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A new Cu–Hf–Al ternary bulk metallic glass with high glass forming ability and ductility
Scripta Materialia 54 (2006) 2165–2168 www.actamat-journals.com
A new Cu–Hf–Al ternary bulk metallic glass with high glass forming ability and ductility P. Jia a, H. Guo a, Y. Li b, J. Xu
a,*
, E. Ma
c
a
b
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China Department of Materials Science and Engineering, National University of Singapore, Engineering Drive 1, Singapore 117675, Singapore c Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, MD 21218, USA Received 27 January 2006; accepted 21 February 2006 Available online 27 March 2006
Abstract We have discovered a new Cu-based bulk metallic glass (BMG). Although of a simple Cu49Hf42Al9 ternary composition, the as-cast alloy is a monolithic, uniform BMG with a critical diameter as large as 10 mm. The width of the supercooled liquid region DTx and the reduced glass transition temperature Trg for this glass are 85 K and 0.62, respectively. In addition to its high glass-forming ability and high density of 11 g/cc, this BMG exhibits high ductility with a compressive plastic strain of 11–13%, making it a good candidate for applications as well as for studies of deformation behavior of Cu-based BMGs. 2006 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. Keywords: Metallic glasses; Bulk amorphous alloys; Glass-forming ability; Copper; Plasticity; Ductility
1. Introduction For practical applications, it is important for a bulk metallic glass (BMG) to simultaneously have strong glass-forming ability (GFA), high strength, decent ductility, and low cost. So far, very few BMGs have possessed these four attributes all at the same time. In this regard, Cu-based BMGs are promising [1–22]. First of all, compared with other BMGs discovered so far, e.g., those in Pd-, Pt-, Zr-, Y-based systems, the Cu-based alloys offer similar or even higher strength, but are of lower cost. Secondly, the Cu-based BMGs often appear ‘‘ductile’’ in compression tests [11–16]. As summarized in Fig. 1, most of the Cu-based BMGs exhibit a compressive plastic strain at room temperature, in contrast to several other families of BMGs that border on brittle behavior [23–26]. However, the Cu-based BMGs that show appreciable plastic strains often exhibit small critical size (Dc) for glass
*
Corresponding author. Tel.: +86 24 23971950; fax: +86 24 23971215. E-mail address: [email protected] (J. Xu).
formation by copper mold casting, i.e., an inadequate GFA. Conversely, the Cu46Zr42Al7Y5 [3] and Cu44.25Ag14.75Zr36Ti5 [18] BMGs with a record Dc of 10 mm showed rather low ductility. The general trend is depicted in Fig. 1 (the area under the dashed line), showing that there is rarely a case of coexisting high GFA and high ductility. Of course, one could improve GFA further by adding Be [16], but the toxic element Be is undesirable for processing and applications. One could also improve ductility significantly by processing a composite structure made of metallic glass and nanometer-sized crystals [14,15], but in this case one cannot speak of a critical diameter for BMG formation. These cases are thus excluded from Fig. 1, which only includes monolithic, uniform BMGs that contained no obvious pre-existing inhomogeneities before deformation (at least according to the published reports). With significant heterogeneities and/or nanocrystals intentionally introduced during processing, the ductility was reported to reach as high as 16–50% in compression tests [14,15]. We embarked on a search of a BMG with simultaneously high GFA (e.g., forming 1 cm diameter rods via
1359-6462/$ - see front matter 2006 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. doi:10.1016/j.scriptamat.2006.02.042
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P. Jia et al. / Scripta Materialia 54 (2006) 2165–2168
Fig. 1. Critical size (diameter in mm) for BMG formation vs. compressive plastic strain of monolithic/uniform Cu-based BMGs. The plastic strains quoted are all for compression tests of rod samples 1–2 mm in diameter. 1 – Cu64Zr36 [19], 2 – Cu66Hf34 [22], 3 – Cu55Hf45 [20], 4 – Cu50Zr43Al7 [16], 5 – Cu45Zr45Ag10 [17], 6 – Cu60Hf25Ti15 [2], 7 – Cu52.5Hf40Al7.5 [7], 8 – Cu44.25Ag14.75Zr36Ti5 [18], 9 – Cu60Zr20Hf10Ti10 [16], 10 – Cu43Zr43Al7Ag7 [16], 11 – Cu54Zr27Ti9Be10 [5], 12 – Cu47Ti33Zr11Ni8Si1 [13], 13 – Cu47Ti33Zr7Nb4Ni8Si1 [13].
copper mold casting) and good plasticity (plastic strain in excess of 10% in compression). Recall that even in binary systems such as Cu–Zr and Cu–Hf, BMG rods 1–2 mm in diameter can be fabricated [19–22]. Also note that ductility has been reported for simple ternary BMGs, Cu–Zr–Al [14], although there the reported Dc is only a couple of millimeters and the glass is non-uniform. Cu–Hf–Al compositions are likely to be promising in yielding a BMG with large Dc and good mechanical properties, even through a BMG with a Dc of only 3 mm has been found in this ternary system at Cu50Hf42.5Al7.5 and Cu52.5Hf40Al7.5 without pinpointing the composition at 1 at.% interval [7]. The density (q) and Poisson’s ratio (t) for the Cu50Hf43Al7 were determined to be 11.0 g/cc and 0.358, respectively [24]. In the following, we report the discovery of a simple ternary BMG former in this system that not only reached Dc = 10 mm, but also exhibited a plastic strain as large as 11–13%.
calorimeter (Diamond-DSC) under flowing purified argon. A heating rate of 40 K min1 was employed. To confirm the reproducibility of the results, at least three samples were measured. All the glass transition temperature Tg and onset temperature of crystallization Tx measurements were reproducible within an error of ±1 K. The heat of crystallization DHx for the glassy phase was determined by integrating the area under the DSC curve. The melting behavior of the alloys was measured in a Netzsch 404 DSC with alumina container, using a heating rate of 20 K min1. Samples for conventional and high-resolution transmission electron microscopy (TEM) observations in a FEI Tecnai F30 electron microscope were prepared by twin-jet electropolishing, with a solution of 9% nitric acid in a mixture of methanol and butoxyethanol (2:1). The compression test samples 2 mm in height were cut from the as-cast rods of 1 mm in diameter, prepared by suction casting. The loading surfaces were polished to be parallel to an accuracy of less than 10 lm. Room temperature compression tests were carried out using a strain rate of 1 · 104 s1. At least five samples were measured to ensure that the results were reproducible. The strain was determined from the platen displacement after correction for machine compliance. 3. Results and discussion In this work, we tested the GFA of a number of Cu–Hf–Al compositions. The mapping was done at 1 at.% intervals to capture the strong composition dependence of GFA. Fig. 2 displays the XRD patterns of several as-cast alloys 8–12 mm in diameter. The Cu49Hf42Al9 alloy exhibits the best GFA: both the 8 mm and 10 mm rods are apparently glassy, while at 12 mm the glassy matrix contained some crystals such as CuHf2 compound. It was concluded that the Dc is as large as 10 mm, equaling that of the best Cu-based BMGs reported previously [3,18]. Interestingly, the GFA of the alloy we located is significantly stronger than that of Cu50Hf42.5Al7.5 and Cu52.5Hf40Al7.5
2. Experimental Elemental pieces with purity better than 99.9% were used as starting materials. The master alloy ingots with the nominal composition (in at.%) were prepared by arc melting under a Ti-gettered argon atmosphere in a watercooled copper crucible. The alloy ingots were melted several times to ensure compositional homogeneity. To produce rods with different diameters, the master alloy re-melted in a tilting water-cooled copper hearth was cast into the copper mold that has internal rod-shaped cavities of about 60 mm in length. The cross-sectional surfaces of the as-cast rods were analyzed by X-ray diffraction (XRD) using a Rigaku D/max 2400 diffractometer with monochromated CuKa radiation. The glass transition and crystallization of the cast samples were investigated in a Perkin–Elmer differential scanning
Fig. 2. XRD patterns taken from the cross-sectional surface of as-cast rods 8, 10 and 12 mm in diameters for the Cu49Hf42Al9 alloy.
P. Jia et al. / Scripta Materialia 54 (2006) 2165–2168
previously reported by Inoue and Zhang [7]. The TEM micrograph and selected area electron diffraction (SAED) pattern are shown in Fig. 3(a) and (b), and the high-resolution image is presented in Fig. 3(c). The sample was taken from the center of the 10 mm as-cast rod. The homogeneous contrast in the images and a broad halo in the SAED pattern indicate the formation of a single amorphous phase. There are no visible crystals remaining in the sample, corroborating the XRD result that the material is fully amorphous. We did not detect obvious contrast in all the images, indicating that the glass is rather uniform in composition and structure. Fig. 4(a) shows the DSC curves of the as-cast BMG rod with D = 1 and 10 mm. A sharp exothermic crystallization peak and a clear endothermic event associated with glass transition are observed. The Tg, Tx, DTx (DTx = Tx Tg) and DHx values are listed in Table 1. The alloy exhibits a large supercooled liquid region, with DTx = 85 K. The crystallization is completed in a single step. The DHx value of 1- and 10-mm BMGs is nearly the same within experimental error, supporting that an as-cast 10-mm rod is fully glassy. Fig. 4(b) shows the DSC heating and cooling curves of the 10 mm BMG, near its melting temperature. These curves show at least two events, indicating that these alloys are at off-eutectic compositions. The Tm (eutectic tempera-
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Fig. 4. (a) DSC scans of as-cast rods of the Cu49Hf42Al9 BMG with 1- and 10-mm diameters. (b) DSC heating and cooling curves of the Cu49Hf42Al9 alloy near its melting temperatures (at a heating rate of 20 K min1).
Table 1 Thermal properties determined using DSC of Cu49Hf42Al9 BMGs fabricated using copper mold casting
Fig. 3. (a) TEM bright field image and (b) corresponding selected area electron diffraction pattern and (c) high-resolution TEM image for the Cu49Hf42Al9 rods of 10 mm in diameter. The sample was taken from the center of the as-cast rod.
Diameter of as-cast rods (mm)
Tg (K)
Tx (K)
DTx (K)
DHx (kJ/mol)
Tm (K)
TL (K)
Trg
1 10
777 778
863 863
86 85
4.16 4.12
– 1212
– 1249
– 0.622
ture), TL (liquidus temperature) and the calculated reduced glass transition temperature Trg (Trg = Tg/TL) are included in Table 1 as well. The Trg value is 0.62, reflecting that the alloy is a good glass former. The eutectic temperature of the ternary alloy (1212 K) is about 40 K lower than that of the related Cu–Hf binary eutectic reaction (L ! Cu10Hf7 + CuHf2, 1253 K), suggesting that the liquid of the ternary alloy is more stable than that of a simple binary system, thereby favoring the glass formation. From the uniaxial compressive stress–strain curves, such as that in Fig. 5(a), the yield stress ry, fracture stress rf, elastic strain ee, plastic strain ep and elastic modulus E for the 1-mm Cu49Hf42Al9 glass are obtained as 2408 MPa, 2620 MPa, 1.9–2.1%, 10.5–13.0%, 102 GPa, respectively. Note that the plastic strain is among the largest of all the Cu-based BMGs that are monolithically and uniformly glassy in the as-cast state before testing [2,5,7, 13,16–20,22], see Fig. 1. The large plasticity is reflected by the many and closely spaced shear bands seen in the scanning electron microscopy (SEM) image of the compression-tested sample, as displayed in the inset of Fig. 5. As mentioned above, the only Cu-based BMG materials
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13% for the several samples we tested, is the largest of all monolithic, uniform Cu-based BMGs so far. Fourth, the fact that this Cu-based BMG has simultaneously achieved high GFA and ductility sets the new alloy apart from all previous Cu-based BMGs, as demonstrated in Fig. 1. Acknowledgement This research was supported by the National Natural Science Foundation of China (Grant Nos. 50323009 and 50021101). The authors are also with the Multi-component Amorphous and Nanocrystalline Systems (MANS) Research team, supported by the Chinese Academy of Sciences. Fig. 5. (a) Compressive stress–strain curve for a typical as-cast 1 mm rod of the Cu49Hf42Al5 glass loaded to failure. (b) The inset is a SEM side view of the fractured sample, showing multiple shear bands.
that had a larger plastic strain than ours were glass/nanocrystal composites or heterogeneous glasses in the as-prepared state [14,15]. Those alloys do not have a good GFA either, as the cast rods obtained were only a couple of millimeters in diameter. As seen in Fig. 1, our glass (the red star) has the best combination of GFA and plasticity compared with all the previous Cu-based BMGs. The only one that comes close (point #10) was a quaternary Cu43Zr43Al7Ag7 BMG reported in Ref.16, where the good plasticity was attributed to stress-assisted nanocrystallization during the compression test. To find out whether or not nanocrystallization occurred during compression testing of our new BMG, particularly localized inside the extremely narrow shear bands, is a non-trivial task that requires extensive TEM work. This subject is being pursued in ongoing studies. 4. Concluding remarks We have discovered a new Cu-based BMG that is interesting for several reasons. First, it has a large size, with a critical diameter (for Cu mold casting) of 10 mm. This equals the best GFA (largest Dc) there is for all Cu-based BMGs. Second, such a high GFA has been achieved in a relatively simple system, a ternary alloy composed of simple metals. Yet the GFA is on par with that achieved in the best quaternary systems, Cu–Zr–Al–Y and Cu– Ag–Zr–Ti. Third, the new heavy-weight BMG has not only a high strength (2.6 GPa), but also appreciable plasticity. In fact, the plastic strain observed, in the range of 11–
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] [26]
Lin XH, Johnson WL. J Appl Phys 1995;78:6514. Inoue A, Zhang W, Zhang T, Kurosaka K. Acta Mater 2001;49:2645. Xu DH, Duan G, Johnson WL. Phys Rev Lett 2004;92:245504. Zhang T, Kurosaka K, Inoue A. Mater Trans 2001;42:2042. Inoue A, Zhang T, Kurosaka K, Zhang W. Mater Trans 2001;42: 1800. Inoue A, Zhang W, Zhang T, Kurosaka K. J Mater Res 2001;16: 2836. Inoue A, Zhang W. J Mater Res 2003;18:1435. Louzguine DV, Kato H, Inoue A. Appl Phys Lett 2004;84:1088. Wang WH, Lewandowski JJ, Greer AL. J Mater Res 2005;20:2307. Wesseling P, Nieh TG, Wang WH, Lewandowski JJ. Scripta Mater 2004;51:151. Lee JC, Kim YC, Ahn JP, Lee S, Lee BJ. Appl Phys Lett 2004;84: 2781. Oh JC, Ohkubo T, Kim YC, Fleury E, Hono K. Scripta Mater 2005; 53:165. Park ES, Kim DH, Ohkubo T, Hono K. J Non-Cryst Solids 2005;351: 1232. Das J, Tang MB, Kim KB, Theissmann R, Baier F, Wang WH, et al. Phys Rev Lett 2005;94:205501. Inoue A, Zhang W, Tsurui T, Yavari AR, Greer AL. Philos Mag Lett 2005;85:221. Lee SW, Huh MY, Fleury E, Lee JC. Acta Mater 2006;54:349. Zhang W, Inoue A. J Mater Res 2006;21:234. Dai CL, Guo H, Shen Y, Li Y, Ma E, Xu J. Scripta Mater 2006;54: 1403. Xu DH, Lohwongwatana B, Duan G, Johnson WL, Garland C. Acta Mater 2004;52:2621. Inoue A, Zhang W. Mater Trans 2004;45:584. Wang D, Li Y, Sun BB, Sui ML, Lu K, Ma E. Appl Phys Lett 2004; 84:4029. Duan G, Xu DH, Johnson WL. Metall Mater Trans 2005;36A: 455. Lewandowski JJ, Wang WH, Greer AL. Philos Mag Lett 2005;85:77. Johnson WL, Samwer K. Phys Rev Lett 2005;95:195501. Ma H, Xu J, Ma E. Appl Phys Lett 2003;83:2793. Xu YK, Ma H, Xu J, Ma E. Acta Mater 2005;53:1857.
A new Cu–Hf–Al ternary bulk metallic glass with high glass forming ability and ductility P. Jia a, H. Guo a, Y. Li b, J. Xu
a,*
, E. Ma
c
a
b
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China Department of Materials Science and Engineering, National University of Singapore, Engineering Drive 1, Singapore 117675, Singapore c Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, MD 21218, USA Received 27 January 2006; accepted 21 February 2006 Available online 27 March 2006
Abstract We have discovered a new Cu-based bulk metallic glass (BMG). Although of a simple Cu49Hf42Al9 ternary composition, the as-cast alloy is a monolithic, uniform BMG with a critical diameter as large as 10 mm. The width of the supercooled liquid region DTx and the reduced glass transition temperature Trg for this glass are 85 K and 0.62, respectively. In addition to its high glass-forming ability and high density of 11 g/cc, this BMG exhibits high ductility with a compressive plastic strain of 11–13%, making it a good candidate for applications as well as for studies of deformation behavior of Cu-based BMGs. 2006 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. Keywords: Metallic glasses; Bulk amorphous alloys; Glass-forming ability; Copper; Plasticity; Ductility
1. Introduction For practical applications, it is important for a bulk metallic glass (BMG) to simultaneously have strong glass-forming ability (GFA), high strength, decent ductility, and low cost. So far, very few BMGs have possessed these four attributes all at the same time. In this regard, Cu-based BMGs are promising [1–22]. First of all, compared with other BMGs discovered so far, e.g., those in Pd-, Pt-, Zr-, Y-based systems, the Cu-based alloys offer similar or even higher strength, but are of lower cost. Secondly, the Cu-based BMGs often appear ‘‘ductile’’ in compression tests [11–16]. As summarized in Fig. 1, most of the Cu-based BMGs exhibit a compressive plastic strain at room temperature, in contrast to several other families of BMGs that border on brittle behavior [23–26]. However, the Cu-based BMGs that show appreciable plastic strains often exhibit small critical size (Dc) for glass
*
Corresponding author. Tel.: +86 24 23971950; fax: +86 24 23971215. E-mail address: [email protected] (J. Xu).
formation by copper mold casting, i.e., an inadequate GFA. Conversely, the Cu46Zr42Al7Y5 [3] and Cu44.25Ag14.75Zr36Ti5 [18] BMGs with a record Dc of 10 mm showed rather low ductility. The general trend is depicted in Fig. 1 (the area under the dashed line), showing that there is rarely a case of coexisting high GFA and high ductility. Of course, one could improve GFA further by adding Be [16], but the toxic element Be is undesirable for processing and applications. One could also improve ductility significantly by processing a composite structure made of metallic glass and nanometer-sized crystals [14,15], but in this case one cannot speak of a critical diameter for BMG formation. These cases are thus excluded from Fig. 1, which only includes monolithic, uniform BMGs that contained no obvious pre-existing inhomogeneities before deformation (at least according to the published reports). With significant heterogeneities and/or nanocrystals intentionally introduced during processing, the ductility was reported to reach as high as 16–50% in compression tests [14,15]. We embarked on a search of a BMG with simultaneously high GFA (e.g., forming 1 cm diameter rods via
1359-6462/$ - see front matter 2006 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. doi:10.1016/j.scriptamat.2006.02.042
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P. Jia et al. / Scripta Materialia 54 (2006) 2165–2168
Fig. 1. Critical size (diameter in mm) for BMG formation vs. compressive plastic strain of monolithic/uniform Cu-based BMGs. The plastic strains quoted are all for compression tests of rod samples 1–2 mm in diameter. 1 – Cu64Zr36 [19], 2 – Cu66Hf34 [22], 3 – Cu55Hf45 [20], 4 – Cu50Zr43Al7 [16], 5 – Cu45Zr45Ag10 [17], 6 – Cu60Hf25Ti15 [2], 7 – Cu52.5Hf40Al7.5 [7], 8 – Cu44.25Ag14.75Zr36Ti5 [18], 9 – Cu60Zr20Hf10Ti10 [16], 10 – Cu43Zr43Al7Ag7 [16], 11 – Cu54Zr27Ti9Be10 [5], 12 – Cu47Ti33Zr11Ni8Si1 [13], 13 – Cu47Ti33Zr7Nb4Ni8Si1 [13].
copper mold casting) and good plasticity (plastic strain in excess of 10% in compression). Recall that even in binary systems such as Cu–Zr and Cu–Hf, BMG rods 1–2 mm in diameter can be fabricated [19–22]. Also note that ductility has been reported for simple ternary BMGs, Cu–Zr–Al [14], although there the reported Dc is only a couple of millimeters and the glass is non-uniform. Cu–Hf–Al compositions are likely to be promising in yielding a BMG with large Dc and good mechanical properties, even through a BMG with a Dc of only 3 mm has been found in this ternary system at Cu50Hf42.5Al7.5 and Cu52.5Hf40Al7.5 without pinpointing the composition at 1 at.% interval [7]. The density (q) and Poisson’s ratio (t) for the Cu50Hf43Al7 were determined to be 11.0 g/cc and 0.358, respectively [24]. In the following, we report the discovery of a simple ternary BMG former in this system that not only reached Dc = 10 mm, but also exhibited a plastic strain as large as 11–13%.
calorimeter (Diamond-DSC) under flowing purified argon. A heating rate of 40 K min1 was employed. To confirm the reproducibility of the results, at least three samples were measured. All the glass transition temperature Tg and onset temperature of crystallization Tx measurements were reproducible within an error of ±1 K. The heat of crystallization DHx for the glassy phase was determined by integrating the area under the DSC curve. The melting behavior of the alloys was measured in a Netzsch 404 DSC with alumina container, using a heating rate of 20 K min1. Samples for conventional and high-resolution transmission electron microscopy (TEM) observations in a FEI Tecnai F30 electron microscope were prepared by twin-jet electropolishing, with a solution of 9% nitric acid in a mixture of methanol and butoxyethanol (2:1). The compression test samples 2 mm in height were cut from the as-cast rods of 1 mm in diameter, prepared by suction casting. The loading surfaces were polished to be parallel to an accuracy of less than 10 lm. Room temperature compression tests were carried out using a strain rate of 1 · 104 s1. At least five samples were measured to ensure that the results were reproducible. The strain was determined from the platen displacement after correction for machine compliance. 3. Results and discussion In this work, we tested the GFA of a number of Cu–Hf–Al compositions. The mapping was done at 1 at.% intervals to capture the strong composition dependence of GFA. Fig. 2 displays the XRD patterns of several as-cast alloys 8–12 mm in diameter. The Cu49Hf42Al9 alloy exhibits the best GFA: both the 8 mm and 10 mm rods are apparently glassy, while at 12 mm the glassy matrix contained some crystals such as CuHf2 compound. It was concluded that the Dc is as large as 10 mm, equaling that of the best Cu-based BMGs reported previously [3,18]. Interestingly, the GFA of the alloy we located is significantly stronger than that of Cu50Hf42.5Al7.5 and Cu52.5Hf40Al7.5
2. Experimental Elemental pieces with purity better than 99.9% were used as starting materials. The master alloy ingots with the nominal composition (in at.%) were prepared by arc melting under a Ti-gettered argon atmosphere in a watercooled copper crucible. The alloy ingots were melted several times to ensure compositional homogeneity. To produce rods with different diameters, the master alloy re-melted in a tilting water-cooled copper hearth was cast into the copper mold that has internal rod-shaped cavities of about 60 mm in length. The cross-sectional surfaces of the as-cast rods were analyzed by X-ray diffraction (XRD) using a Rigaku D/max 2400 diffractometer with monochromated CuKa radiation. The glass transition and crystallization of the cast samples were investigated in a Perkin–Elmer differential scanning
Fig. 2. XRD patterns taken from the cross-sectional surface of as-cast rods 8, 10 and 12 mm in diameters for the Cu49Hf42Al9 alloy.
P. Jia et al. / Scripta Materialia 54 (2006) 2165–2168
previously reported by Inoue and Zhang [7]. The TEM micrograph and selected area electron diffraction (SAED) pattern are shown in Fig. 3(a) and (b), and the high-resolution image is presented in Fig. 3(c). The sample was taken from the center of the 10 mm as-cast rod. The homogeneous contrast in the images and a broad halo in the SAED pattern indicate the formation of a single amorphous phase. There are no visible crystals remaining in the sample, corroborating the XRD result that the material is fully amorphous. We did not detect obvious contrast in all the images, indicating that the glass is rather uniform in composition and structure. Fig. 4(a) shows the DSC curves of the as-cast BMG rod with D = 1 and 10 mm. A sharp exothermic crystallization peak and a clear endothermic event associated with glass transition are observed. The Tg, Tx, DTx (DTx = Tx Tg) and DHx values are listed in Table 1. The alloy exhibits a large supercooled liquid region, with DTx = 85 K. The crystallization is completed in a single step. The DHx value of 1- and 10-mm BMGs is nearly the same within experimental error, supporting that an as-cast 10-mm rod is fully glassy. Fig. 4(b) shows the DSC heating and cooling curves of the 10 mm BMG, near its melting temperature. These curves show at least two events, indicating that these alloys are at off-eutectic compositions. The Tm (eutectic tempera-
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Fig. 4. (a) DSC scans of as-cast rods of the Cu49Hf42Al9 BMG with 1- and 10-mm diameters. (b) DSC heating and cooling curves of the Cu49Hf42Al9 alloy near its melting temperatures (at a heating rate of 20 K min1).
Table 1 Thermal properties determined using DSC of Cu49Hf42Al9 BMGs fabricated using copper mold casting
Fig. 3. (a) TEM bright field image and (b) corresponding selected area electron diffraction pattern and (c) high-resolution TEM image for the Cu49Hf42Al9 rods of 10 mm in diameter. The sample was taken from the center of the as-cast rod.
Diameter of as-cast rods (mm)
Tg (K)
Tx (K)
DTx (K)
DHx (kJ/mol)
Tm (K)
TL (K)
Trg
1 10
777 778
863 863
86 85
4.16 4.12
– 1212
– 1249
– 0.622
ture), TL (liquidus temperature) and the calculated reduced glass transition temperature Trg (Trg = Tg/TL) are included in Table 1 as well. The Trg value is 0.62, reflecting that the alloy is a good glass former. The eutectic temperature of the ternary alloy (1212 K) is about 40 K lower than that of the related Cu–Hf binary eutectic reaction (L ! Cu10Hf7 + CuHf2, 1253 K), suggesting that the liquid of the ternary alloy is more stable than that of a simple binary system, thereby favoring the glass formation. From the uniaxial compressive stress–strain curves, such as that in Fig. 5(a), the yield stress ry, fracture stress rf, elastic strain ee, plastic strain ep and elastic modulus E for the 1-mm Cu49Hf42Al9 glass are obtained as 2408 MPa, 2620 MPa, 1.9–2.1%, 10.5–13.0%, 102 GPa, respectively. Note that the plastic strain is among the largest of all the Cu-based BMGs that are monolithically and uniformly glassy in the as-cast state before testing [2,5,7, 13,16–20,22], see Fig. 1. The large plasticity is reflected by the many and closely spaced shear bands seen in the scanning electron microscopy (SEM) image of the compression-tested sample, as displayed in the inset of Fig. 5. As mentioned above, the only Cu-based BMG materials
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13% for the several samples we tested, is the largest of all monolithic, uniform Cu-based BMGs so far. Fourth, the fact that this Cu-based BMG has simultaneously achieved high GFA and ductility sets the new alloy apart from all previous Cu-based BMGs, as demonstrated in Fig. 1. Acknowledgement This research was supported by the National Natural Science Foundation of China (Grant Nos. 50323009 and 50021101). The authors are also with the Multi-component Amorphous and Nanocrystalline Systems (MANS) Research team, supported by the Chinese Academy of Sciences. Fig. 5. (a) Compressive stress–strain curve for a typical as-cast 1 mm rod of the Cu49Hf42Al5 glass loaded to failure. (b) The inset is a SEM side view of the fractured sample, showing multiple shear bands.
that had a larger plastic strain than ours were glass/nanocrystal composites or heterogeneous glasses in the as-prepared state [14,15]. Those alloys do not have a good GFA either, as the cast rods obtained were only a couple of millimeters in diameter. As seen in Fig. 1, our glass (the red star) has the best combination of GFA and plasticity compared with all the previous Cu-based BMGs. The only one that comes close (point #10) was a quaternary Cu43Zr43Al7Ag7 BMG reported in Ref.16, where the good plasticity was attributed to stress-assisted nanocrystallization during the compression test. To find out whether or not nanocrystallization occurred during compression testing of our new BMG, particularly localized inside the extremely narrow shear bands, is a non-trivial task that requires extensive TEM work. This subject is being pursued in ongoing studies. 4. Concluding remarks We have discovered a new Cu-based BMG that is interesting for several reasons. First, it has a large size, with a critical diameter (for Cu mold casting) of 10 mm. This equals the best GFA (largest Dc) there is for all Cu-based BMGs. Second, such a high GFA has been achieved in a relatively simple system, a ternary alloy composed of simple metals. Yet the GFA is on par with that achieved in the best quaternary systems, Cu–Zr–Al–Y and Cu– Ag–Zr–Ti. Third, the new heavy-weight BMG has not only a high strength (2.6 GPa), but also appreciable plasticity. In fact, the plastic strain observed, in the range of 11–
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