Caracterización de aleaciones base Ni-Ti producidas por solidificación rápida (melt-spinning)

Abstract

In the present work, the shape memory effect and the thermal stability of the martensitic transformation under various thermal treatments have been characterized on several Ni-Ti based alloys cast by melt-spinning, a novel technique on shape memory alloys. The studied alloys belong to the Ni-Ti based system, with eventual additions of Cu, Hf or Zr and have been characterized by means of DSC, mechanical testing, thermomechanical cycling, X-ray diffraction and transmission electron microscopy. The sequences of the martensitic transformation and the lattice parameters of the melt spun ribbons have been firstly determined. Additionally, the initial microstructures show a big amount of defects (mainly dislocations and small precipitates) and small grain size in comparison to the bulk alloys. Among all the alloys, the near-equiatomic one show the best thermal stability under severe ageing at 500 ºC, followed by the alloys with Cu and finally by the ones with Hf and Zr additions. The latter ones are the only ones exhibiting transformation temperatures well above room temperature, but their poor thermal stability is shown even at 300 ºC. Moreover, thermal treatments in martensite carried out on the the alloys with Hf and Zr reveal thermal martensite stabilization, which was a novel phenomenon in Ni-Ti based alloys; the activation energies of this process have been estimated. Ageing in mixtures of martensite and austenite on these alloys also promote the thermal martensite stabilization by pinning. Partially amorphous melt spun ribbons can be crystallized by either thermal treatments above the crystallization temperature shown in the DSC or slightly below, in a controlled way. The crystallization promotes the apparition of small crystals from the initial amorphous regions, which transform at very low temperatures due to changes in the non-chemical terms from the energy balance equation respect to the big crystalline regions that are present in the initial state. Finally, the shape memory effect characterized by thermomechanical cycling show recoverable strains similar to the ones in bulk alloys, which demonstrate that the melt spun ribbons may be used in potential applications when using reduced tensional stress in order to avoid an early fracture.