Crystallization in Partially Amorphous Ni50Ti32Hf18 Melt Spun Ribbon
Rubén Santamarta, Alexander Pasko, Jaume Pons, Eduard Cesari, Patrick Ochin
A partially amorphous Ni50Ti32Hf18 melt spun ribbon has been characterized by means of calorimetry, X-ray diffraction and transmission electron microscopy, showing that the amorphous regions are mostly concentrated in the wheel side as a consequence of a higher cooling rate during the fast solidification (i.e. higher solidification rate). Special emphasis has been given to the crystallization process of the amorphous regions, studying the evolution of the microstructure and the martensitic transformation. Although several crystallization procedures have been carried out by thermal treatments, either slightly under or over the crystallization temperature measured by DSC, TC, the final microstructure and calorimetric behavior of the fully crystalline samples does not depend on the applied temperature. The fully crystalline samples contain regions with small crystallites produced during the thermal treatment (showing that the crystal nucleation energy is rather low for this alloy) together with the large crystals originated during the melt-spinning. At room temperature (RT), both types of crystals are in martensitic state. The new crystallized regions with small grain size show notably lower transformation temperatures and higher hysteresis in comparison to the big crystals already existing before the crystallization treatments. However, an additional mechanism, apart from the crystal size, affects the transformation temperatures, likely related to the presence of more defects (mainly grain boundaries) in the crystallites created by thermal treatments. These defects could act as nucleation sites for the martensite and increase the transformation temperatures with respect to the as cast spherical crystallites of similar sizes.