Materials Science Forum Online: 2004-03-15 ISSN: 1662-9752, Vols. 449-452, pp 945-948 doi:10.4028/www.scientific.net/msf.449-452.945 2004 Trans Tech Publications, Switzerland New Cu-based Bulk Metallic Glasses with High Strength of 2000 MPa S. Y. Shin 1, J. H. Kim 1, D. M. Lee 1, J. K. Lee 2, H.J. Kim 2, H. G. Jeong 3 and J. C. Bae 2 1 Advanced Welding & Joining Technology Team, Production Technology Center 2 Nano Material Team, Advanced Materials R&D Center, 3 Digital Production Processing Team, Manufacturing Process Research Center, Korea Institute of Industrial Technology, 35-3, HongChonRi, IbJangMyun, CheonAn 330-825, Korea Keywords: Cu-based alloy, Glass forming ability, Amorphous, Supercooled liquid region Abstract. New Cu-based bulk amorphous alloys exhibiting a large supercooled liquid region and good mechanical properties were formed in a quaternary Cu-Ni-Zr-Ti systems consisting of only metallic elements. The compositional range for the formation of the amorphous alloys that have high glass forming ability (GFA) (> 3 mm diameter) and large supercooled liquid region (> 50 K) is defined in the pseudo-ternary phase diagram Cu-Ni-(Zr, Ti). A bulk amorphous alloy with the diameter of 6 mm can be prepared by copper mold casting. The alloy shows glass transition temperature (T g ) of 712 K, crystallization temperature (T x ) of 769 K and supercooled liquid region (T x ) of 57 K. The alloy exhibits high compressive fracture strength of about 2130 MPa with a plastic strain of about 1.5 %. The new Cu-based bulk amorphous alloy with high GFA and good mechanical properties allows us to expect the extension of application fields as a new engineering material. Introduction Bulk amorphous alloys exhibit many unique properties associated with the atomic structure. These unique properties that can be rarely found in crystalline materials are attractive for the practical applications as new classes of structural as well as functional materials. Several bulk amorphous alloys have been developed mainly in the Pd-, Zr-, Ti-, Mg- and Ni-based alloy systems [1-5]. Development of Cu-based bulk amorphous alloys with high strength and low price can expand the application fields of the amorphous alloys. Some Cu-based bulk amorphous alloys have been obtained in Cu-Ti-Zr-Ni and Cu-Ti-Zr-Ni-Si systems containing Cu less than 50 at. % [6,7]. Recently, fully amorphous rods containing more than 50 at % Cu have been prepared in the Cu- (Zr,Hf)-Ti [8] and Cu-Zr-Ti-(Y,Be) [9,10] alloy systems exhibiting the high strength exceeding 2000 MPa. The maximum sample diameter of these bulk amorphous alloys is 4 mm in the Cu- (Zr,Hf)-Ti and 5 mm in the Cu-Zr-Ti-(Y,Be) systems. However, the addition of Y or Be is accompanied by some economical and technological demerits. In this study, we report new quaternary Cu-based bulk amorphous alloys having high glass forming ability (GFA) and large supercooled liquid region (T x ) that consist of only metallic elements, Cu, Ni, Zr and Ti. The thermal and mechanical properties of the bulk amorphous alloy are investigated. Corresponding author : ldm0922@kitech.re.kr All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (#69712719, Pennsylvania State University, University Park, USA-13/09/16,02:21:45)
946 Designing, Processing and Properties of Advanced Engineering Materials Experimental procedures Alloys with compositions of Cu 49-56 Ni 5-11 Zr 20-24 Ti 16-20 were prepared by arc melting a mixture of pure metals Cu(99.9%), Ni(99.9%), Zr(99.9%), Ti(99.9%) under an argon atmosphere. For rapid solidification of the alloys, alloy specimens were remelted in quartz tubes and ejected with an over pressure of 35 kpa through a nozzle onto a Cu wheel surface rotating with a surface velocity of 40 m/s under an argon atmosphere. The resulting ribbons exhibit thickness of about 30 µm and width of about 2 mm. Bulk amorphous alloys in a rod form were prepared by a suction casting method using copper molds that have cylindrical cavities of varying diameters from 3 to 6 mm and a height of around 50 mm. Thermal analysis of the samples was carried out by differential scanning calorimetry (DSC) and differential thermal analysis (DTA), respectively. Structure of the amorphous specimens was characterized by X-ray diffractometry (XRD). Room temperature uniaxial compressive tests were conducted on the cylindrical cast rods with a diameter of 3 mm and a height of 6 mm using the Instron-type test machine under the constant strain rate of 5 10-4 s -1. Microstructure of the tested specimens was observed using scanning electron microscopy (SEM). Results and discussion Fig. 1 shows a pseudo-ternary phase diagram showing two regions of compositional range in which fully amorphous rods (> 3 mm diameter) with large T x (> 50 K) in DSC heating were fabricated in the present study. The large supercooled liquid region before crystallization indicates that the alloy exhibits a high thermal stability against crystallization. Although T x is not directly related to the glass forming ability of an alloy, most bulk amorphous alloys have T x larger than 50 K [11]. The formation of fully amorphous structure of the alloys in Fig. 1 was confirmed by XRD analysis. A model alloy is selected to study properties of the bulk amorphous alloys in the quaternary system Cu-Ni-Zr-Ti, since the alloy can be cast into fully amorphous rods of 6 mm in diameter with a large T x of about 57 K. The existence of large T x is of technological interest. For example, using the viscous flow of the supercooled liquid, near net shaping as well as consolidation to form bulk amorphous materials can be effectively performed at temperatures within T x. 0.67 K/s 600 Exothermic (W/g) melt-spun ribbon bulk 3 mm bulk 6 mm T g T x 0.5 W/g Fig. 1. Pseudo-ternary phase diagram showing compositional ranges of the bulk amorphous alloys. ( : 5-6 mm, : 3-4 mm) 600 700 800 900 Temperature (K) Fig. 2. DSC traces obtained from the ribbon and bulk amorphous alloys. Fig. 2 shows DSC traces obtained from the bulk amorphous alloy, together with the data for the corresponding melt-spun ribbon. The DSC trace of the melt-spun alloy exhibits an endothermic event corresponding to glass transition to supercooled liquid, and two overlapped
Materials Science Forum Vols. 449-452 947 exothermic peaks corresponding to crystallization process. The melt-spun alloy shows glass transition temperature (T g ) of 712 K, onset temperature of first crystallization (T x ) of 769 K and integrated heat of crystallization of 60. 8 J/g. The supercooled liquid range, defined as T x = T x - T g, is about 57 K of the alloy. The crystallization behaviors of the bulk amorphous alloy are the same as those of the ribbon. The integrated heat of the exotherms of the bulk specimen is almost same as that of the ribbon specimen, indicating fully amorphous structure was obtained in the bulk specimen. The formation of fully amorphous structure in the ribbon and bulk specimens was also confirmed by XRD study. The XRD patterns of melt-spun and bulk specimens are shown in Fig. 3. The XRD patterns of the ribbon and bulk specimens show only a broad halo diffraction peak in the 2 range of 34 o -46 o, which is characteristic of the amorphous structure. CuK 0.33 K/s Intensity (a.u.) melt spun ribbon bulk 3 mm bulk 6 mm ( Endothermic) T T m sol T m liq 20 30 40 50 60 70 2 (degree) Fig. 3. XRD patterns obtained from the ribbon and bulk amorphous alloys. 1200 1250 1300 1350 Temperature (K) Fig. 4. DTA traces obtained from the bulk amorphous alloy. Fig. 4 shows DTA trace obtained from the bulk amorphous alloy. The solidus temperature (T m sol ) and liquidus temperature (T m liq ) assumed to be the onset and end temperature, respectively, of the melting endotherm. The alloy shows T m sol and T m liq of 1240 and 1287 K, respectively. The reduced glass transition temperature (T rg = T g /T m liq ) value, as a parameter that determines the GFA of amorphous alloy, is calculated to be 0.55. Considering that a number of Cubased bulk amorphous alloy have the high T rg above 0.60 [8-10], this value is by comparison not a very high value. The high GFA of the alloy exceeding what is expected based on the T rg is not yet clearly understood. Some bulk amorphous alloys, such as Mg-Cu-Y [4] and Ti-Zr- Cu-Ni [6] also show better GFA than expected from T rg. In the Ti-Zr-Cu-Ni system, it has been reported that
948 Designing, Processing and Properties of Advanced Engineering Materials 2500 2000 = 3 mm Stress (MPa) 1500 1000 500 Uniaxial Compression Initial Strain rate = 5x10-4 s -1 0 0 1 2 3 4 5 Strain (%) 50 µm Fig. 5. Compressive stress-strain curve of the Fig. 6. Fracture surface of the bulk bulk amorphous alloy. amorphous alloy. the excellent GFA is a result of simultaneously minimizing the nucleation rate of competing crystalline phases [6]. An investigation on the extraordinary GFA of alloy is underway. Fig. 5 shows the compressive stress-strain curve of the bulk amorphous specimen with diameter of 3 mm. The curve shows linear elastic behavior up to yielding, followed by plastic strain of about 1.5 % and then final fracture, indicating that the new Cu-based bulk amorphous alloys have rather good ductility. The compressive fracture strength and fracture strain are approximately 2130 MPa and 3.3 %, respectively. The secondary electron image in Fig. 6 was obtained from the fracture surface of the alloy. Vein patterns, a typical fracture characteristic of amorphous alloys, are clearly observed on the fracture surface as shown in Fig. 6. It is noticed that the fracture strength value of the Cu-based bulk amorphous alloy is about twice higher than the highest strength of conventional Cu-Be base crystalline alloys. Therefore, the finding of the new Cu-based bulk amorphous alloys with high GFA and good mechanical properties is promising for future uses as a new type of high strength material. Conclusion Cu-based bulk amorphous alloys having high GFA as well as large T x are developed in a quaternary Cu-Ni-Zr-Ti system, consisting of only metallic elements. The T g, T x and T x of the bulk amorphous alloy are 712, 769 and 57 K, respectively. A fully alloy ( 6 mm) can be prepared by copper mold suction casting. The compressive fracture strength and fracture strain of the alloy are approximately 2130 MPa and 3.3 %, respectively. Acknowledgements This work was jointly supported by the Center for Nanostructured Materials Technology under the 21st Century Frontier R&D Program of the Ministry of Science and Technology and the Next Generation - New Technology Development Program of the Ministry of Commerce, Industry and Energy in Korea. References [1] A. Inoue, N. Nishiyama and T. Matsuda: Mater. Trans. JIM Vol. 37 (1996), p. 181 [2] A. Peker and W.L. Johnson: Appl. Phys. Lett. Vol. 63 (1993), p. 2342
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