Small atom diffusion and breakdown on Stokes-Einstein relation in the supercooled liquid state of Zr-Ti-Cu-Ni-Be alloys

Abstract

Be diffusivity data in the bulk metallic glass forming alloys Zr_(41.2)Ti_(13.8)Cu_(12.5)Ni_(10)Be_(22.5) and Zr_(46.7)Ti_(8.3)Cu_(7.5)Ni_(10)Be_(27.5) are reported for temperatures between 530K and 710K, extending up to 80K into the supercooled liquid states of the alloys. At the glass transition temperature, T_g, a change in temperature dependence of the data is observed in both alloys, and above T_g the diffusivity increases faster with temperature than below. The data in the supercooled liquid can be described by a modified Arrhenius expression containing the communal entropy of the supercooled liquid and based on a diffusion mechanism suggested earlier. The comparison with viscosity data in the supercooled liquid state of Zr_(46.7)Ti_(8.3)Cu_(7.5)Ni_(10)Be_(27.5) reveals a breakdown of the Stokes- Einstein relation, whereas D(T) and η(T) follow a relation close to van den Beukel's. The breakdown of the Stokes- Einstein relation indicates a cooperative diffusion mechanism in the supercooled liquid state of the ZrTiCuNiBe alloys.

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