Small atom diffusion in the glassy and supercooled liquid states of bulk amorphous ZrTiCuNiBe alloys

New bulk metallic ZrTiCuNiBe glass forming alloys exhibit an excellent stability over a large supercooled liquid region and offer the opportunity to investigate atomic transport in the supercooled liquid state. We present measurements of atomic diffusion of beryllium in Zr41.2Ti13.8Cu12.5Ni10Be22.5 and Zr46.7Ti8.3Cu7.5Ni10Be27.5 in the temperature range between 530 K and 710 K. Below the glass transition temperature, Tg∼625 K, the data suggest a single atomic jump diffusion behavior of Be with a migration enthalpy of about 1 eV/atom in both alloys. Below Tg, Be diffusivity in Zr46.7Ti8.3Cu7.5Ni10Be27.5 is by one order of magnitude higher than in Zr41.2Ti13.8Cu12.5Ni10Be27.5 which can be explained by a larger fraction of free volume in this alloy. Above the glass transition the temperature dependence of Be diffusivity increases in both alloys, leading to apparent activation energies of 4.5 eV/atom in Zr41.2Ti13.8Cu12.5Ni10Be22.5 and 1.9 eV/atom in Zr46.7Ti8.3Cu7.5Ni10Be27.5. By taking into account the change in configurational entropy due to the glass transition, we explain the mechanism for diffusion of Be in the supercooled liquid states of the ZrTiNiCuBe alloys by single atomic jumps in a continously changing configuration of neighboring atoms. The corresponding migration enthalpies are the same as in the solid states, and the larger activation energies above Tg are mimiced by the increase of configurational entropy with temperature which is smaller in Zr46.7Ti8.3Cu7.5Ni10Be27.5 than in Zr41.2Ti13.8Cu12.5Ni10Be22.5. A comparison of Be diffusivity and viscosity of Zr46.7Ti8.3Cu7.5Ni10Be27.5 above Tg reveals a breakdown of the Stokes-Einstein relation, indicating cooperative processes in the supercooled liquid state. (orig.)