Structural features of the extraordinary low glass transition temperature for Au₆₅Cu₁₈Si₁₇ bulk metallic glass

Au₆₅Cu₁₈Si₁₇ bulk metallic glass (BMG) exhibits an extraordinary low glass transition temperature (<100°C) and a wide supercooled liquid region. In order to elucidate the structural features of the extraordinary low glass transition temperature and high glass forming ability of the Au₆₅Cu₁₈Si₁₇ BMG, high-energy X-ray diffraction (HEXRD) and Extended X-ray Absorption Fine Structure (EXAFS) measurements were carried out using synchrotron radiation. A sharp first peak and oscillation of the structure factor S(Q) up to the high wave number Q can be attributed to the dominant weighting factor of the Au-Au correlation over others. A reverse Monte Carlo (RMC) simulation was applied simultaneously to the HEXRD and EXAFS data. The obtained RMC model shows a quite highly dense packed structure with large amount of icosahedral type clusters around Au atoms while small ones around Si atoms. The partial Cu-Cu pair distribution function g _Cu,Cu(r) shows a sharp peak at a short interatomic distance, i.e., 2.38 Å. The Cu-Cu pairs with short interatomic distances and the large thermal fluctuation around Cu atoms are considered to be one of the structural characteristics of the low glass transition temperature of the Au ₆₅Cu₁₈Si₁₇ BMG.