Short communication Fractal in fracture of bulk metallic glass M.Q. Jiang a, b , J.X. Meng a , J.B. Gao c , X.-L. Wang d , T. Rouxel e , V. Keryvin e , Z. Ling a , L.H. Dai a, b, * a State key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, No 15 Beisihuan Road, Beijing 100190, People’s Republic of China b State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, People’s Republic of China c PMB Intelligence LLC, P.O. Box 2077, West Lafayette, IN 47996, USA d Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA e Applied Mechanics Laboratory of the University of Rennes 1, LARMAUR, ERL CNRS 6274, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes cedex, France article info Article history: Received 4 July 2010 Accepted 1 August 2010 Available online 9 September 2010 Keywords: A. Bulk metallic glass B. Dynamic fracture C. Nanoscale periodic corrugation C. Fractal abstract We investigate the nanoscale periodic corrugation (NPC) structures on the dynamic fracture surface of a typical tough bulk metallic glass, submitted to high-velocity plate impact and scanned by atomic force microscopy (AFM). The detrended fluctuation analysis (DFA) of the recorded AFM profiles reveals that the valley landscapes of the NPC are nearly memoryless, characterized by Hurst parameter of 0.52 and exhibiting a self-similar fractal character with the dimension of about 1.48. Our findings confirm the existence of the “quasi-cleavage” fracture underpinned by tension transformation zones (TTZs) in metallic glasses. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction In crystalline metals, atomic bonds are particularly susceptible to rupture in shear (slip) and tension (cleavage) across preferred crystallographic planes, leading to ductile and brittle fracture, respectively. Unlike long-range slip in crystals, the ductile fracture of glassy metals is guided by the nanoscale shear banding ahead of the crack [1e5], where the blunt tip advances via continuous operations of “flow defect” [6] or “shear transformation zone” (STZ) [7]. The STZ is essentially a local atomic cluster that undergoes an inelastic shear distortion from one relatively low local energy basin to a second, crossing an activated configuration of higher energy and volume [8e10]. The ductile crack propagation can leave cell or river-like vein patterns on final fracture surfaces [11e 13], due to the fluid meniscus instability [1,2] initially discussed by Taylor [14]. Recently, however, a new pattern e the nanoscale periodic corru- gation (NPC), has been widely observed in various metallic glasses (MGs) covering ideally brittle Mg-based [15e19], Fe-based [17,19], Co-based [17], and rare earth-based [19], less brittle Ni-based [20] and even tough Zr-based [21e25], indicating its universality. The characteristic size, i.e. the spacing of NPC is usually smaller than the critical wavelength of the fluid meniscus instability [16,18,21]. This poses a big challenge to the STZ-mediated ductile fracture mechanism. Based on a broad overview of fracture patterns [21,22,26], we have previously proposed that the NPC forms via periodically activation of tension transformation zones (TTZs) with local plastic flow in the background ahead of the crack tip. The TTZ can be envisioned as a transient transition from a STZ that experiences significant tension/dilatation fracture [21,24,26]. Compared to a STZ being the elementary process of ductile cracking, a TTZ can be regarded as the basis of brittle “quasi-cleavage” fracture through which energy dissipates mainly by forming new surfaces and little by accompanying weaker plastic flow. The fracture mechanism via STZ versus TTZ is of paramount importance in understanding the ductile-to-brittle transition in MGs [21,24e26]. The STZ-under- pinned ductile fracture plane has been widely studied [1,2,11e 13], but a quantitative analysis on the TTZs-mediated “quasi-cleavage” surface is not yet available. Therefore, in this paper, we perform an exhaustive investigation of the NPC in a typical Zr-based bulk MG by atomic force microscopy (AFM). A detrended fluctuation analysis (DFA) method is adopted to quantitatively analyze the AFM topo- graphical profiles recorded, which allowed us to identify our proposed “quasi-cleavage” fracture mechanism on the basis of TTZs in metallic glasses. 2. Experimental observations Fig. 1a is a high-resolution scanning electron microscope (HRSEM) image of representative NPC that has been observed on * Corresponding author at: State key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, No 15 Beisihuan Road, Beijing 100190, People’s Republic of China. Tel.: þ86 10 82543958; fax: þ86 10 82543977. E-mail address: lhdai@lnm.imech.ac.cn (L.H. Dai). Contents lists available at ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet 0966-9795/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2010.08.003 Intermetallics 18 (2010) 2468e2471