Structures at Glassy, Supercooled Liquid, and Liquid States in La-Based Bulk Metallic Glasses
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UNTIL now, about six alloy systems, forming bulk metallic glasses (BMGs) with diameters larger than 20 mm by copper mold injection, have been developed.[1–6] The underlying mechanisms for the high glass forming ability (GFA) of these alloys are not completely understood. From a structural point of view, BMGs should have similar structures as their corresponding liquids. So far, very few structural data in liquids for multicomponent BMGs are available,[7–11] resulting in few studies for the correlation of the local atomic structures of liquid with GFA. The coordination number Nc is a key parameter used to evaluate the short-range atomic packing density, which is not simultaneously available, to the best of our knowledge, for any BMG systems at glassy, supercooled liquid, and liquid states. On the other hand, the temperature dependences of the coordination number for some nonmetallic amorphous materials have been studied by several groups.[12–14] Ansell et al.[12] and Jakse et al.[13] reported the coordination number decreases with decreasing temperature for supercooled liquid silicon, while Kim et al.[14] presented evidence to emphasize the temperature independence of Q.K. JIANG and Z.Y. CHANG, Postdoctoral Students, X.D. WANG, Lecturer, and J.Z. JIANG, Professor, are with the International Center for New-Structured Materials (ICNSM), Zhejiang University and Laboratory of New-Structured Materials (LNSM), Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China. Contact e-mail: [email protected] This article is based on a presentation given in the symposium ‘‘Bulk Metallic Glasses VI,’’ which occurred during the TMS Annual Meeting, February 15–19, 2009, in San Francisco, CA, under the auspices of TMS, the TMS Structural Materials Division, TMS/ASM: Mechanical Behavior of Materials Committee. Article published online November 24, 2009 1634—VOLUME 41A, JULY 2010
the coordination number for the same material. In this work, local atomic structures at glassy, supercooled liquid, and liquid states for La62Al14Cu11.7Ag2.3Ni5Co5 BMG are investigated by in-situ high-temperature X-ray diffraction (XRD). There are three reasons to select this alloy: (1) it exhibits excellent GFA, which can be cast into amorphous rods with a diameter of 35 mm; (2) it has a low glass-transition temperature of 430 K and low melting temperature of about 640 K; and (3) it provides an excellent prototype to investigate the glass transition behavior and liquid properties of BMGs.[5,15] For comparison, the ternary La62Al14Cu24 and quaternary La62Al14Cu20Ag4 BMGs are also studied here. It is found that the coordination number for the La62Al14Cu11.7Ag2.3Ni5Co5 BMG does not depend on temperature up to liquid temperature, while those for the La62Al14Cu24 and La62Al14Cu20Ag4 alloys show a slight decrease with temperature. For the three alloys, the volume expansion coefficient and the slopes of radii variation for the first to third nearest neighboring coordination shells show differences at glassy-to-super
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