Chinese Academy of Sciences, Shenyang, China. Baltimore, MD 21218, USA. First Published: May 2008 PLEASE SCROLL DOWN FOR ARTICLE

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1 This article was downloaded by:[national Institute for Materials Science] On: 21 May 2008 Access Details: [subscription number ] Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Philosophical Magazine Letters Publication details, including instructions for authors and subscription information: Bulk metallic glass formation near the TiCu-TiNi pseudo-binary eutectic composition Yan-Ling Wang a ; Evan Ma b ; Jian Xu a a Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China b Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, MD 21218, USA First Published: May 2008 To link to this article: DOI: / URL: To cite this Article: Wang, Yan-Ling, Ma, Evan and Xu, Jian (2008) 'Bulk metallic glass formation near the TiCu-TiNi pseudo-binary eutectic composition', Philosophical Magazine Letters, 88:5, PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article maybe used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 Philosophical Magazine Letters Vol. 88, No. 5, May 2008, Bulk metallic glass formation near the TiCu TiNi pseudo-binary eutectic composition Yan-Ling Wang a, Evan Ma b and Jian Xu a * a Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, , China; b Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, MD 21218, USA (Received 13 October 2007; final version received 1 February 2008) No bulk metallic glass (BMG) has been observed so far in the Ti Cu binary system. It has been found that by choosing a pseudo-binary eutectic (L! TiCu þ TiNi) deeper than that in the binary, reachable via only small substitution of Cu with Ni, the glass-forming ability can be effectively elevated. BMGs of mm diameter have been formed over a composition range around Ti 50 Cu 43 Ni 7. The new BMGs are reproducibly malleable with significant compressive plastic strains. Keywords: bulk amorphous materials; metallic glasses; pseudo-binary eutectic; plasticity; titanium 1. Introduction Recently, it has been discovered that bulk metallic glasses (BMGs, with critical size exceeding 1 mm) can be obtained even in simple binary systems [1 6]. A curious finding is that both Zr Cu [1 3] and Hf Cu [3,4] systems form BMGs, whereas the chemically similar Ti Cu system does not, although melt spinning was able to produce glassy ribbons over a relatively wide composition range [7,8]. Since compositions with high glass-forming ability (GFA) are often associated with the eutectics in the corresponding binaries, it seems that the Ti Cu eutectics are not yet deep enough for BMG formation, when compared with Zr Cu and Hf Cu. As such, Ti Cu needs some additional help, which can come in various ways [9,10]. In the following, we present an approach of partially substituting the Cu in the Ti Cu alloy with a small amount of Ni, to approach a pseudo-binary eutectic that promotes GFA. This Ni substitution is interesting because, even though Ni is chemically and topologically similar to Cu, the resulting Ti Cu Ni is a true ternary system. Figure 1 displays the vertical section of the ternary Ti Cu Ni phase diagram at the tie line between TiCu and TiNi [11]. We see that Ni has limited solubility in TiCu and the two equiatomic compounds, TiCu and TiNi, have very different crystal structures, tetragonal (a ¼ nm and c ¼ nm) for TiCu [12] and CsCl type (B2, a ¼ nm) for TiNi [13], respectively. A pseudo-binary eutectic (L!TiCuþTiNi) is present, exhibiting a eutectic temperature 26 K lower than the eutectic (L! TiCu þ Ti 2 Cu) in the Ti Cu binary *Corresponding author. jianxu@imr.ac.cn. ISSN print/issn online ß 2008 Taylor & Francis DOI: /

3 320 Y.-L. Wang et al. Figure 1. Vertical section of the ternary Ti Cu Ni phase diagram [11]. Notes: The composition region of BMG formation with critical diameter D c ¼ 1 mm is marked as a double-ended arrow, Ti 50 Cu 50 x Ni x (6 x 10). The best glass-forming composition with 1.5 mm diameter is marked by the vertical line. subsystem [14]. We will demonstrate that by taking advantage of this increased stability of the liquid, BMGs can be obtained near the pseudo-eutectic composition. Hints for improved liquid stability have in fact been revealed before, via the presence of a detectable supercooled liquid region T x (T x ¼ T x T g, where T x and T g are the onset temperature of crystallization and the glass transition temperature, respectively) in some melt-spun Ti Cu Ni ribbons [15,16]. The usual BMG-forming alloys are multicomponent, such that phase selection during solidification of the liquid is quite complex. This makes it difficult to establish the one-toone correspondence between the BMG formation and the known eutectic reactions. In many ternary cases, whether the easy glass formation is necessarily correlated with a ternary invariant eutectic reaction remains questionable. In our case, however, this direct correlation is possible, similar to the scenario in simple binary systems. This will be demonstrated below with a detailed monitoring of the compositional dependence of the GFA near the pseudo-binary eutectic composition. In addition, the TiCu TiNi glasses can be used as a model to examine the hypothesis that the BMGs formed in a system where the crystalline competing phases all have simple structures (such as B2 structure) will be more malleable than those in a system containing complex intermetallics, such as Laves phases or Frank Kasper phases. 2. Experimental procedure Ternary Ti-based alloy ingots with nominal compositions of Ti 50 Cu 50-x Ni x (5 x 12, in at.%) were prepared by arc melting a mixture of Ti, Cu and Ni metals with purity better than 99.9 wt% under a Ti-gettered argon atmosphere in a water-cooled copper crucible. The alloy ingots were melted several times to ensure compositional homogeneity. Cylindrical rods of about 20 mm in length and 1 2 mm in diameter were fabricated using

4 Philosophical Magazine Letters 321 Figure 2. XRD patterns of as-cast Ti 50 Cu 50 x Ni x (5 x 12) rods with different diameters: (a) 1 mm; (b) 1.5 mm. Notes: The crystalline TiNi phase can have different forms, depending on the Cu content [17]. This is known from previous studies on the transformation sequence in TiNi(Cu) [17]. For Cu content below 10 at.%, the monoclinic B19 0 martensite is formed on cooling from the cubic B2 austenite, as in the binary Ni Ti. The intermediate orthorhombic B19 martensite appears when the Cu concentration is approximately between 10 and 15 at.%, the transformation sequence becoming B2! B19! B19 0. Higher Cu additions shift the second step to lower temperatures and usually only the B2! B19 transformation is observed above room temperature. copper mould suction casting in a mini-arc melter. The cross-sectional surfaces of the as-cast rods were analysed by X-ray diffraction (XRD) using a Rigaku D/max 2400 diffractometer with monochromated Cu-Ka radiation. The glass transition and crystallization of the cast samples were investigated using differential scanning calorimetry (DSC, Perkin Elmer Diamond DSC) under flowing purified argon. A heating rate of 40 K min 1 was employed. To confirm the reproducibility of the results, at least three samples were measured for each composition. All the T g and T x measurements were reproducible within an error of 1 K. The heat of crystallization, H x, for the glassy phase was determined by integrating the area under the DSC curve. Compression test samples 2 mm in height were cut from the as-cast rods of 1 mm diameter, for characterization of the mechanical properties of the BMGs obtained. The loading surfaces were polished to be parallel to an accuracy of better than 10 mm. Room temperature compression tests were carried out using a strain rate of s 1. At least seven 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 Figure 2a shows XRD patterns obtained from the as-cast rods with 1 mm diameter for Ti 50 Cu 50 x Ni x (5 x 12) alloys. The patterns for x ¼ 6, 7, 8 and 10 exhibit broad halos in the 2 ¼ range, characteristic of an amorphous phase. There is no evidence of any crystalline peaks within the XRD resolution. On the Ni-lean side, the 5 at.% Ni-containing alloy contains an amorphous phase plus a small amount of the TiCu crystalline phase. The 12 at.% Ni-containing alloy, on the other side, is mostly crystalline, with a mixture of the TiCu and TiNi phases expected from the pseudo-eutectic reaction.

5 322 Y.-L. Wang et al. Figure 3. DSC scans of Ti 50 Cu 50 x Ni x (6 x 10) as-cast rods with different diameters: (a) 1 mm; (b) 1.5 mm (at a heating rate of 40 K min 1 ). Table 1. Thermal properties of Ti 50 Cu 50 x Ni x (6 x 10) series BMGs with rod diameters of 1 and 1.5 mm, obtained from DSC measurements at a heating rate of 40 K min 1. Ni content (at. %) Rod diameters (mm) T g (K) T x1 (K) T x (K) H x (kj mol 1 ) x ¼ x ¼ x ¼ x ¼ The corresponding DSC curves of the fully amorphous alloys in continuous heating mode are shown in Figure 3a. All curves exhibit an endothermic reaction associated with the glass transition, followed by the supercooled liquid region and then two exothermic reactions due to crystallization. The T g and T x1 are marked using arrows in the DSC curves. With increasing Ni content from 6 to 10, both T g and T x1 slightly increased from 667 to 672 K and from 704 to 715 K, respectively. The T g, T x1, T x and H x for the Ti 50 Cu 50-x Ni x (6 x 10) series BMGs are listed in Table 1. The BMG-forming composition zone with 1 mm diameter, within the Ti 50 Cu 50 x Ni x (6 x 10), is marked as a double-ended arrow in Figure 1. To locate the best glass former, 1.5 mm diameter samples were investigated. As shown in Figure 2b, the XRD patterns for x ¼ 7 and 8 show fully amorphous features. The corresponding DSC curves are shown in Figure 3b. The thermal properties from the DSC measurements for the 1.5 mm BMGs at x ¼ 7 and 8 are also summarized in Table 1. These properties are quite similar to the results of the glassy rods at 1 mm diameter. For the x ¼ 6 and 9 rods, several crystalline phases, including TiCu and TiNi phases, appear in the patterns. By comparing the data listed in Table 1 of the x ¼ 7 samples, no differences can be found within experimental error for the different diameters, further confirming that the

6 Philosophical Magazine Letters 323 Figure 4. Compressive engineering stress strain curves for the as-cast Ti 50 Cu 43 Ni 7 BMG with a 1 mm diameter. The curves are shifted relative to each other for clarity. Figure 5. SEM images of the fracture morphology of as-cast Ti 50 Cu 43 Ni 7 BMG with a 1 mm diameter: (a) side-view of the fracture surface; (b) side-view under higher magnification of the framed rectangle in (a); (c) top-view of the fracture surface. 1.5 mm cast rod of the x ¼ 7 alloy is fully amorphous. This best BMGforming composition is indicated by a vertical line in Figure 1. Its T g and T x1 were determined to 667 K and 705 K, respectively, resulting in a 38 K width for T x. For the x ¼ 8 sample at 1.5 mm diameter, the enthalpy of crystallization is a bit smaller than that of the 1 mm rod, suggesting that a small fraction of the crystalline phases may be present in the 1.5 mm rod. Figure 4 shows the room-temperature uniaxial compressive stress strain curves of the 1mmTi 50 Cu 43 Ni 7 glass. All the samples tested exhibit high fracture strengths and obvious compressive plastic strains. The yield stress, y, fracture stress, f, and compressive plastic strain, " p, were measured to be MPa, MPa and 5 11%, respectively. It is important to note that this BMG shows reproducible malleability; this is very different from many BMGs that show malleability only occasionally, or only when there is sample miscut leading to deviation from orthogonality [18]. Figure 5 shows SEM images of the fracture morphology of the as-cast glassy Ti 50 Cu 43 Ni 7 rod with a diameter of 1 mm. The large plasticity is apparently provided by

7 324 Y.-L. Wang et al. the many and closely-spaced shear bands seen in Figure 5b. Figure 5c displays the typical fracture surface, which shows a vein-like pattern characteristic of BMGs. 4. Summary In summary, new BMGs are found in Ni-containing Ti Cu alloys, near the pseudo-binary eutectic point (L! TiCu þ TiNi) in the Ti Cu Ni system. The best glass former was determined to be Ti 50 Cu 43 Ni 7, where glassy rods with a diameter of 1.5 mm can be fabricated using copper mould casting. Our results demonstrate that pseudo-binary eutectics can be taken advantage of, to boost the GFA. In the present case, the increased stability (lowered eutectic temperature) of the pseudo-binary eutectic liquid relative to the corresponding binary liquid is due to the effects of Ni. The heat of mixing in the liquid state between the Ni Ti pair is 35 kj mol 1, significantly larger than that of the Cu Ti pair ( 9 kj mol 1 ) [19]. Our results suggest that in addition to locating the ternary invariant eutectic compositions in a ternary system, which is sometimes a difficult task in the absence of a detailed phase diagram, pseudo-binary eutectics, if available, can be alternative regions where better GFA can be expected. Our repeated compression tests also indicate that the new Ti Cu Ni BMGs are consistently malleable, with significant plastic strains. This finding points to the possibility that ductile BMGs are more likely in systems involving only crystalline phases with a relatively simple structure, rather than complex intermetallics such as Laves phases or Frank Kasper phases, even though the GFA of the former may not be as strong as the latter. Acknowledgements This research was supported by the National Natural Science Foundation of China under contract No and National Basic Research Program of China (973 Program) under contract No. 2007CB References [1] D. Wang, Y. Li, B.B. Sun et al., Appl. Phys. Lett. 84 (2004) p [2] D.H. Xu, B. Lohwongwatana, G. Duan et al., Acta Mater. 52 (2004) p [3] A. Inoue and W. Zhang, Mater. Trans. 45 (2004) p.584. [4] G. Duan, D.H. Xu and W.L. Johnson, Metall. Mater. Trans. A 36 (2005) p.455. [5] M. Leonhardt, W. Lo ser and H.G. Lindenkreuz, Acta Mater. 47 (1999) p [6] L. Xia, W.H. Li, S.S. Fang et al., J. Appl. Phys. 99 (2006) p (1 3). [7] T.B. Massalski, C.G. Woychik and J. Dutkiewicz, Metall. Trans. A 19 (1988) p [8] K.H.J. Buschow, Acta Metall. 31 (1983) p.155. [9] H. Men, S.J. Pang, A. Inoue et al., Mater. Trans. 46 (2005) p [10] X.F. Wu, Z.Y. Suo, Y. Si et al., J. Alloys Compd. 452 (2008) p [11] S.P. Alisova, N.V. Volynskaya, P.B. Budbery et al., Handbook of Ternary Alloy Phase Diagrams, Vol. 8, P. Villars, A. Prince and H. Okamoto, eds., ASM International, Materials Park, OH, 1995, p [12] N. Karlsson, J. Inst. Metals 79 (1951) p.391. [13] K.M. Knowles and D.A. Smith, Acta Metall. 29 (1981) p.101.

8 Philosophical Magazine Letters 325 [14] J.L. Murray, Binary Alloy Phase Diagrams, Vol. 2, T.B. Massalski, H. Okamoto, P.R. Subramanian and L. Kacprzak, eds., ASM International, Materials Park, OH, 1990, p [15] T. Zhang and A. Inoue, Mater. Trans. JIM 39 (1998) p [16] Y.C. Kim, S. Yi, W.T. Kim et al., Mater. Sci. Forum (2001) p.67. [17] W.J. Moberly and K.N. Melton, Engineering Aspects of Shape Memory Alloys, Butterworth- Heinemann, Essex, [18] W.F. Wu, Y. Li and C.A. Schuh, Phil. Mag. 88 (2008) p.71. [19] F.R. De Boer, R. Boom, W.C.M. Mattens et al., Cohesion in Metals, North-Holland, Amsterdam, 1988.