Inherent strength of zirconium-based bulk metallic glass A.S. Bakai a , A.P. Shpak b , N. Wanderka c , S. Kotrechko b , T.I. Mazilova a , I.M. Mikhailovskij a, ⁎ a National Scientific Center “Kharkov Institute of Physics and Technology”, Kharkov, 61108, Ukraine b Kurdyumov Institute for Metal Physics, National Academy of Sciences of the Ukraine, Kyiv, 142, 03680, Ukraine c Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin, D-14109, Germany abstract article info Article history: Received 18 May 2009 Received in revised form 10 February 2010 Keywords: Atomic structure; Bulk amorphous metals; Strength The polycluster morphology and inherent tensile strength of bulk metallic glass Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 in the as-cast state were determined by means of high-field mechanical loading using field ion microscopy. The two-level cluster structure with the length scale of the order of 2 and 10 nm was revealed. It was disclosed that the strength of this alloy is characterized by a strong size-effect in a nanometer scale range. It is shown that within this size region the Weibull distribution is not suitable to describe the scale effect. It is ascertained that the limit level of shear strength of the examined glass is 5.8 GPa, and shear strength of a separate cluster may be estimated as 6.7 GPa. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The inherent strength is the highest achievable strength of defect- free material at 0 K. Conventional crystalline materials fracture at stresses far below its theoretical strength and the macroscopic strength is highly sensitive to the specimen size. As it was shown in earlier experiments, the tensile strengths of Fe whiskers [1] and W nanotips [2] are more than an order of magnitude higher than that of the bulk crystals. Recently the determination of theoretical strength became possible using quantum-mechanical electronic structure calculations based on the density functional theory. The calculation results are in harmony with these experiments [3–5]. Metallic glasses provide higher strength and hardness compared to crystalline solids. The origin of the very high tensile strength of the amorphous alloys is shown to be due to a uniform distribution of some kind of atomic clusters [6–8]. In the absence of intrinsic defects such as stacking faults and dislocations, the strength of metallic glasses has been considered to be close to the ideal strength of solids [9,10] However, the existence of extrinsic casting defects (voids, oxides, inclusions, inner boundaries, etc.) makes it difficult to determine the elastic limit of metallic glasses. Our understanding of the elastic–plastic transition and strain- releasing mechanism on the nanoscale has been dramatically advanced by nanoindentation studies of plastic yielding in defect- free regions under the indenter nanotip. The recent nanoindentation experiments showed that the onset of yielding of metallic glasses in the small volume beneath the indenter is observed at the stresses near the theoretical strength [10,11]. Another promising approach in the determination of the ideal strength of metallic glasses is the use of nanosized specimens, which may be defect-free if the native defect density is low enough. An experimental determination of the tensile strength of nanospecimens is rather challenging, due to the difficulties in measuring the mechanical response of nanoscale objects under tensile load. Because of these problems, there are only a few data about mechanical properties of individual nano-objects, and until now the direct tensile strength measurements of metallic glasses are lacking. In this paper we present results of determination of the tensile strength of zirconium-based bulk metallic glass by the high- field method of mechanical loading using field ion microscopy (FIM) [2,12]. 2. Experimental details and material A zirconium-based bulk metallic glass (BMG) Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5, Vitreloy 1 (V1) was used in the current investigation. Amorphous ingots of 12 mm in diameter were produced by alloying of pure constituents in an inductive levitation furnace under the argon atmosphere by re-melting several times. The amorphous nature and large-scale homogeneity of the as-quenched material were verified by means of X-ray diffraction, small angle neutron scattering and transmission electron microscopy. Thin disks of 0.2 mm were cut from the ingots. In order to perform FIM analysis the samples were cut into the rods of 10 × 0.2 × 0.2 mm using a diamond wire saw. Needle-like specimens with an initial radius of the curvature r 0 of about 10 nm at a hemispherical top (Fig. 1(a)) were electrolytically polished in a solution of 10% HClO 4 + 90% CH 3 COOH at room temperature and at DC voltage of 12–15 V. The specimen surface was cleaned and polished in situ by the methods of field desorption and by low- temperature field evaporation in a FIM [12]. FIM experiments were performed at 77 K with a two-chamber field emission microscope operating in the ion and electron regimes. The Journal of Non-Crystalline Solids 356 (2010) 1310–1314 ⁎ Corresponding author. Tel.: + 380 57 700 2676; fax: + 380 57 335 1688. E-mail address: mikhailovskij@kipt.kharkov.ua (I.M. Mikhailovskij). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.03.009 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol