Glass transition T_g, and quenched-in free volume in bulk metallic glasses measured by X-ray diffraction

In this work we show that through thermal expansion am measured by real-time diffraction using synchrotron light in transmission, the "glass transition T_g" and for the first time, the "quenched-in free volume" can be quantitatively measured in bulk metallic glasses. This was impossible previously as metallic glasses with crystallisation temperature T_x near T_g crystallized during real-time diffraction experiments. This limitation does not hold for newly developed bulk metallic glasses (BMGs) with large supercooled regions ΔT = T_x-T _g. Under such conditions, the variation, Δα_th, of the volume coefficient of thermal expansion has been measured 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 measured by DSC calorimetry in particular for Pd-based, Zr-based and Hf-based bulk glasses. The directly measured quenched-in free-volume content V_f is reported here for the first time is found, as expected, to depend on the quench rate and anneals out just below T_g.