Structural relaxation, glass transition and crystallization of Cu-Zr amorphous ribbons studied by dilatometry and differential scanning calorimetry

Abstract

The structural relaxation, glass transition and crystallization of Cu 33Zr 67, Cu 40Zr 60 and Cu 60Zr 40 amorphous ribbons were studied by dilatometry and differential scanning calorimetry (DSC). A highly sensitive dilatometer (built by us) was used which allowed samples in the form of thin ribbon to be heated at a slow heating rate (1.7°C min −1) in an ultrahigh vacuum (10 −9 Torr). Thermal expansion coefficients were determined on heating and cooling. Isothermal annealings were also performed in situ in the dilatometer. The mean coefficients of linear thermal expansion (α × 10 6) measured for the relaxed amorphous alloys and for the crystalline alloys were the same between room temperature and the crystallization temperature: 9.2 ± 0.2 for Cu 33Zr 67, 9.9 ± 0.2 for Cu 40Zr 60, and 11.8±0.2 for Cu 60Zr 40. The increase in the coefficient of thermal expansion is thus strongly correlated with the increase in copper content. These values are very close to those calculated assuming a linear relationship between the mean composition and the values of α m of the individual components. This suggests that the cohesion varies smoothly with composition without any substantial change in the nature of bonding. After the initial heating of the as-quenched ribbons below the glass transition, structural relaxation resulted in shrinkage of the samples after cooling to room temperature. The glass transition was indicated by the creep of the specimen. Crystallization resulted in an important shrinkage. DSC was used to aid in the interpretation of the dilatometric events, in particular for the characterization of the enthalpy relaxation associated with structural relaxation and of the enthalpy increase at the glass transition. The irreversibility of the structural relaxation below the glass transition was clearly demonstrated by DSC as were the reversible enthalpy changes at the glass transition.