THERMAL-ANALYSIS OF SOLID-STATE AMORPHIZATION - PREDICTED AND MEASURED REACTION ENTHALPIES AND REACTION-KINETICS

Measurements of heat released upon solid state amorphization (SSA) hav e been performed to evaluate quantitatively a recently proposed thermo dynamic model for solid state amorphization in binary systems. Three t ypes of Ni-Ti systems have been investigated: crystalline Ni/crystalli ne Ti multilayers with incoherent interfaces, crystalline Ni/crystalli ne Ti multilayers with coherent interfaces and amorphous Ni/crystallin e Ti multilayers. The heat released during SSA has been determined by differential scanning calorimetry (DSC) measurements. For the crystall ine/crystal line multilayers with incoherent interfaces the reaction h eats measured agree with the reaction heats derived from the thermodyn amic model. For the crystalline Ni/crystalline Ti multilayers with coh erent interfaces and the amorphous Ni/crystalline Ti multilayers the e xperimental reaction heats differ 30 to 40% from the calculated values . The activation energies for SSA in the three Ni/Ti systems have been determined, too, from the DSC measurements. Within experimental accur acy the same value for the activation energy was obtained for the thre e types of Ni/Ti multilayers The activation energy was found to be 130 +/- 15 kJ/mol. This value agrees with the one obtained previously fro m X-ray diffraction analysis. The evolution of heat during SSA could b e described as a function of annealing time upon isochronal annealing adopting a Johnson-Mehl-Avrami type reaction rate equation.