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High strength tungsten wire reinforced Zr-based bulk metallic glass matrix composites prepared by continuous infiltration process
Materials Letters 93 (2013) 210–214
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
High strength tungsten wire reinforced Zr-based bulk metallic glass matrix composites prepared by continuous infiltration process B.Y. Zhang, X.H. Chen n, S.S. Wang, D.Y. Lin, X.D. Hui State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2012 Accepted 20 November 2012 Available online 29 November 2012
Tungsten fiber reinforced Zr41.25Ti13.75Cu12.5Ni10Be22.5 bulk metallic glass matrix composites (BMGMC) were prepared by continuously drawing a bunch of preform from molten alloys. The novel processing makes the interface of composites well combined and the mechanical properties enhanced, e.g., the tensile strength of BMGMC rod with 61.4 vol% tungsten fibers reaches 2867 MPa, which is the highest among this kind of composites. It is found that the strengthening of tungsten is attributed to confinement of the propagation of shear bands. & 2012 Elsevier B.V. All rights reserved.
Keywords: Bulk metallic glass matrix composite (BMGMC) Zr41.25Ti13.75Cu12.5Ni10Be22.5 (Vit 1) Tungsten Continuous infiltration process
1. Introduction Bulk metallic glasses (BMGs) have been considered as new generation of structural materials due to their high specific strength, hardness, toughness and corrosion resistance [1–3]. The feasibility of deformation in supercooled region makes BMGs applicable for micro- or nano-scale processing of complex parts [4]. Besides the above merits, BMGs show special dynamic mechanical properties and fracture mode. In contrast with crystalline materials, BMGs exhibit sharp increase in the fracture toughness at the high rate of compress deformation [5]. Moreover, it has been found that the fracture ends of BMGs rods always exhibit self-sharpening effect, which is especially needed for armor-piercing projectiles [6]. Most monolithic metallic glasses tend to form localized shear band and fail catastrophically upon yielding in tension or compression test, resulting in the softness and low plasticity. Compared with the current tungsten alloys, the densities of BMGs are relatively low, which is one of the issues to restrict their application for kinetic energy penetrator. To overcome these drawbacks of BMGs, great effort has been paid to the development of bulk metallic glasses matrix composites (BMGMCs) reinforced by heavy fibers or particles. For example, Zr41.25Ti13.75Cu12.5Ni10Be22.5 (Vit 1) and (Zr55Al10Ni5Cu30)98.5Si1.5 BMG matrix composites reinforced by tungsten and steel wires have been fabricated by infiltration process [7–10]. It is shown that the mechanical properties of these BMGs matrix composites are
n
Corresponding author. Tel.: þ86 1062333066. E-mail address: [email protected] (X.H. Chen).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.11.086
obviously improved. Especially, the penetrator performance of Vit 1 matrix composites with 85 vol% of tungsten wires is 10–20% better than tungsten heavy alloy penetrators with comparable aspect ratio [11]. By using injection casting, Zr57Nb5Al10Cul5.4Ni12 (Vit 106) BMGs matrix composites reinforced by Mo, Nb, Ta, W and WC particles were also prepared [12,13]. The compressive strain-to-failure is increased by up to a factor of 12 compared to the unreinforced BMG matrix alloy. For certain BMGs matrix alloys and the reinforcement, the methods to prepare these BMGs matrix composites are crucial. So far as present, the methods based on the melt quenching of BMGs alloys include pressure injection [14] and infiltration processes [8]. The former is only applicable for the BMGMCs with the low content of particulates. The latter is thought as the most ideal process to prepare BMGMCs, and has been widely used for various BMGMCs. This process is carried out by melting the BMGs alloy firstly in a crucible, then the molten BMG alloy is filled into the densely aligning tungsten wires under high pressure, at last form BMGMC by quenching the crucible into water. However, the infiltration process needs long time, and easily causes interfacial reaction, leading to the decrease of mechanical properties. Therefore, the length of the composite prepared by this process is usually restricted to centimeter to decimeter scale. Recently, we developed a continuous processing method to prepare BMGs coated composite wires [15,16]. By using this method, we fabricated continuously single and double tungsten wires coated with Vit 1 BMG. These composite wires possess excellent ultimate strength and ductility under tensile loading [15]. It is seen that the length of composites is no longer limited, and the productivity can be enormously increased due to the continuous drawing and dynamic coating of composite wires. The short infiltration term
B.Y. Zhang et al. / Materials Letters 93 (2013) 210–214
reduces the possibility of interfacial reaction. However, one more step is needed through hot pressing of these coated wires in supercooled liquid region to become live BMGMC materials. In this paper, we report our finding in the high strength tungsten reinforced Vit 1 matrix composites prepared by a new continuous infiltration process. This process includes continuous drawing of a tungsten bunch which contains 24 wires into Vit 1 molten alloy, dynamic infiltration of the bunch and at last the forcible cooling of the composite by argon. The tensile strength of this BMGMC prepared in this work is as high as 2867 MPa, which is the highest among the Vit 1 matrix composites reinforced by tungsten.
2. Experimental procedures The schematic of the continuous infiltration system for BMGMCs is shown in Fig. 1a. The system consists of a vacuum, heat, cooling, and motor drive unit. In this work, prealloy ingots of Zr 41.25 Ti 13.75 Cu 12.5 Ni10 Be 22.5 (Vit 1, at%) were prepared by arc melting the mixed high purity elements (99.5–99.9 wt%) under a Ti-gettered Ar atmosphere. The master ingots were remelted four times for homogenizing the alloy elements. Alloy ingots were placed in a crucible and heated to a melt temperature of 1023 K. Tungsten wires with
211
diameter of 80 mm were polished by a sand paper and cleaned by a solution of acetone and ethanol, respectively. The tungsten bunch composed of 24 wires was fed continuously from a series of supply wheels. This bunch passed through preheating unit, and then immersed into molten metal for infiltration at a drawing velocity of 10 mm/s. As the bunch was drawn out the molten pool, it was cooled forcibly by argon. At last, BMGMC rods with diameter of 0.5 mm were prepared continuously. For comparison, Vit 1 BMG rods was also prepared by copper mold casting The structure of the BMGMC was examined by Rigaku D/max-rB type of X-ray diffraction (XRD) with Cu Ka radiation. The thermal stability was investigated by using the Netzch STA449 differential scanning calorimeter (DSC) at a heating rate of 20 K/min. The room temperature tensile properties of BMGMC rods were tested by the Instron 5969 with the strain rate of 1 104 s1 , using electron extensometer with gage length of 50 mm. The BMGMC rods, 0.5 mm in diameter and 20 cm long, were mounted in a collet holder which was clamped in a parallel mode. The morphologies of cross-section and fracture surface of BMGMC rods were observed by scanning electron microscopy (SEM) using a Carl Zeiss Auriga Crossbeam Workstation. Transmission electron microscope (TEM) specimen was prepared with Carl Zeiss Auriga Crossbeam Workstation and was characterized by a Philip F200 field emission TEM.
Fig. 1. (a) Schematic of the new continuous infiltration process; (b) a bunch of BMGMC prepared in this work; (c) and (d) SEM images of the cross-section of BMGMC bunch containing 61.4 vol% tungsten wires; (e) TEM image of the interface between the tungsten and the Vit 1 is shown in the inset of d.
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B.Y. Zhang et al. / Materials Letters 93 (2013) 210–214
W wires 24W/vit1 vit1BMG
10
20
30
40 50 60 2 theta (degree)
70
80
90
1.2 Exothemic Heatflow (W/g)
W/vit 1
0.8
Tx1 ∆H=-15.1 J/g
Tg
0.4 Tx1
Tg
vit1
∆H=79.4 J/g
0.0 -0.4 -0.8 450
500
550
600 650 700 Temperature (K)
750
800
850
Fig. 2. (a) XRD and (b) DSC patterns of the BMGMC bunch containing 61.4 vol% tungsten wires, monolithic Vit 1 BMG and uncoated tungsten.
3500 3000
W wire
2500 Stress (MPa)
The coil wound by a BMGMC rod prepared with continuous infiltration is shown in Fig. 1b. It is seen that the outer surface of the BMGMC rod is smooth and has a good metallic luster. The SEM and TEM micrographs of the cross-section of the BMGMC rod are shown in Fig. 1c–e. The interfacial region is clean. No contrast corresponding to the crystalline phase can be found at the interface between matrix and wire, indicating that no reactant was formed during the infiltration process. The clean interface resulted from the dynamic infiltration process. In traditional infiltration process, the tungsten perform is static. The tungsten wires are usually held for several minutes to half an hour in the molten pool at the temperature over the melting point. Therefore, interfacial reaction easily takes place. In our previous work, it is found that the W5Zr3 and Zr2Cu phases are formed when tungsten wires are immersed into Vit 1 molten alloy for infiltration for 10 min at 1223 K [17]. The interfacial reaction is reduced as the Vit 1 alloy is modified by the addition of Nb. In this work, it is easily understood that the infiltration time is greatly shorten due to the tungsten preform is drawn continuously in the molten pool. The perform takes no more than 1 min for passing through the Vit 1 melt. Therefore, the possibility of reaction is decreased. The XRD and DSC traces of the BMGMC rod, monolithic BMG sample and tungsten wire are shown in Fig. 2. The XRD pattern of BMGMC rod exhibits four sharp crystalline peaks superimposed on the broad halo peak, indicating that the composite is composed of alpha tungsten and glassy phase. The DSC curve of BMGMC rod shows the characteristics of glass transition and multiple stage of crystallization. The glass transition temperature (T g ), the onset crystallization temperature (T x1 ), the extend of the supercooled liquid region (DT ¼ T x1 T g ) and crystallization enthalpy (DH) are 640 K, 703 K, 63 K and 15.1 J/g, respectively, which are in consistence with those of the monolithic BMG sample [18]. From the XRD and DSC patterns, it is confirmed that the composite consists of BMG and tungsten phase, which also agree with the results as shown in Fig. 1c. Fig. 3 shows the uniaxial tension curves of the BMGMC rod with 24 tungsten wires, monolithic BMG sample and tungsten wire. The tensile strength, elastic deformation and tensile plasticity of these three kinds of materials can be determined from these curves. It is seen that the BMGMC rod firstly undergoes elastic deformation before the strain is lower than 0.5%. Then plastic deformation takes place. Work-hardening can be observed during this stage, which is caused by the deformation of tungsten wires in the composite. As the strain reaches 1.75%, the BMGMC rod is fractured. From Fig. 3, the yielding strength, tensile strength and tensile plasticity for BMGMC rod are 1230 MPa, 2867 MPa, 0.8%, respectively. The Vit 1 BMG shows the tensile strength of 1965 MPa and nearly zero tensile plasticity [7]. It is seen that BMGMC wires show improved tensile properties compared with the monolithic Vit 1 BMG. It has been reported that the tensile strength of Vit 1 and (Zr55Al10Ni5Cu30)98.5Si1.5 BMGMCs with a volume fraction of 60% tungsten fiber prepared by traditional infiltration process reaches about 1500 MPa and 1400 MPa, respectively [7,9]. The compressive strength of Vit 106 BMG matrix composite reinforced by 15 vol% tungsten particulates reaches 1.96 GPa [12]. However, the tensile strength of Vit 106 matrix composites containing 5% tungsten particulate is only about 1400 MPa [12]. It is seen that the tensile strength of BMGMC rod obtained in this work is the highest among the Vit 1 matrix composites reinforced with similar fraction of tungsten prepared by to date. According to the ideal mixing rule of Young’s modulus for the composites, the Young modulus of the BMGMC can be predicted
Intensity (arbitray unit)
3. Results and discussion
W/Vit 1
2000 1500
Vit 1 BMG
1000 500 0.0
0.5
1.0
1.5
2.0
2.5
Strain (%) Fig. 3. Quasi-static tensile stress–strain curves of the BMGMC wires containing 61.4 vol% tungsten wires, uncoated tungsten wire and monolithic BMG Vit 1.
by the empirical equation EBMGMC ¼ EW V W þ EBMG V BMG
ð1Þ
where EBMGMC ,EW and EBMG are the Young modulus of the BMGMC, tungsten and Vit 1 BMG, respectively, V W and V BMG are the volume
B.Y. Zhang et al. / Materials Letters 93 (2013) 210–214
fractions of the tungsten and Vit 1 BMG, respectively. By using following data, EW ¼410 GPa [7],EBMG ¼ 96 GPa [7], V W ¼61.4%, EBMGMC is calculated as 289 GPa, which is close to the experimental value of 293 GPa obtained from Fig. 3. This result reveals that the cohesion between BMG matrix and the tungsten wire is so perfect that each component in the composite can make the most of its advantage. The fracture surface of the BMGMC rod and its side images is shown in Fig. 4. It is seen that the fracture surface of the BMGMC rod is composed of smooth shear offset zone (indicated by white arrows) and vein pattern zones. No pulling out or ungluing of tungsten wires from the Vit 1 alloy is observed. Cracks were formed in tungsten wires and at the interface between matrix and tungsten wires. This feature of crack can be also seen from the side image of the BMGMC as shown in Fig. 4b. From Fig. 4b, it is also seen that the BMGMC fractured in several shear steps. The fracture surface of Vit 1 alloy still takes approximately 481 with the tension axis. There is localized necking with the average reduction in area of 9%, indicating that the composite undergone plastic deformation before fractured. Shear bands are densely formed in Vit 1 BMG matrix. Primary shear bands are intersected with secondary shear band. It is noticeable that shear bands are stopped at the interface, implying that shear deformation can be strictly confined by tungsten wires. As shown in Fig. 1, the interface of the composite is clean and well combined by the continuous infiltration process, which ensures the transferred from matrix to reinforcement, and vice versa. Therefore, it is easy understood that the beneficial effect of tungsten wire to the shear deformation results from the perfect cohesion at the interface. Based on the tensile test and SEM observation of the fracture morphologies, the strengthening mechanism of BMGMC prepared
213
by this new process can be clearly understood. As shown in Fig. 3, the yield strengths of tungsten, Vit 1 BMG and BMGMC are 1120 MPa, 1960 MPa and 1230 MPa, respectively. Elastic deformation takes place firstly in the BMG matrix and tungsten wires as the sample is exerted a tensile stress. When the stress reaches the yielding stress of tungsten plastic deformation emerges in tungsten wires. The plastic deformation in tungsten is necessary to coordinate the deformation of BMG matrix and tungsten wires because the elastic constants of these two parts are different. On the other hand, the work hardening effect in tungsten wires ensures that the composite deform homogeneously before the stress reaches the yielding stress of Vit 1 glassy matrix. As the stress is further increased to the yielding stress of the matrix, primary shear bands are initiated and heterogeneous shear deformation is formed in the matrix. The propagation of primary shear band is hindered ahead of the interface due to the well cohesion between the wire and matrix. Accordingly, the stress is accumulated at the interface, and causes the initiation of secondary shear bands. With the progress of plastic deformation, more shear bands, i.e. local shear deformation is produced in Vit 1 matrix. As the local shear band cannot be coordinated with the plastic deformation in tungsten wires, the sample is fractured. Under the joint effect of plastic deformation of tungsten and matrix, localized necking emerges. Therefore, it is seen that the strengthening mechanism of the composite is attributed to the confinement to the propagation of shear bands by the interface. For the enhanced strength of this kind of composites, the key point is the ideal cohesion of interface.
4. Conclusions Vit 1 matrix BMGMCs containing 24 tungsten wires was prepared by a new continuous infiltration process. The BMGMCs show improved tensile properties compared with the monolithic Vit 1 BMG. The BMGMC rod containing 61.4 vol% wires exhibit tensile strength as high as 2867 MPa and plasticity of 0.75%, respectively. The strengthening mechanism of the composite is interpreted as the confinement to the propagation of shear bands by the interface. The key point is the ideal cohesion of interface.
Acknowledgments The authors are grateful for the financial support of the Specialized Research Fund for the Doctoral Program of Higher Education of China (no. 20100006120020), National Natural Science Foundation of China (nos. 51010001, 51071018 and 51001009). The authors also thank the support of the Carl Zeiss Auriga Crossbeam Workstation in the State Key Laboratory of Advanced Metals and Materials. References
Fig. 4. (a) Fracture surface and (b) side image of the BMGMC containing 61.4 vol% tungsten wires. In this letter, tungsten fiber reinforced Zr41.25Ti13.75Cu12.5Ni10Be22.5 bulk metallic glass matrix composites (BMGMC) were prepared by continuously drawing a bunch of preform from molten alloys. The novel processing makes the interface of composites well combined and the mechanical properties enhanced.
[1] Inoue A. Stabilization of metallic supercooled liquid and bulk amorphous alloys. Acta Mater 2000;48:279–306. [2] Wang WH, Dong C, Shek CH. Bulk metallic glasses. Mater Sci Eng R 2004;44:45–89. [3] Greer AL, Ma E. Bulk metallic glasses: at the cutting edge of metals research. MRS Bull 2007;32:611–9. [4] Schroers J. Processing of bulk metallic glass. Adv Mater 2010;22:1566–97. [5] Owen DM, Rosakis AJ, Johnson WL. Dynamic failure mechanisms in beryllium-bearing bulk metallic glasses. MRS Proceedings 1999:419–30. [6] Pampillo CA. Flow and fracture in amorphous alloys. J Mater Sci 1975;10:1194–227. [7] Conner RD, Dandliker RB, Johnson WL. Mechanical properties of tungsten and steel fiber reinforced Zr41.25Ti13.75Cu12.5Ni10Be22.5 metallic glass matrix composites. Acta Mater 1998;46:6089–102. [8] Dandliker RB, Conner RD, Johnson WL. Melt infiltration casting of bulk metallic-glass matrix composites. J Mater Res 1998;13:2896–901.
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[9] Qiu KQ, Wang AM, Zhang HF, Ding BZ, Hu ZQ. Mechanical properties of tungsten fiber reinforced ZrAlNiCuSi metallic glass matrix composite. Intermetallics 2002;10:1283–8. [10] Wang ML, Hui X, Chen GL. Interfacial characteristics and mechanical properties of a modified Nb-containing Zr-based bulk metallic glass composite reinforced with W fiber. J Mater Res 2008;23:320–7. [11] Conner RD, Dandliker RB, Scruggs V, Johnson WL. Dynamic deformation behavior of tungsten–fiber/metallic-glass matrix composites. Int J Imp Eng 2000;24:435–44. ¨ [12] Choi-Yim H, Busch R, Koster U, Johnson WL. Synthesis and characterization of particulate reinforced Zr57Nb5Al10Cu15.4Ni12.6 bulk metallic glass composites. Acta Mater 1999;47:2455–62. [13] Conner RD, Choi-Yim H, Johnson WL. Mechanical properties of Zr57Nb5Al10Cu15.4Ni12.6 metallic glass matrix particulate composites. J Mater Res 1999;14:3292–7.
[14] Choi-Yim H, Johnson WL. Bulk metallic glass matrix composites. Appl Phys Lett 1997;71:3808–10. [15] Chen XH, Zhang BY, Chen GL, Zhang Y, Hui XD, Lu ZP, et al. Continuously manufacturing of bulk metallic glass-coated wire composite. Intermetallics 2010;18:2034–8. [16] Zhang Y, Chen XH, Chen GL, Zhang XC, Wang ZD China patent, ZL200710120355.4; 2009. [17] Wang ML, Chen GL, Hui X, Zhang Y, Bai ZY. Optimized interface and mechanical properties of W fiber/Zr-based bulk metallic glass composites by minor Nb addition. Intermetallics 2007;15:1309–15. [18] Peker A, Johnson WL. A highly processable metallic glass: Zr41.2Ti13.8Cu12.5Ni10.0Be22.5. Appl Phys Lett 1993;63:2342–4.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
High strength tungsten wire reinforced Zr-based bulk metallic glass matrix composites prepared by continuous infiltration process B.Y. Zhang, X.H. Chen n, S.S. Wang, D.Y. Lin, X.D. Hui State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 September 2012 Accepted 20 November 2012 Available online 29 November 2012
Tungsten fiber reinforced Zr41.25Ti13.75Cu12.5Ni10Be22.5 bulk metallic glass matrix composites (BMGMC) were prepared by continuously drawing a bunch of preform from molten alloys. The novel processing makes the interface of composites well combined and the mechanical properties enhanced, e.g., the tensile strength of BMGMC rod with 61.4 vol% tungsten fibers reaches 2867 MPa, which is the highest among this kind of composites. It is found that the strengthening of tungsten is attributed to confinement of the propagation of shear bands. & 2012 Elsevier B.V. All rights reserved.
Keywords: Bulk metallic glass matrix composite (BMGMC) Zr41.25Ti13.75Cu12.5Ni10Be22.5 (Vit 1) Tungsten Continuous infiltration process
1. Introduction Bulk metallic glasses (BMGs) have been considered as new generation of structural materials due to their high specific strength, hardness, toughness and corrosion resistance [1–3]. The feasibility of deformation in supercooled region makes BMGs applicable for micro- or nano-scale processing of complex parts [4]. Besides the above merits, BMGs show special dynamic mechanical properties and fracture mode. In contrast with crystalline materials, BMGs exhibit sharp increase in the fracture toughness at the high rate of compress deformation [5]. Moreover, it has been found that the fracture ends of BMGs rods always exhibit self-sharpening effect, which is especially needed for armor-piercing projectiles [6]. Most monolithic metallic glasses tend to form localized shear band and fail catastrophically upon yielding in tension or compression test, resulting in the softness and low plasticity. Compared with the current tungsten alloys, the densities of BMGs are relatively low, which is one of the issues to restrict their application for kinetic energy penetrator. To overcome these drawbacks of BMGs, great effort has been paid to the development of bulk metallic glasses matrix composites (BMGMCs) reinforced by heavy fibers or particles. For example, Zr41.25Ti13.75Cu12.5Ni10Be22.5 (Vit 1) and (Zr55Al10Ni5Cu30)98.5Si1.5 BMG matrix composites reinforced by tungsten and steel wires have been fabricated by infiltration process [7–10]. It is shown that the mechanical properties of these BMGs matrix composites are
n
Corresponding author. Tel.: þ86 1062333066. E-mail address: [email protected] (X.H. Chen).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.11.086
obviously improved. Especially, the penetrator performance of Vit 1 matrix composites with 85 vol% of tungsten wires is 10–20% better than tungsten heavy alloy penetrators with comparable aspect ratio [11]. By using injection casting, Zr57Nb5Al10Cul5.4Ni12 (Vit 106) BMGs matrix composites reinforced by Mo, Nb, Ta, W and WC particles were also prepared [12,13]. The compressive strain-to-failure is increased by up to a factor of 12 compared to the unreinforced BMG matrix alloy. For certain BMGs matrix alloys and the reinforcement, the methods to prepare these BMGs matrix composites are crucial. So far as present, the methods based on the melt quenching of BMGs alloys include pressure injection [14] and infiltration processes [8]. The former is only applicable for the BMGMCs with the low content of particulates. The latter is thought as the most ideal process to prepare BMGMCs, and has been widely used for various BMGMCs. This process is carried out by melting the BMGs alloy firstly in a crucible, then the molten BMG alloy is filled into the densely aligning tungsten wires under high pressure, at last form BMGMC by quenching the crucible into water. However, the infiltration process needs long time, and easily causes interfacial reaction, leading to the decrease of mechanical properties. Therefore, the length of the composite prepared by this process is usually restricted to centimeter to decimeter scale. Recently, we developed a continuous processing method to prepare BMGs coated composite wires [15,16]. By using this method, we fabricated continuously single and double tungsten wires coated with Vit 1 BMG. These composite wires possess excellent ultimate strength and ductility under tensile loading [15]. It is seen that the length of composites is no longer limited, and the productivity can be enormously increased due to the continuous drawing and dynamic coating of composite wires. The short infiltration term
B.Y. Zhang et al. / Materials Letters 93 (2013) 210–214
reduces the possibility of interfacial reaction. However, one more step is needed through hot pressing of these coated wires in supercooled liquid region to become live BMGMC materials. In this paper, we report our finding in the high strength tungsten reinforced Vit 1 matrix composites prepared by a new continuous infiltration process. This process includes continuous drawing of a tungsten bunch which contains 24 wires into Vit 1 molten alloy, dynamic infiltration of the bunch and at last the forcible cooling of the composite by argon. The tensile strength of this BMGMC prepared in this work is as high as 2867 MPa, which is the highest among the Vit 1 matrix composites reinforced by tungsten.
2. Experimental procedures The schematic of the continuous infiltration system for BMGMCs is shown in Fig. 1a. The system consists of a vacuum, heat, cooling, and motor drive unit. In this work, prealloy ingots of Zr 41.25 Ti 13.75 Cu 12.5 Ni10 Be 22.5 (Vit 1, at%) were prepared by arc melting the mixed high purity elements (99.5–99.9 wt%) under a Ti-gettered Ar atmosphere. The master ingots were remelted four times for homogenizing the alloy elements. Alloy ingots were placed in a crucible and heated to a melt temperature of 1023 K. Tungsten wires with
211
diameter of 80 mm were polished by a sand paper and cleaned by a solution of acetone and ethanol, respectively. The tungsten bunch composed of 24 wires was fed continuously from a series of supply wheels. This bunch passed through preheating unit, and then immersed into molten metal for infiltration at a drawing velocity of 10 mm/s. As the bunch was drawn out the molten pool, it was cooled forcibly by argon. At last, BMGMC rods with diameter of 0.5 mm were prepared continuously. For comparison, Vit 1 BMG rods was also prepared by copper mold casting The structure of the BMGMC was examined by Rigaku D/max-rB type of X-ray diffraction (XRD) with Cu Ka radiation. The thermal stability was investigated by using the Netzch STA449 differential scanning calorimeter (DSC) at a heating rate of 20 K/min. The room temperature tensile properties of BMGMC rods were tested by the Instron 5969 with the strain rate of 1 104 s1 , using electron extensometer with gage length of 50 mm. The BMGMC rods, 0.5 mm in diameter and 20 cm long, were mounted in a collet holder which was clamped in a parallel mode. The morphologies of cross-section and fracture surface of BMGMC rods were observed by scanning electron microscopy (SEM) using a Carl Zeiss Auriga Crossbeam Workstation. Transmission electron microscope (TEM) specimen was prepared with Carl Zeiss Auriga Crossbeam Workstation and was characterized by a Philip F200 field emission TEM.
Fig. 1. (a) Schematic of the new continuous infiltration process; (b) a bunch of BMGMC prepared in this work; (c) and (d) SEM images of the cross-section of BMGMC bunch containing 61.4 vol% tungsten wires; (e) TEM image of the interface between the tungsten and the Vit 1 is shown in the inset of d.
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B.Y. Zhang et al. / Materials Letters 93 (2013) 210–214
W wires 24W/vit1 vit1BMG
10
20
30
40 50 60 2 theta (degree)
70
80
90
1.2 Exothemic Heatflow (W/g)
W/vit 1
0.8
Tx1 ∆H=-15.1 J/g
Tg
0.4 Tx1
Tg
vit1
∆H=79.4 J/g
0.0 -0.4 -0.8 450
500
550
600 650 700 Temperature (K)
750
800
850
Fig. 2. (a) XRD and (b) DSC patterns of the BMGMC bunch containing 61.4 vol% tungsten wires, monolithic Vit 1 BMG and uncoated tungsten.
3500 3000
W wire
2500 Stress (MPa)
The coil wound by a BMGMC rod prepared with continuous infiltration is shown in Fig. 1b. It is seen that the outer surface of the BMGMC rod is smooth and has a good metallic luster. The SEM and TEM micrographs of the cross-section of the BMGMC rod are shown in Fig. 1c–e. The interfacial region is clean. No contrast corresponding to the crystalline phase can be found at the interface between matrix and wire, indicating that no reactant was formed during the infiltration process. The clean interface resulted from the dynamic infiltration process. In traditional infiltration process, the tungsten perform is static. The tungsten wires are usually held for several minutes to half an hour in the molten pool at the temperature over the melting point. Therefore, interfacial reaction easily takes place. In our previous work, it is found that the W5Zr3 and Zr2Cu phases are formed when tungsten wires are immersed into Vit 1 molten alloy for infiltration for 10 min at 1223 K [17]. The interfacial reaction is reduced as the Vit 1 alloy is modified by the addition of Nb. In this work, it is easily understood that the infiltration time is greatly shorten due to the tungsten preform is drawn continuously in the molten pool. The perform takes no more than 1 min for passing through the Vit 1 melt. Therefore, the possibility of reaction is decreased. The XRD and DSC traces of the BMGMC rod, monolithic BMG sample and tungsten wire are shown in Fig. 2. The XRD pattern of BMGMC rod exhibits four sharp crystalline peaks superimposed on the broad halo peak, indicating that the composite is composed of alpha tungsten and glassy phase. The DSC curve of BMGMC rod shows the characteristics of glass transition and multiple stage of crystallization. The glass transition temperature (T g ), the onset crystallization temperature (T x1 ), the extend of the supercooled liquid region (DT ¼ T x1 T g ) and crystallization enthalpy (DH) are 640 K, 703 K, 63 K and 15.1 J/g, respectively, which are in consistence with those of the monolithic BMG sample [18]. From the XRD and DSC patterns, it is confirmed that the composite consists of BMG and tungsten phase, which also agree with the results as shown in Fig. 1c. Fig. 3 shows the uniaxial tension curves of the BMGMC rod with 24 tungsten wires, monolithic BMG sample and tungsten wire. The tensile strength, elastic deformation and tensile plasticity of these three kinds of materials can be determined from these curves. It is seen that the BMGMC rod firstly undergoes elastic deformation before the strain is lower than 0.5%. Then plastic deformation takes place. Work-hardening can be observed during this stage, which is caused by the deformation of tungsten wires in the composite. As the strain reaches 1.75%, the BMGMC rod is fractured. From Fig. 3, the yielding strength, tensile strength and tensile plasticity for BMGMC rod are 1230 MPa, 2867 MPa, 0.8%, respectively. The Vit 1 BMG shows the tensile strength of 1965 MPa and nearly zero tensile plasticity [7]. It is seen that BMGMC wires show improved tensile properties compared with the monolithic Vit 1 BMG. It has been reported that the tensile strength of Vit 1 and (Zr55Al10Ni5Cu30)98.5Si1.5 BMGMCs with a volume fraction of 60% tungsten fiber prepared by traditional infiltration process reaches about 1500 MPa and 1400 MPa, respectively [7,9]. The compressive strength of Vit 106 BMG matrix composite reinforced by 15 vol% tungsten particulates reaches 1.96 GPa [12]. However, the tensile strength of Vit 106 matrix composites containing 5% tungsten particulate is only about 1400 MPa [12]. It is seen that the tensile strength of BMGMC rod obtained in this work is the highest among the Vit 1 matrix composites reinforced with similar fraction of tungsten prepared by to date. According to the ideal mixing rule of Young’s modulus for the composites, the Young modulus of the BMGMC can be predicted
Intensity (arbitray unit)
3. Results and discussion
W/Vit 1
2000 1500
Vit 1 BMG
1000 500 0.0
0.5
1.0
1.5
2.0
2.5
Strain (%) Fig. 3. Quasi-static tensile stress–strain curves of the BMGMC wires containing 61.4 vol% tungsten wires, uncoated tungsten wire and monolithic BMG Vit 1.
by the empirical equation EBMGMC ¼ EW V W þ EBMG V BMG
ð1Þ
where EBMGMC ,EW and EBMG are the Young modulus of the BMGMC, tungsten and Vit 1 BMG, respectively, V W and V BMG are the volume
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fractions of the tungsten and Vit 1 BMG, respectively. By using following data, EW ¼410 GPa [7],EBMG ¼ 96 GPa [7], V W ¼61.4%, EBMGMC is calculated as 289 GPa, which is close to the experimental value of 293 GPa obtained from Fig. 3. This result reveals that the cohesion between BMG matrix and the tungsten wire is so perfect that each component in the composite can make the most of its advantage. The fracture surface of the BMGMC rod and its side images is shown in Fig. 4. It is seen that the fracture surface of the BMGMC rod is composed of smooth shear offset zone (indicated by white arrows) and vein pattern zones. No pulling out or ungluing of tungsten wires from the Vit 1 alloy is observed. Cracks were formed in tungsten wires and at the interface between matrix and tungsten wires. This feature of crack can be also seen from the side image of the BMGMC as shown in Fig. 4b. From Fig. 4b, it is also seen that the BMGMC fractured in several shear steps. The fracture surface of Vit 1 alloy still takes approximately 481 with the tension axis. There is localized necking with the average reduction in area of 9%, indicating that the composite undergone plastic deformation before fractured. Shear bands are densely formed in Vit 1 BMG matrix. Primary shear bands are intersected with secondary shear band. It is noticeable that shear bands are stopped at the interface, implying that shear deformation can be strictly confined by tungsten wires. As shown in Fig. 1, the interface of the composite is clean and well combined by the continuous infiltration process, which ensures the transferred from matrix to reinforcement, and vice versa. Therefore, it is easy understood that the beneficial effect of tungsten wire to the shear deformation results from the perfect cohesion at the interface. Based on the tensile test and SEM observation of the fracture morphologies, the strengthening mechanism of BMGMC prepared
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by this new process can be clearly understood. As shown in Fig. 3, the yield strengths of tungsten, Vit 1 BMG and BMGMC are 1120 MPa, 1960 MPa and 1230 MPa, respectively. Elastic deformation takes place firstly in the BMG matrix and tungsten wires as the sample is exerted a tensile stress. When the stress reaches the yielding stress of tungsten plastic deformation emerges in tungsten wires. The plastic deformation in tungsten is necessary to coordinate the deformation of BMG matrix and tungsten wires because the elastic constants of these two parts are different. On the other hand, the work hardening effect in tungsten wires ensures that the composite deform homogeneously before the stress reaches the yielding stress of Vit 1 glassy matrix. As the stress is further increased to the yielding stress of the matrix, primary shear bands are initiated and heterogeneous shear deformation is formed in the matrix. The propagation of primary shear band is hindered ahead of the interface due to the well cohesion between the wire and matrix. Accordingly, the stress is accumulated at the interface, and causes the initiation of secondary shear bands. With the progress of plastic deformation, more shear bands, i.e. local shear deformation is produced in Vit 1 matrix. As the local shear band cannot be coordinated with the plastic deformation in tungsten wires, the sample is fractured. Under the joint effect of plastic deformation of tungsten and matrix, localized necking emerges. Therefore, it is seen that the strengthening mechanism of the composite is attributed to the confinement to the propagation of shear bands by the interface. For the enhanced strength of this kind of composites, the key point is the ideal cohesion of interface.
4. Conclusions Vit 1 matrix BMGMCs containing 24 tungsten wires was prepared by a new continuous infiltration process. The BMGMCs show improved tensile properties compared with the monolithic Vit 1 BMG. The BMGMC rod containing 61.4 vol% wires exhibit tensile strength as high as 2867 MPa and plasticity of 0.75%, respectively. The strengthening mechanism of the composite is interpreted as the confinement to the propagation of shear bands by the interface. The key point is the ideal cohesion of interface.
Acknowledgments The authors are grateful for the financial support of the Specialized Research Fund for the Doctoral Program of Higher Education of China (no. 20100006120020), National Natural Science Foundation of China (nos. 51010001, 51071018 and 51001009). The authors also thank the support of the Carl Zeiss Auriga Crossbeam Workstation in the State Key Laboratory of Advanced Metals and Materials. References
Fig. 4. (a) Fracture surface and (b) side image of the BMGMC containing 61.4 vol% tungsten wires. In this letter, tungsten fiber reinforced Zr41.25Ti13.75Cu12.5Ni10Be22.5 bulk metallic glass matrix composites (BMGMC) were prepared by continuously drawing a bunch of preform from molten alloys. The novel processing makes the interface of composites well combined and the mechanical properties enhanced.
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