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Processing of bulk metallic glasses with high strength and large compressive plasticity in Cu50Zr50
Scripta Materialia 54 (2006) 1145–1149 www.actamat-journals.com
Processing of bulk metallic glasses with high strength and large compressive plasticity in Cu50Zr50 Z.W. Zhu a
a,b
, H.F. Zhang
a,*
, W.S. Sun a, B.Z. Ding a, Z.Q. Hu
a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received 1 October 2005; received in revised form 21 November 2005; accepted 22 November 2005 Available online 27 December 2005
Abstract Fully metallic glassy rods of 1.5 mm diameter were successfully prepared in binary Cu50Zr50 alloy by copper mould injection casting method. The samples were characterized by X-ray diffraction, differential scanning calorimetry and high-resolution transmission electron microscopy (HRTEM). Room temperature uniaxial compression tests for the Young’s modulus, fracture strength and total strain yield values of about 88 GPa, 2550 MPa and 11.7%, respectively. Significant work hardening is also observed from the stress–strain curves. The 1–2 nm scale medium-range ordering revealed by HRTEM investigations is thought to be responsible for the high strength, significant work hardening and large plasticity of the samples. 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Bulk metallic glasses; Compression test; Plasticity; 1–2 nm scale medium-range ordering
1. Introduction Tremendous work has been done to pursue the high glass-forming ability and excellent mechanical properties of amorphous alloys since the first glassy metal was prepared directly from the melt by Duwez [1] several decades ago. Bulk metallic glasses (BMGs, the minimum dimension >1 mm) have been developed one by one in a variety of alloy systems, including Ln- [2], Mg- [3], Zr- [4,5], Fe- [6], Ti- [6], Cu-based [7] BMGs. Most of these BMGs display very low plastic flow (0–2%) under uniaxial compressive loading and none in single axis tension on a macro-scale at ambient temperature, which constrains their applications as structural materials. Plastic deformation is highly limited in a localized area, where shear bands initiate and rapidly propagate prior to catastrophic failure [6,8]. Thus, BMG matrix composites were prepared by introducing a
*
Corresponding author. Tel.: +86 24 23971783. E-mail address: [email protected] (H.F. Zhang).
second phase (ex situ and in situ) of particles, fibers, etc., to act as barriers to hinder the initiation and propagation of shear bands. The idea proved to be very effective to improve the ductility of BMGs, especially in Zr-, Ti- and Cu-based BMGs [9–15]. Recently, large values of the compression plastic deformation of 18% and 20.5% were measured for Cu47.5Zr47.5Al5 [16] and Zr62Cu15.4Ni12.6Al10 [17] BMGs, respectively. Multiplication, intersection and branching of shear bands were observed on the surface of the fractured samples [9–17]. Up to now, although the increased fracture strength and the enhanced ductility of BMGs have been attributed to these shear band features, the intrinsic reason is still unknown and under investigation. Three possibilities have been proposed as follows: (a) the precipitation of nanocrystallites in the metallic glassy matrix [13,17,18]; (b) 1–2 nm scale medium-range ordering [16,19]; (c) phase separation into regions of different compositions [20]. Although amorphous phase was formed in the binary Cu–Zr alloy system some decades ago, it is of great interest that BMG rods up to 2 mm were successfully fabricated in
1359-6462/$ - see front matter 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.11.063
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Z.W. Zhu et al. / Scripta Materialia 54 (2006) 1145–1149
that alloy system by some different research groups [21–24], as it offers an opportunity to thoroughly study the formation and deformation mechanisms of BMGs because of the simplicity of the constituent elements. For 1-mm-diameter Cu50Zr50 as-cast rods, Inoue et al. [25] reported a yield strength of 1860 MPa and an unusual plastic deformation exceeding 50%, which was attributed to the dispersion of 5–10 nm Cu5Zr phase in the amorphous matrix. Das et al. [16] reported that the cylindrical specimens of 2 mm diameter and 4 mm length of Cu50Zr50 exhibited a yield strength of 1272 MPa, a maximum strength of 1794 MPa and a total strain of 7.9%. These authors considered that the large plastic flow resulted from the combined effect of nanoscale crystallites and 1–2 nm scale medium-range ordering clusters. From these two studies, a question arises: which scale range of inhomogeneity plays the more important role during the process of plastic deformation. So far, no evidence has been produced to clarify this. In this paper, 1.5 mm-diameter Cu50Zr50 fully metallic glassy samples were successfully prepared, which possess a much higher fracture strength of 2550 MPa and still have a large plastic strain of 9.6% in the uniaxial compression tests at room temperature. The samples show no evidence for the precipitation of nanocrystallites and long-range atomic ordering in either X-ray diffraction or high-resolution transmission electron microscopy (HRTEM), and retain a distinct glass transition and a wide supercooled liquid region. The increased fracture strength and enhanced plasticity are attributed to the 1–2 nm scale medium-range ordering revealed by HRTEM.
3. Results The transverse cross-section of the as-cast rods was checked by X-ray diffraction. The estimated cooling rates for melt-spinning and copper mould casting techniques were 105 K/s and 102 K/s, respectively. The XRD patterns of the melt-spun ribbons with 60 lm in thickness and the as-cast cylindrical rod of 1.5 mm diameter are shown in Fig. 1. The patterns only consist of a broad diffraction halo and display no distinct crystalline reflection, indicating that the investigated materials are mostly amorphous. To further determine the nature of the as-cast rods, DSC was also used to compare the 1.5 mm-diameter rod with the melt-spun ribbon. Fig. 2 shows DSC traces of the as-cast rod and melt-spun ribbon obtained by continu-
2. Experimental Alloy ingots of nominal composition Cu50Zr50 at.% were prepared by arc melting a mixture of ultrasonically cleansed Cu and Zr with high purity (above 99.99%) on a water-cooled copper hearth under Ti-gettered high purity argon atmosphere. The chemical homogeneity was obtained by repeating melting four times. The ingot was then remelted under high vacuum in a quartz tube by using an induction heating coil and then injected through a nozzle with 0.5–1 mm diameter into the copper mould with a cavity of 1.5 mm diameter. In addition, ribbon samples with 60 lm in thickness were prepared by single-roller melt spinning method. The structure and glass transition were characterized by X-ray diffraction (XRD, Philips PW1050, Cu-Ka radiation), high-resolution transmission electron microscopy (HRTEM, JEOL2010) and differential scanning calorimetry (DSC, Netzsch DSC 404C). Mechanical properties were measured with the samples of 1.5 mm diameter and 3 mm length on a servo-hydraulic materials testing system (MTS 810). To perform compression tests under constant strain rate of 2 · 10 4 s 1, a MTS strain gauge was used. Fracture surface was examined by scanning electron microscopy (SEM, Supra 35).
Fig. 1. XRD patterns of melt-spun ribbons and as-cast cylindrical rods.
Fig. 2. DSC traces obtained from as-cast rods and melt-spun ribbons during continuous heating with a rate of 0.667 K/s.
Z.W. Zhu et al. / Scripta Materialia 54 (2006) 1145–1149
ous heating at a heating rate of 0.667 K/s. Each DSC profile exhibits an obvious glass transition at Tg with an onset of crystallization at Tx. The Tg, Tx and supercooled liquid region, DT (=Tx Tg), are, respectively, 685 K, 729 K, 44 K for the melt-spun ribbon and 680 K, 726 K, 46 K for the as-cast rod. The supercooled liquid region of the as-cast was found to be slightly shifted toward lower temperature compared with that of the melt-spun ribbon. The exothermic heat flow of the first crystallization peak of both samples was also measured and found to be 54 J/g for melt-spun ribbons and 53 J/g for as-cast rods. It is indicated that the as-cast rods are fully amorphous. Fig. 3 shows a high-resolution transmission electron micrograph and selected area electron diffraction pattern from the 1.5 mm-diameter as-cast rods. Once again, there is no evidence for any crystalline phases. However, there was an abundance of 1–2 nm medium-range ordering clusters marked by the circles in the Fig. 3, which may be pre-existing nuclei probably remaining from the liquid during rapid solidification into the 1.5 mm-diameter as-cast rods. The analogous structure was also observed in ZrTaCuNiAl and CuZrAl alloys by Xing et al. [19] and Das et al. [16], respectively. The mechanical behaviors of at least 10 as-cast samples of 1.5 mm diameter and 3 mm in length were investigated under uniaxial compression tests at a strain rate of 2 · 10 4 s 1 at room temperature. The stress–strain curves, as exemplified in Fig. 4, indicated that the materials deformed elastically up to about 2.1%, followed by a large plastic flow characterized by a significant work hardening resulting in a 35% increase of the stress prior to ultimate fracture and a plastic strain of about 9.6%. Moreover, one of the true stress–strain curves is also presented in the inset of Fig. 4. It is very clear that the flow stress increases after yielding at about 2.1% strain, which was not indicated in the Inoue et al. [25] data for 1 mm-diameter as-cast Cu50Zr50 samples. From Fig. 4, values of the
Fig. 3. HRTEM image of the 1.5 mm-diameter as-cast rods of Cu50Zr50 inset with the selected area electron diffraction pattern.
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Fig. 4. Compressive strain–stress curves of the as-cast 1.5 mm-diameter rod with a strain rate of 2 · 10 4 s 1 at room temperature. The inset shows the true stress–strain curve as obtained by conversion of the curve starting from about 2%.
Young’s modulus, yield stress, ultimate strength and total deformation are about 88 GPa, 1830 MPa, 2550 MPa and 11.7%, respectively. It is worth noting that the mechanical properties determined from the 1.5 mm-diameter as-cast specimens are different from those of 1 mm- and 2 mmdiameter rods with the same composition reported in Refs.
Fig. 5. SEM micrographs of (a) the fractured sample inset with the fracture surface and (b) multiple shear bands on the surface of the fractured sample.
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Z.W. Zhu et al. / Scripta Materialia 54 (2006) 1145–1149
Table 1 Mechanical properties of the 1 mm-, 1.5 mm- and 2 mm-diam. as-cast rods in Cu50Zr50 alloy Values
E (GPa)
ry (MPa)
ey (%)
rmax (MPa)
ef (%)
1 mm-diam. as-cast rods [25] 1.5 mm-diam. as-cast rods 2 mm-diam. as-cast rods [16]
104 88 ± 3 86
1860 1830 ± 150 1272
2 2.1 ± 0.2 1.7
>2200 2550 ± 110 1794
>52 11.7 ± 1.0 7.9
[16,25], particularly the working hardening in the early stage of the plastic deformation which results in high strength. To gain a better understanding of the deformation behaviors, the fracture surface of the tested specimens was examined by using SEM (Fig. 5). The as-cast sample fractured along a single plane (showed in Fig. 5(a)), revealing that a major shear band dominated the final fracture. The inclination angle is about 46.1, which indicates the plasticity in the investigated sample deviates from the classical von Mises yield criterion, as is usual in most of bulk metallic glasses [26]. The typical vein patterns were welldeveloped (showed in Fig. 5(a)), with some droplets of the melt. Multiple shear bands are observed on the surface of the fractured sample (shown in Fig. 5(b)). A large number of visible serrated branches of the primary shear bands along a certain angle to the primary shear bands are marked by the arrows in the Fig. 5(b), together with some intersections of shear bands. 4. Discussion Vitrification is a process suppressing crystal nucleation and growth [6,8]. Theoretically, a large number of nuclei under the critical size exist in the supercooled melt, especially for Cu–Zr alloys, owing to the strong attractive interaction between copper and zirconium atoms as indicated by the large negative mixing enthalpy of about 23 kJ/ mol [27]. It has been considered that the presence of the icosahedral atomic configuration results in the high glassforming ability through the achievement of high stability of the supercooled liquid against crystallization even for the Cu–Zr binary alloy [22]. However, these nuclei do not grow because of the lack of a sufficient driving force. Under rapid solidification, the atomic configuration is frozen into the solid, as illustrated in Fig. 3. From the experimental results, the special structure of 1–2 nm scale medium-range ordering seems to result in remarkable mechanical properties including the strength and plastic deformation of the bulk amorphous alloys. It is believed that this special structure ultimately changes the free volume distribution and therefore promotes the nucleation of shear bands throughout the bulk material, enabling their branching and leading to a large global plastic strain. Possible mechanisms responsible for the work hardening might include shear bands interactions, as mentioned by Das et al. [16,19,28]. The mechanical properties of 1 mm- and 2 mm-diameter as-cast rods of Cu50Zr50 were also reported in Refs. [16,25]. Table 1 summarizes the values of the mechanical properties
of the as-cast rods of Cu50Zr50 of 1, 1.5 and 2 mm diameter. It was explained that the large plastic flow (about 6.2%) of the 2 mm-diameter rods comes from the combined effect of 2–5 nm crystallites and 1–2 nm scale medium-range ordering [16]. The perfect elastic–plastic behavior and unusual plastic deformation reported for 1 mm diameter specimen could be attributed to the precipitation of 5–10 nm crystallites [25]. However, by correlating the structure of the material and the deformation and fracture behaviors in our experiments, it is thought that the higher fracture strength (2550 MPa) and the large plasticity (9.6%) exhibited by the 1.5 mm-diameter as-cast rods arise from 1 to 2 nm scale medium-range ordering. It is generally accepted that nanocrystallites are well bonded with the glassy matrix because they are formed in situ upon cooling and the intimate interfaces between them may allow an efficient stress transfer under mechanical loading [13,17,18]. But the differences in elastic properties may produce stress concentration that would initiate shear bands. The larger the number of nanocrystals, the larger the number of shear bands and the larger the plastic deformation. Inoue et al. attributed the compensation of shear softening traditionally observed in BMGs to the nanocrystals coalescence and pinning of shear bands. Such mechanisms could not take place in 1.5 mm diameter specimens; however, it is believed that at the 1–2 nm scale, medium-range ordering is more conducive than nanocrystallites to promoting work hardening in Cu50Zr50 BMGs. Investigations are currently ongoing to elucidate the work hardening mechanism. 5. Conclusions Fully metallic glassy rods of 1.5 mm diameter were successfully prepared in Cu50Zr50 alloy by copper mould injection casting method, possessing Young’s modulus of 88 ± 3 GPa, high fracture strength of 2550 ± 110 MPa and large plastic strain of 9.6 ± 1.0% in the uniaxial compression tests at room temperature. 1–2 nm medium-range ordering revealed by HRTEM is thought to be responsible for the high strength, significant work hardening and large plasticity of the 1.5 mm diameter Cu50Zr50 samples. Acknowledgements The authors gratefully acknowledge the financial support of this work by the National Natural Science Foundation of China (Grant No. 50274064 and 50471077) and the Foundation of the Chinese Academy of Sciences (Grant No. KGCX2-SW-214).
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Clement W, Willens RH, Duwez P. Nature 1960;187:869. Inoue A, Zhang T, Masunoto T. Mater Trans JIM 1989;30:965. Inoue A. Mater Trans JIM 1995;36:2342. Inoue A, Zhang T, Nishiyama N, Ohba K, Masumoto T. Mater Trans JIM 1993;34:1234. Peker A, Johnson WL. Appl Phys Lett 1993;63:2342. Inoue A. Acta Mater 2000;48:279. Inoue A, Zhang W, Zhang T, Kurosaka K. Acta Mater 2001;49:2645. Johnson WL. MRS Bull 1999;24:42. Yim HC, Johnson WL. Appl Phys Lett 1997;71:3808. Hays CC, Kim CP, Johnson WL. Phys Rev Lett 2000;84:2901. Fan C, Ott RT, Hufnagel TC. Appl Phys Lett 2002;81:1020. Qin CL, Zhang W, Kimura H, Inoue A. Mater Trans 2004;45:2936. Calin M, Eckert J, Schultz L. Scripta Mater 2003;48:653. Zhang QS, Zhang HF, Zhu ZW, Hu ZQ. Mater Trans 2005;46:730. Kim YC, Fleury E, Lee JC, Kim DH. J Mater Res 2005;20:2474. Das J, Tang MB, Kim KB, Theissmann R, Baier F, Wang WH, et al. Phys Rev Lett 2005;94:205501. Dong WB, Zhang HF, Ding BZ, Hu ZQ. Appl Phys Lett, submitted for publication.
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[18] Fan C, Li CF, Inoue A. Phys Rev B 2000;61:R3761. [19] Xing LQ, Li Y, Ramesh KT, Li J, Hufnagel TC. Phys Rev B 2001;64:180201. [20] Oh JC, Ohkubo T, Kim YC, Fleury E, Hono K. Scripta Mater 2005; 53:165. [21] Tang MB, Zhao DQ, Pan MX, Wang WH. Chin Phys Lett 2004;21: 901. [22] Inoue A, Zhang W. Mater Trans 2004;45:584. [23] Xu DH, Lohwongwatana B, Duan G, Johnson WL, Garland C. Acta Mater 2004;52:2621. [24] Wang D, Li Y, Sun BB, Sui ML, Lu K, Ma E. Appl Phys Lett 2004; 84:4029. [25] Inoue A, Zhang W, Tsurui T, Yavari AR, Greer AL. Philos Mag Lett 2005;85:221. [26] Zhang ZF, He G, Eckert J, Schultz L. Phys Rev Lett 2003;91:045505. [27] de Boer FR, Bloom R, Mattens WC, Miedema AR, Niessen AK. Cohesion in metals: transition metal alloy. Amsterdam: Ed. NorthHolland; 1989. [28] Spaepen F. Acta Metall 1977;25:407.
Processing of bulk metallic glasses with high strength and large compressive plasticity in Cu50Zr50 Z.W. Zhu a
a,b
, H.F. Zhang
a,*
, W.S. Sun a, B.Z. Ding a, Z.Q. Hu
a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received 1 October 2005; received in revised form 21 November 2005; accepted 22 November 2005 Available online 27 December 2005
Abstract Fully metallic glassy rods of 1.5 mm diameter were successfully prepared in binary Cu50Zr50 alloy by copper mould injection casting method. The samples were characterized by X-ray diffraction, differential scanning calorimetry and high-resolution transmission electron microscopy (HRTEM). Room temperature uniaxial compression tests for the Young’s modulus, fracture strength and total strain yield values of about 88 GPa, 2550 MPa and 11.7%, respectively. Significant work hardening is also observed from the stress–strain curves. The 1–2 nm scale medium-range ordering revealed by HRTEM investigations is thought to be responsible for the high strength, significant work hardening and large plasticity of the samples. 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Bulk metallic glasses; Compression test; Plasticity; 1–2 nm scale medium-range ordering
1. Introduction Tremendous work has been done to pursue the high glass-forming ability and excellent mechanical properties of amorphous alloys since the first glassy metal was prepared directly from the melt by Duwez [1] several decades ago. Bulk metallic glasses (BMGs, the minimum dimension >1 mm) have been developed one by one in a variety of alloy systems, including Ln- [2], Mg- [3], Zr- [4,5], Fe- [6], Ti- [6], Cu-based [7] BMGs. Most of these BMGs display very low plastic flow (0–2%) under uniaxial compressive loading and none in single axis tension on a macro-scale at ambient temperature, which constrains their applications as structural materials. Plastic deformation is highly limited in a localized area, where shear bands initiate and rapidly propagate prior to catastrophic failure [6,8]. Thus, BMG matrix composites were prepared by introducing a
*
Corresponding author. Tel.: +86 24 23971783. E-mail address: [email protected] (H.F. Zhang).
second phase (ex situ and in situ) of particles, fibers, etc., to act as barriers to hinder the initiation and propagation of shear bands. The idea proved to be very effective to improve the ductility of BMGs, especially in Zr-, Ti- and Cu-based BMGs [9–15]. Recently, large values of the compression plastic deformation of 18% and 20.5% were measured for Cu47.5Zr47.5Al5 [16] and Zr62Cu15.4Ni12.6Al10 [17] BMGs, respectively. Multiplication, intersection and branching of shear bands were observed on the surface of the fractured samples [9–17]. Up to now, although the increased fracture strength and the enhanced ductility of BMGs have been attributed to these shear band features, the intrinsic reason is still unknown and under investigation. Three possibilities have been proposed as follows: (a) the precipitation of nanocrystallites in the metallic glassy matrix [13,17,18]; (b) 1–2 nm scale medium-range ordering [16,19]; (c) phase separation into regions of different compositions [20]. Although amorphous phase was formed in the binary Cu–Zr alloy system some decades ago, it is of great interest that BMG rods up to 2 mm were successfully fabricated in
1359-6462/$ - see front matter 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.11.063
1146
Z.W. Zhu et al. / Scripta Materialia 54 (2006) 1145–1149
that alloy system by some different research groups [21–24], as it offers an opportunity to thoroughly study the formation and deformation mechanisms of BMGs because of the simplicity of the constituent elements. For 1-mm-diameter Cu50Zr50 as-cast rods, Inoue et al. [25] reported a yield strength of 1860 MPa and an unusual plastic deformation exceeding 50%, which was attributed to the dispersion of 5–10 nm Cu5Zr phase in the amorphous matrix. Das et al. [16] reported that the cylindrical specimens of 2 mm diameter and 4 mm length of Cu50Zr50 exhibited a yield strength of 1272 MPa, a maximum strength of 1794 MPa and a total strain of 7.9%. These authors considered that the large plastic flow resulted from the combined effect of nanoscale crystallites and 1–2 nm scale medium-range ordering clusters. From these two studies, a question arises: which scale range of inhomogeneity plays the more important role during the process of plastic deformation. So far, no evidence has been produced to clarify this. In this paper, 1.5 mm-diameter Cu50Zr50 fully metallic glassy samples were successfully prepared, which possess a much higher fracture strength of 2550 MPa and still have a large plastic strain of 9.6% in the uniaxial compression tests at room temperature. The samples show no evidence for the precipitation of nanocrystallites and long-range atomic ordering in either X-ray diffraction or high-resolution transmission electron microscopy (HRTEM), and retain a distinct glass transition and a wide supercooled liquid region. The increased fracture strength and enhanced plasticity are attributed to the 1–2 nm scale medium-range ordering revealed by HRTEM.
3. Results The transverse cross-section of the as-cast rods was checked by X-ray diffraction. The estimated cooling rates for melt-spinning and copper mould casting techniques were 105 K/s and 102 K/s, respectively. The XRD patterns of the melt-spun ribbons with 60 lm in thickness and the as-cast cylindrical rod of 1.5 mm diameter are shown in Fig. 1. The patterns only consist of a broad diffraction halo and display no distinct crystalline reflection, indicating that the investigated materials are mostly amorphous. To further determine the nature of the as-cast rods, DSC was also used to compare the 1.5 mm-diameter rod with the melt-spun ribbon. Fig. 2 shows DSC traces of the as-cast rod and melt-spun ribbon obtained by continu-
2. Experimental Alloy ingots of nominal composition Cu50Zr50 at.% were prepared by arc melting a mixture of ultrasonically cleansed Cu and Zr with high purity (above 99.99%) on a water-cooled copper hearth under Ti-gettered high purity argon atmosphere. The chemical homogeneity was obtained by repeating melting four times. The ingot was then remelted under high vacuum in a quartz tube by using an induction heating coil and then injected through a nozzle with 0.5–1 mm diameter into the copper mould with a cavity of 1.5 mm diameter. In addition, ribbon samples with 60 lm in thickness were prepared by single-roller melt spinning method. The structure and glass transition were characterized by X-ray diffraction (XRD, Philips PW1050, Cu-Ka radiation), high-resolution transmission electron microscopy (HRTEM, JEOL2010) and differential scanning calorimetry (DSC, Netzsch DSC 404C). Mechanical properties were measured with the samples of 1.5 mm diameter and 3 mm length on a servo-hydraulic materials testing system (MTS 810). To perform compression tests under constant strain rate of 2 · 10 4 s 1, a MTS strain gauge was used. Fracture surface was examined by scanning electron microscopy (SEM, Supra 35).
Fig. 1. XRD patterns of melt-spun ribbons and as-cast cylindrical rods.
Fig. 2. DSC traces obtained from as-cast rods and melt-spun ribbons during continuous heating with a rate of 0.667 K/s.
Z.W. Zhu et al. / Scripta Materialia 54 (2006) 1145–1149
ous heating at a heating rate of 0.667 K/s. Each DSC profile exhibits an obvious glass transition at Tg with an onset of crystallization at Tx. The Tg, Tx and supercooled liquid region, DT (=Tx Tg), are, respectively, 685 K, 729 K, 44 K for the melt-spun ribbon and 680 K, 726 K, 46 K for the as-cast rod. The supercooled liquid region of the as-cast was found to be slightly shifted toward lower temperature compared with that of the melt-spun ribbon. The exothermic heat flow of the first crystallization peak of both samples was also measured and found to be 54 J/g for melt-spun ribbons and 53 J/g for as-cast rods. It is indicated that the as-cast rods are fully amorphous. Fig. 3 shows a high-resolution transmission electron micrograph and selected area electron diffraction pattern from the 1.5 mm-diameter as-cast rods. Once again, there is no evidence for any crystalline phases. However, there was an abundance of 1–2 nm medium-range ordering clusters marked by the circles in the Fig. 3, which may be pre-existing nuclei probably remaining from the liquid during rapid solidification into the 1.5 mm-diameter as-cast rods. The analogous structure was also observed in ZrTaCuNiAl and CuZrAl alloys by Xing et al. [19] and Das et al. [16], respectively. The mechanical behaviors of at least 10 as-cast samples of 1.5 mm diameter and 3 mm in length were investigated under uniaxial compression tests at a strain rate of 2 · 10 4 s 1 at room temperature. The stress–strain curves, as exemplified in Fig. 4, indicated that the materials deformed elastically up to about 2.1%, followed by a large plastic flow characterized by a significant work hardening resulting in a 35% increase of the stress prior to ultimate fracture and a plastic strain of about 9.6%. Moreover, one of the true stress–strain curves is also presented in the inset of Fig. 4. It is very clear that the flow stress increases after yielding at about 2.1% strain, which was not indicated in the Inoue et al. [25] data for 1 mm-diameter as-cast Cu50Zr50 samples. From Fig. 4, values of the
Fig. 3. HRTEM image of the 1.5 mm-diameter as-cast rods of Cu50Zr50 inset with the selected area electron diffraction pattern.
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Fig. 4. Compressive strain–stress curves of the as-cast 1.5 mm-diameter rod with a strain rate of 2 · 10 4 s 1 at room temperature. The inset shows the true stress–strain curve as obtained by conversion of the curve starting from about 2%.
Young’s modulus, yield stress, ultimate strength and total deformation are about 88 GPa, 1830 MPa, 2550 MPa and 11.7%, respectively. It is worth noting that the mechanical properties determined from the 1.5 mm-diameter as-cast specimens are different from those of 1 mm- and 2 mmdiameter rods with the same composition reported in Refs.
Fig. 5. SEM micrographs of (a) the fractured sample inset with the fracture surface and (b) multiple shear bands on the surface of the fractured sample.
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Z.W. Zhu et al. / Scripta Materialia 54 (2006) 1145–1149
Table 1 Mechanical properties of the 1 mm-, 1.5 mm- and 2 mm-diam. as-cast rods in Cu50Zr50 alloy Values
E (GPa)
ry (MPa)
ey (%)
rmax (MPa)
ef (%)
1 mm-diam. as-cast rods [25] 1.5 mm-diam. as-cast rods 2 mm-diam. as-cast rods [16]
104 88 ± 3 86
1860 1830 ± 150 1272
2 2.1 ± 0.2 1.7
>2200 2550 ± 110 1794
>52 11.7 ± 1.0 7.9
[16,25], particularly the working hardening in the early stage of the plastic deformation which results in high strength. To gain a better understanding of the deformation behaviors, the fracture surface of the tested specimens was examined by using SEM (Fig. 5). The as-cast sample fractured along a single plane (showed in Fig. 5(a)), revealing that a major shear band dominated the final fracture. The inclination angle is about 46.1, which indicates the plasticity in the investigated sample deviates from the classical von Mises yield criterion, as is usual in most of bulk metallic glasses [26]. The typical vein patterns were welldeveloped (showed in Fig. 5(a)), with some droplets of the melt. Multiple shear bands are observed on the surface of the fractured sample (shown in Fig. 5(b)). A large number of visible serrated branches of the primary shear bands along a certain angle to the primary shear bands are marked by the arrows in the Fig. 5(b), together with some intersections of shear bands. 4. Discussion Vitrification is a process suppressing crystal nucleation and growth [6,8]. Theoretically, a large number of nuclei under the critical size exist in the supercooled melt, especially for Cu–Zr alloys, owing to the strong attractive interaction between copper and zirconium atoms as indicated by the large negative mixing enthalpy of about 23 kJ/ mol [27]. It has been considered that the presence of the icosahedral atomic configuration results in the high glassforming ability through the achievement of high stability of the supercooled liquid against crystallization even for the Cu–Zr binary alloy [22]. However, these nuclei do not grow because of the lack of a sufficient driving force. Under rapid solidification, the atomic configuration is frozen into the solid, as illustrated in Fig. 3. From the experimental results, the special structure of 1–2 nm scale medium-range ordering seems to result in remarkable mechanical properties including the strength and plastic deformation of the bulk amorphous alloys. It is believed that this special structure ultimately changes the free volume distribution and therefore promotes the nucleation of shear bands throughout the bulk material, enabling their branching and leading to a large global plastic strain. Possible mechanisms responsible for the work hardening might include shear bands interactions, as mentioned by Das et al. [16,19,28]. The mechanical properties of 1 mm- and 2 mm-diameter as-cast rods of Cu50Zr50 were also reported in Refs. [16,25]. Table 1 summarizes the values of the mechanical properties
of the as-cast rods of Cu50Zr50 of 1, 1.5 and 2 mm diameter. It was explained that the large plastic flow (about 6.2%) of the 2 mm-diameter rods comes from the combined effect of 2–5 nm crystallites and 1–2 nm scale medium-range ordering [16]. The perfect elastic–plastic behavior and unusual plastic deformation reported for 1 mm diameter specimen could be attributed to the precipitation of 5–10 nm crystallites [25]. However, by correlating the structure of the material and the deformation and fracture behaviors in our experiments, it is thought that the higher fracture strength (2550 MPa) and the large plasticity (9.6%) exhibited by the 1.5 mm-diameter as-cast rods arise from 1 to 2 nm scale medium-range ordering. It is generally accepted that nanocrystallites are well bonded with the glassy matrix because they are formed in situ upon cooling and the intimate interfaces between them may allow an efficient stress transfer under mechanical loading [13,17,18]. But the differences in elastic properties may produce stress concentration that would initiate shear bands. The larger the number of nanocrystals, the larger the number of shear bands and the larger the plastic deformation. Inoue et al. attributed the compensation of shear softening traditionally observed in BMGs to the nanocrystals coalescence and pinning of shear bands. Such mechanisms could not take place in 1.5 mm diameter specimens; however, it is believed that at the 1–2 nm scale, medium-range ordering is more conducive than nanocrystallites to promoting work hardening in Cu50Zr50 BMGs. Investigations are currently ongoing to elucidate the work hardening mechanism. 5. Conclusions Fully metallic glassy rods of 1.5 mm diameter were successfully prepared in Cu50Zr50 alloy by copper mould injection casting method, possessing Young’s modulus of 88 ± 3 GPa, high fracture strength of 2550 ± 110 MPa and large plastic strain of 9.6 ± 1.0% in the uniaxial compression tests at room temperature. 1–2 nm medium-range ordering revealed by HRTEM is thought to be responsible for the high strength, significant work hardening and large plasticity of the 1.5 mm diameter Cu50Zr50 samples. Acknowledgements The authors gratefully acknowledge the financial support of this work by the National Natural Science Foundation of China (Grant No. 50274064 and 50471077) and the Foundation of the Chinese Academy of Sciences (Grant No. KGCX2-SW-214).
Z.W. Zhu et al. / Scripta Materialia 54 (2006) 1145–1149
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