Investigation of specific heat and thermal expansion in the glass-transition regime of Pd-based metallic glasses

Abstract

The alloys Pd–Ni–P and Pd–Ni–Cu–P have an outstanding glass-forming ability and stability against crystallisation in the undercooled liquid state at temperatures greater than the glass-transition temperatures. Samples of the copper containing alloys can be vitrified readily by cooling the liquid at rates ⩾0.1 K/s. This rate allows us to investigate the thermodynamic quantities of this alloy system in the liquid as well as in the glassy state during continuous cooling or heating without interference from crystallisation. In the present work, the specific heat and the volume expansion coefficient in the glassy and in the undercooled liquid state were determined at temperatures less than and greater than the glass-transition temperature by applying differential scanning calorimetry (DSC) and sessile drop (SD) technique, respectively. The glass temperatures of the samples resulting from the cooling rate were investigated during re-heating by both the methods. By comparing the results, a correlation between enthalpy and specific volume as a function of temperature could be demonstrated experimentally for these metallic alloys. We attribute the characteristic course of the enthalpy and the specific volume in the range of the glass-transition to structural relaxation processes and to the fraction of free volume in the glass as a function of temperature and time.