MARTENSITIC NUCLEATION ON DISLOCATIONS IN Cu-Al-Ni SHAPE MEMORY ALLOYS STUDIED BY IN SITU TEM

摘要

Martensitic Transformation (MT) in shape memory alloys (SMA) takes place by heterogeneous nucleation, but there is no clear description about the role that lattice defects play in the nucleation process. In this situation, we have performed a study on the mechanisms of martensite nucleation in Cu-Al-Ni SMA single crystals by in situ transmission electron microscopy (In situ TEM). This method supply information about the martensite nucleation site, allowing to determine simultaneously the martensite kind and the character of dislocations. In the present research work, Cu-Al-Ni SMA have been cycled inside the TEM at a temperature higher than Af. The in situ experiences show that dislocations in β phase can be nucleation sites for martensites when a stress is applied during superelastic tests. The martensite plates have been characterized by Select Area Diffraction Pattern (SADP) during the in situ experience in TEM. Later on, when the stress is withdrawn, the reverse transformation takes place and the martensite plates disappear on the dislocation. It has been observed that forward and reverse transformations are completely reproducible on the same dislocation during cycling. Finally the results are discussed concluding that the atomic configuration of the dislocation core promotes the martensite nucleation.