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

The "isentropic" glass transition T_g 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.