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INTRODUCTION

BULK metallic glasses (BMGs) are generally recognized as advanced engineering materials because of their superior physical, chemical, and mechanical properties compared to their crystalline counterparts.[1–3] Among these BMGs, Zr-(Ti, Nb)-Cu-Ni-(Al, Be) alloys (Vitreloy series) are well known due to their high glassforming ability (GFA), excellent corrosion resistance, large plastic deformability in the supercooled liquid region, and good mechanical properties, which are manifested by a high yield strength and a large elastic strain limit.[4–8] However, these Zr-based alloys contain late transition metals (LTMs) (e.g., Ni and Cu) with a high mass density and, hence, are not attractive for applications where light weight is required. Seen from this angle, Ti-based alloys have a larger potential owing to their lower density and relatively low cost.[9,10] Therefore, extensive research has been conducted in the Ti-rich corner of the (Zr,Ti)-(Cu,Ni)-Be pseudoternary glass-forming system,[11,12] and only recently, low density Ti-Zr-Be–based BMGs with improved specific strength excluding LTMs (e.g., Fe, Ni, Cu, and Sn) have been developed.[13,14] These results imply that the adjustment of the LTM content in alloys of the Vitreloy series allows tailoring of the yield strength while still maintaining a large specific strength.[15] On the other hand, plasticity is also a desirable property for BMGs for widespread practical application.

J.M. PARK and G. WANG, Guest Scientists, S. PAULY, Research Staff Member, N. MATTERN, Head of Department, and J. ECKERT, Director and Professor, are with the Institute for Complex Materials, IFW Dresden, D-01171 Dresden, Germany. Contact e-mail: [email protected] D.H. KIM, Professor, is with the Center for Non-Crystalline Materials, Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Republic of Korea and the Institute of Materials Science, TU Dresden, D-01062 Dresden, Germany. Manuscript submitted March 25, 2010. Article published online September 22, 2010 1456—VOLUME 42A, JUNE 2011

Since the plastic deformation of BMGs is highly inhomogeneous at room temperature through the formation of shear bands, they exhibit limited plastic strain ( 0.35 or G/K < 0.33).[27–29] When BMGs have a high m value or a low G/K ratio, shear bands are easily initiated and multiplied during deformation and the plastic strain therefore increases. This reveals that the tendency of the shear stress to concentrate in shear bands rather than in cracks plays a crucial role in the enhancement of the deformation behavior.[20,42] In this study, we selected the Ti-rich Ti65Cu9Ni8Be18 glassy alloy in the (Zr,Ti)-(Cu,Ni)-Be pseudo-ternary glass-forming system as a starting composition having a high specific strength.[17] By changing the composition of these alloys, our focus is on the influence of the Zr content on the mechanical properties, especially on plasticity. We will discuss the mechanism underlying plastic flow in the framework of self-organized criticality behavior, which will be