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

The “isentropic” glass transition Tg in metallic glasses can be studied by calorimetric techniques which show an endothermic event corresponding to a specific heat ΔCp difference between the glassy and supercooled liquid states. In this work we show that isostructural thermal expansion coefficient αth is easily obtained by diffraction data hence an “isochoric Tg” should be detectable by diffraction. Real-time diffraction studies of phenomena occurring on a time scale of τt can only be effective if acquisition time τa<<τt. The problem was that near Tg, 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=Tx−Tg using high energy, high intensity monochromatic light from synchrotron sources that allow τa<<τt. Under such conditions, the variation of αth, the volume coefficient of thermal expansion has been measured for the first time by diffraction near Tg for a number of bulk metallic glasses. A clear Δαth has been detected in the same glass transition temperature range of ΔCp for Pd-based metal–metalloid bulk glasses but not in Zr-based metal–metal type bulk glasses where Δαth occurs well above the calorimetric Tg.