantages for engineering designing of amorphous matrix composites. As shown in Fig. 4, the LM1 alloy does not show the yielding phenomenon, but the B specimen shows the yielding at about 1540 MPa. This is attributed to the yielding of tungsten foams. When tungsten foams and matrix do not show the anisotropy and tungsten/matrix interfaces are excellent, as in the B specimen, tungsten foams and matrix show the homogeneous deformation. Here, the yielding is not localized at one place, but occurs homogeneously throughout tungsten foams according to the constraint of the adjacent amorphous matrix. In order to examine the deformation behavior of the B specimen, the side region of a compressive specimen (rectangular bar of 3 mm × 3 mm × 6 mm) was polished, and the microstructural change of the same region was observed at the (1)–(5) stress stages as marked in Fig. 4. The resultant SEM micrographs are shown in Fig. 7(a)–(h). At the (1)–(2) stages of the elastic deformation region, the microstructural variation is not observed (Fig. 7(a) and (b)). At the (3) stage where the yielding takes place, deformation bands are initiated at some tungsten foams as indicated by arrows in Fig. 7(c) and (d), while the amorphous matrix is hardly deformed. At the (4) stage where the work hardening occurs after the yielding, deformation bands are formed at most of tungsten foams, while shear bands are formed in the amorphous matrix (Fig. 7(e) and (f)). Deformation bands formed at tungsten foams and shear bands formed at the matrix are not connected, and tend to develop in different directions. Tungsten/matrix interfacial separations, cracks, or voids are not observed at all. At the (5) stage where a considerable amount of strain is shown before the fracture, deformation bands or shear bands are developed well at all tungsten foams and amorphous matrix, which shows the homogeneous deformation (Fig. 7(g) and (h)). At this stage, cracks are initiated as some tungsten foams are broken or some tungsten prior-powder boundaries are separated (arrow-marked in Fig. 7(h)). Under further loading, it is expected
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Fig. 7. (a)–(h) SEM micrographs of the side region of the compressive test specimen of the amorphous matrix composite reinforced with tungsten porous foams. Each micrograph corresponds to (1)–(5) stages marked in Fig. 4. (d), (f), and (h) are high-magnification micrographs of (c), (e), and (g), respectively.
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that these cracks propagate and coalesce to reach the final fracture. Thus, the fracture surface shows mostly the intergranular fracture mode in the tungsten foam region, and tungsten priorpowder boundaries are exposed, while vein patterns are observed in the matrix region (Fig. 6(c)). Abrupt fracture in the LM1 alloy occurs due to the absence of obstacles when the applied stress is concentrated at one or two shear bands to reach the fracture. On the contrary, in the B specimen, shear bands or deformation bands are formed almost simultaneously in the amorphous matrix and tungsten foams, respectively. The deformation is not localized because of excellent tungsten/matrix interfaces, but takes place homogeneously throughout the specimen as the applied stress can be effectively dispersed. The amorphous matrix and tungsten foams hardly show any plastic strains, but when they are combined, the composite shows the plastic strain of 39.4%, which is much higher ductility than that of either one alone. This distinctive compressive property of the B specimen is a synergy effect arising from the mixing of amorphous matrix and tungsten foams. The maximum compressive strength reaches 2764 MPa as the work hardening occurs after the yielding and the stress continues to rise until the plastic strain of 39.4%. The present study on the amorphous matrix composites reinforced with tungsten continuous fibers or porous foams not only provides better understanding of the fabrication process of the composites, but also confirms the possibility to simultaneously enhance the strength and ductility while maintaining merits of amorphous alloys. The composites show the strength above 2.4 GPa under compressive loading, while showing the considerable ductility. In particular, the B specimen shows the excellent maximum compressive strength of 2764 MPa as well as the high plastic strain of about 40%. Since the B specimen has outstanding properties of high strength and ductility, which have been unreported in previous studies on amorphous matrix composites, it presents new applications to structural materials requiring excellent properties. Particularly, it can be applied to penetrators, in which the self-sharpening should be well promoted while keeping high specific gravity, sufficient strength, and fracture toughness. Cracks can be initiated by the formation of shear bands in the amorphous matrix or by the separation of tungsten prior-powder boundaries, and thus can work for easy removal of edge parts of the penetrator, thereby leading to the well-promoted self-sharpening and the subsequent improvement of penetration performance. In order to further enhance properties of the B specimen, intensive studies to select or develop new reinforcements and matrix alloys by new alloy designing, to establish conditions for the liquid pressing process, and to clarify deformation mechanisms involved in volume fraction of porous foams should be continued. 5. Summary In the present study, the Zr-based amorphous matrix composites reinforced with tungsten fibers or tungsten porous foams were fabricated by the liquid pressing process, and their microstructures and compressive properties were investigated.
(1) The composites with strong tungsten/matrix interfaces were successfully fabricated without pores and misinfiltration by the liquid pressing process. About 60–70 vol.% of fibers or foams were homogeneously distributed in the amorphous matrix of the composites. (2) According to the compressive test results of the tungsten-fiberreinforced composite, the fracture did not take place at one time after the maximum strength point, but proceeded as the applied load was sustained by fibers, thereby leading to the maximum strength of 2477 MPa and the plastic strain of 1.5%. Fibers played an important role in improving the strength and ductility by interrupting the radical propagation of shear cracks initiated at the amorphous matrix. (3) Though the yielding took place at 1540 MPa in the tungstenfoam-reinforced composite, the compressive stress continued to increase according to the work hardening after the yielding, thereby leading to the maximum strength of 2764 MPa and the plastic strain of 39.4%. This dramatic increase in ductility was attributed to the simultaneous and homogeneous deformation at tungsten foams and amorphous matrix since tungsten foams did not show anisotropy and tungsten/matrix interfaces were excellent. Acknowledgements This work was supported by the Center for Advanced Materials Processing (CAMP) of the 21st Century Frontier R&D Program (No. F00030492007-311006000115) funded by Ministry of Knowledge Economy, Korea, and the National Research Laboratory Program (No. M10400000361-06J0000-36110) funded by the Korea Science and Engineering Foundation (KOSEF), Korea. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
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