The glass transition of bulk metallic glasses studied by real-time diffraction in transmission using high-energy synchrotron radiation

Abstract

The isentropic glass transition Tg in metallic glasses can be studied by calorimetric techniques which show an endothermic event corresponding to a specific heat ΔC p difference between the glassy and supercooled liquid states. In this work we show that isostructural thermal expansion coefficient α th is easily obtained by diffraction data hence an isochoric T g should be detectable by diffraction. Real-time diffraction studies of phenomena occurring on a time scale of τ t can only be effective if acquisition time τ a < < τ t . The problem was that near T g , metallic glasses are unstable and crystallize during real-time diffraction experiments. This limitation does not occur in diffraction experiments on bulk metallic glasses with large supercooled regions ΔT = T x - T g using high energy, high intensity monochromatic light from synchrotron sources that allow τ a << τ t . Under such conditions, the variation of α th , the volume coefficient of thermal expansion has been measured for the first time by diffraction near T g for a number of bulk metallic glasses. A clear Δα th has been detected in the same glass transition temperature range of ΔC p for Pd-based metal-metalloid bulk glasses but not in Zr-based metal-metal type bulk glasses where Δα th occurs well above the calorimetric T g .