ENTHALPIES OF FORMATION AND CRYSTALLIZATION OF AMORPHOUS ZR1-XALX AND ZR1-XNIX ALLOYS - CALCULATIONS COMPARED WITH MEASUREMENTS

The enthalpies of formation of amorphous Zr1-xAlx and Zr1-xNix alloys have been calculated based on CALPHAD data and compared with experimentally determined values. A direct extrapolation of the thermodynamic functions of the undercooled liquid significantly underestimates the stability of the amorphous phase. When including the effects of the excess specific heat of the undercooled liquid, DELTAC(p), good quantitative agreement between the calculations and experimental data has been achieved, in particular for the Zr-Ni case. Using the same DELTAC(p) functions, the enthalpy of crystallization of the amorphous alloys in both systems has been computed and found to agree well with the same quantity directly obtained from calorimetric measurements. These calculated enthalpy of crystallization data have also been employed in an alternative approach to calculating the enthalpy of formation of the amorphous alloys; such calculations are in excellent agreement with experimental values in both Zr-Al and Zr-Ni cases. It is shown that the enthalpy-composition diagram measured or calculated using our approaches can be used as a good approximation to the free energy-composition diagram in predicting the homogeneity range of the amorphous phase for low synthesis temperatures.