Materials Science and Engineering A 526 (2009) 69–73 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Strain rate effects on the deformation behavior of (Zr 55 Al 10 Ni 5 Cu 30 ) 99 Y 1 and Cu 45 Zr 47.5 Ag 7.5 bulk metallic glasses Dongchun Qiao a,∗ , Lu Huang a,b , Gongyao Wang a , Peter K. Liaw a a Department of Material Science and Engineering, The University of Tennessee, Knoxville, TN 37996, USA b Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, China article info Article history: Received 18 May 2009 Accepted 25 June 2009 Keywords: Metallic Glass Compression Deformation Softening Hardening abstract The strain rate effects on the deformation behavior of bulk-metallic glasses (BMGs) are investigated. The loading–unloading–reloading tests were performed with the same strain rates for the loading and reloading. The deformation for both (Zr 55 Al 10 Ni 5 Cu 30 ) 99 Y 1 and Cu 45 Zr 47.5 Ag 7.5 BMGs shows the strain softening at the low strain rate. However, at the high strain rate, the (Zr 55 Al 10 Ni 5 Cu 30 ) 99 Y 1 BMG exhibits more obvious strain softening while Cu 45 Zr 47.5 Ag 7.5 BMG demonstrates the strain hardening instead. This could be rationalized by the free volume increase and the structural instability during plastic deformation process. Published by Elsevier B.V. 1. Introduction Dislocations are a common defect in crystalline materials formed during the cooling process from liquid to solid state. During plastic deformation, the dislocation can be generated and multi- plied, and dislocation density could increase. The imposed force for the further plastic deformation will increase caused by the dislo- cating gliding and climbing, which is known as strain hardening [1]. The strain-hardening exponent is equal to the strain when the necking happens. Thus, higher strain-hardening exponent cor- responds to better ductility. For the normal face-centered-cubic materials, the strain-hardening exponent could be very large. It is about 0.45 for an austenite stainless steel, which has very good ductility [1]. As the grain size becomes small, the dislo- cation effect will decrease. When the grain size reaches a nano scale (∼20 nm), the grain boundary becomes dominant during the plastic deformation. The strain-hardening phenomenon will be very weak. The stain-hardening exponent could be lower than 0.1, which makes nano-materials to be relatively brittle [2–4]. If the grain size continuously become finer and reach 1–2 nm, ∗ Corresponding author at: Washington State University, Applied Science Labo- ratory/Institute of Shock Physics, 120 N Pine Street, Spokane, WA 99210, USA. Tel: +1 865 604 3994; fax: +1 509 358 7728. E-mail address: dcqiao@gmail.com (D. Qiao). the materials are considered to be amorphous materials. Metallic glasses are a kind of amorphous materials. There are no dislo- cations and grain boundaries, which results in an expectation of the absence of strain hardening in metallic glasses. The strain- hardening exponent could be zero and even sometimes negative [5]. High-purity metallic-glass materials only show the elastic defor- mation or very limited plastic deformation under un-constrained conditions. The plastic deformation mainly concentrates in the very thin shear bands surrounded by the un-deformed materials, which leads to the strain softening of metallic glass [6,7]. However, the metallic glass is metastable, the crystal nucleation and growth of which are restricted by rapid cooling. Therefore, the structure of metallic glass could be changed from the high energy amorphous state to low energy crystalline state during deformation, which leads to the formation of the nano-crystalline phase. The exist- ing nano-crystalline phase could act as a strengthen phase which restricts the propagation and promotes the multiplication of shear bands, and hence, causes the strain hardening. The crystalline phase formation is affected by the strain rate. Thus different strain rates could result in various deformation behavior of metallic glasses [8,9]. In this paper, the strain rates effects on the plastic deformation and shear-band operation of metallic glasses will be investigated. The large plastic deformation is obtained by a small height/width (H/W) ratio of 1/2. 0921-5093/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.msea.2009.06.062