Effect of temperature on the devitrification kinetics of NiTi films

The effect of isothermal devitrification temperature on the reaction kinetics and the microstructure of freestanding NiTi films have been studied using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The minimum temperature for complete crystallization of amorphous sputtered films was found to be 400 °C, and analysis of the nucleation kinetics yielded Avrami exponents between 2 and 3 for all films. Crystalline grains always nucleated first at the surface, grew laterally until impingement, then grew inward to form columnar grains. Surface roughness delayed the onset of surface nucleation. In very smooth surfaced films, multiple DSC exotherms heralded repeated nucleation of new wide, flat grains within the film interior. This secondary microstructure exhibited different kinetics than the columnar grains, and Avrami exponents consistent with a continuous nucleation mode.