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Zhuoxi Li and Baoyi Wang Key Laboratory of Nuclear Analysis Techniques, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China (Received 10 April 2012; accepted 17 May 2012)

The effects of hydrogen on the structure of Zr-based bulk metallic glasses were investigated by positron annihilation lifetime spectroscopy. Three lifetime components are identified, indicating the presence of three distinct size ranges for open volume defects in the glass. The concentration of the smallest sites identified as tetrahedral interstitial holes in the densely packed and the intermediate sites identified as flow defects, changes with hydrogen addition. The concentration of tetrahedral interstitial holes in Zr55Cu30Ni5Al10 alloys initially increases with the increase of hydrogen content. When Zr55Cu30Ni5Al10 alloys were prepared in Ar 1 10%H2 atmospheres, the concentration of tetrahedral interstitial holes reaches a maximum, which may provide a more dense random-packed structure. For Zr57Al10Cu15.4Ni12.6Nb5 alloys, the increase of hydrogen content causes a decrease in the concentration of tetrahedral interstitial holes and an increase in the concentration of flow defects. I. INTRODUCTION

Compared with crystalline counterparts, bulk metallic glasses (BMGs) exhibit some favorable mechanical properties including high strength, high elastic strain limits, high hardness, and high fracture toughness, which are important qualities for engineering and structural materials.1,2 Among all BMGs, the Zr-based BMGs have the widest commercial success due to the superior glass formation ability (GFA) and good mechanical properties. Prior studies show that the GFA and plasticity of BMGs are sensitive to the alloy compositions.3–5 Thus, control and optimization of the GFA and plasticity require an understanding of atomic-level structure. Numerous experimental and theoretical studies have examined short- and medium-range order in metallic glasses. Fewer studies have addressed the details of the free volume distribution. Recently, Flores et al.6 investigated the evolution of the open volume sites in a Zr-based BMG subjected to increasing amounts of inhomogeneous deformation by positron annihilation lifetime spectroscopy (PALS). Three lifetime components were observed in their work, suggesting the presence of three sites ranges of open volume sites, which are more complex than that described by Cohen and Turnbull.7 While Flores’ work provides interesting insight into possible atomic

Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2012.209 J. Mater. Res., Vol. 27, No. 20, Oct 28, 2012

arrangements, PALS provides the experimental capability to examine open volume sites with subatomic length scales and is useful for more directly analyzing the open volume distribution in metallic glasses. As a unique gas element, hydrogen can be generally used as a probe to explore the structure of metallic glassy alloys and distribution of interstitial sites. Most glassy