STUDY OF DISLOCATIONS GENERATED BY THERMAL CYCLING IN NI-TI-CO SHAPE-MEMORY ALLOYS

In the present work we report some results concerning the evolutions of transformation temperatures and microstructure during thermal cycling in Ti-47%Ni-3% Co (atomic percent). In the case of hulk material (obtained hy a conventional solidification method) high amounts of dislocations from during cycling, which produce a considerable evolution of the transformation temperatures (up to similar to 40K), while in meltspun material containing a dispersion of fine precipitates, the formation of dislocations during thermal cycling is highly reduced, giving rise to stable transformation temperatures. The dislocation characteristics have been analysed by means of two-beam condition observations as well as the Large Angle Convergent Beam Electron Diffraction technique. Some energetical considerations concerning the dislocations are pointed out to interpret the evolution of transformation temperatures.