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INTRODUCTION

BULK metallic glasses (BMGs) are a relatively new class of engineering materials with unique properties that make them potential candidates for many structural applications.[1] Favorable properties include near-theoretical strengths combined with reasonably high fracture toughness, low damping, large elastic strain limits, and the ability to be thermoplastically formed into precisionshaped parts with complex geometries,[2,3] all properties that are generally distinct from, or superior to, corresponding crystalline metals and alloys. One property which has been perceived as a limitation for BMGs has been low fatigue resistance relative to crystalline metallic materials; indeed, the first study on Zr41.25Ti13.75Ni10 Cu12.5Be22.5 (Vitreloy 1),* the most studied BMG, found *All compositions are given in terms of atomic percent.

STEVEN E. NALEWAY and RAWLEY B. GREENE, Graduate Research Assistants, and JAMIE J. KRUZIC, Associate Professor, are with the Materials Science Program, School of Mechanical, Industrial, and Manufacturing Engineering, Oregon State University, Corvallis, OR 97331. Contact e-mail: [email protected] BERND GLUDOVATZ, Postdoctoral Fellow, is with the Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720. NEIL K.N. DAVE, Graduate Research Assistant, and ROBERT O. RITCHIE, Professor, are with the Materials Sciences Division, Lawrence Berkeley National Laboratory, and also with the Department of Materials Science and Engineering, University of California, Berkeley, CA 94720. Manuscript submitted February 8, 2013. Article published online August 13, 2013 5688—VOLUME 44A, DECEMBER 2013

the endurance strength to be only ~4 pct of the ultimate tensile strength (rfat/rUTS  0.04) in four-point bending at a load ratio of minimum to maximum load of R = Pmin/Pmax = 0.1.[4] At the time, such poor fatigue performance was attributed to the lack of microstructure in monolithic glasses which could arrest incipient flaws; however, more recently, a monolithic Zr-based BMG, Vitreloy 105, with composition Zr52.5Cu17.9 Ni14.6Al10Ti5, has been reported to have excellent fatigue resistance, specifically a 107-cycle endurance strength of rfat/rUTS  0.25 (in four-point bending at R = 0.1), which is better than, or comparable to, all other monolithic glasses and indeed even many crystalline metals.[5,6] Since neither of these monolithic BMGs has microstructure, other factors must govern the large difference in fatigue resistance. Recently, it has been shown that there is a significant environmental degradation of fatigue properties for ZrTi-Cu-Ni-Be BMGs in ambient air which causes a low fatigue threshold and a stress intensity-independent regime of fatigue-crack growth.[7] Furthermore, it is has been suggested that the environmental degradation may be the dominant factor explaining the low measured endurance limit for Zr-Ti-Cu-Ni-Be BMGs.[8] Accordingly, the purpose of this paper is to test the hypothesis that the characteristic low fatigue thresholds and stress intensit