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I.

INTRODUCTION

BULK metallic glasses (BMGs) are a relatively new class of engineering materials with unique and unusual properties that make them potential candidates for many structural applications.[1] Favorable properties include near theoretical strengths combined with reasonable fracture toughness, low damping, large elastic strain limits, and the ability to be thermoplastically formed into precision-shaped parts with complex geometries,[2–4] all of which are generally distinct from, or superior to, corresponding crystalline metals and alloys. One property that has been perceived as a limitation for BMGs has been low fatigue resistance relative to crystalline metallic materials; indeed, the first study on Zr41.25Ti13.75Ni10Cu12.5Be22.5,* the most studied BMG, *All compositions are given in terms of atomic percent.

found the endurance limit to be only roughly 4 pct of the ultimate tensile strength (rfat/rUTS  0.04) for fourpoint bending with a load ratio of R = Pmin/Pmax = 0.1.[5] However, not all studies to date have been in agreement on this point. The reported 107 cycle fatigue strengths for Zr41.25Ti13.75Ni10Cu12.5Be22.5 vary by a factor of 7,[5–7] and fatigue thresholds, DKTH, vary by a factor of 3.[5] While some of the reported scatter may be explained by different testing configurations,[8] this does not account for all the observed variations, for example, those within single studies.[5,6] Furthermore, a recent review article has shown that when a broad array of Zrbased BMG chemistries are considered, a wide range of fatigue life behavior is observed and many Zr-based BMGs show behavior comparable to crystalline metals J.J. KRUZIC, Associate Professor, Materials Science, is with the School of Mechanical, Industrial, and Manufacturing Engineering, Oregon State University, Corvallis, OR 97331, USA. Contact e-mail: [email protected] Manuscript submitted March 16, 2010. Article published online September 24, 2010 1516—VOLUME 42A, JUNE 2011

(Figure 1),[9] suggesting initial concerns may have been specific to certain BMG compositions. Furthermore, while early attempts at making BMG matrix composites also resulted in low fatigue limits and fatigue thresholds,[10] the latter even worse than comparable fully amorphous BMGs, more recently developed BMG matrix composites demonstrate excellent fatigue resistance.[11,12] While it is becoming apparent that it is likely possible to produce BMGs and BMG matrix composites with a rich variety of fatigue properties, the fatigue literature to date has been plagued by a high degree of scatter. Such scatter suggests that some important variables affecting the fatigue behavior of BMGs are not adequately understood or controlled in the experiments. Studies over the last few years have suggested that factors such as the thermal history,[6,13–15] mechanical history,[16–18] environment,[19–22] and defects[23–25] all may play a role in affecting the fatigue behavior of BMGs. However, most of those factors were rarely characterized, reported, or controlled in early fatigue