THE THERMODYNAMICS OF AMORPHOUS PHASES IN IMMISCIBLE SYSTEMS - THE EXAMPLE OF SPUTTER-DEPOSITED NB-CU ALLOYS

Amorphous metallic alloys, frequently observed to occur in systems wit h large negative heats of mixing, ate much less common in systems whic h are immiscible in the equilibrium solid stale, such as Nb-Cu. Howeve r, amorphous Nb-Cu alloys can be produced over a wide composition rang e by sputtering. Using isothermal and nonisothermal differential scann ing calorimetry, both the kinetics and the thermodynamics of these amo rphous Nb-Cu alloys were characterized quantitatively. It was found th at the formation enthalpies of the amorphous alloys amounted to only 4 .5-7.6 kJ/g atom. These data were combined with a modeling of the ther modynamic functions of the system. The unexpected low enthalpies and G ibbs energies of the amorphous phase demonstrate the thermodynamic sta bilization of the Liquid phase which develops with undercooling. This is connected with a change of sign in the heal of mixing of the liquid phase, which is positive at high temperatures and negative al low tem peratures. (C) 1997 American Institute of Physics.