Microstructure and properties of crystallized melt-spun Ti50Ni25Cu25 ribbons after current-driven thermal cycling

Abstract

The influence of thermal cycling driven by electrical current under a small and constant applied stress of 50 MPa on martensitic transformation, microstructure and electrical resistance of crystallized melt-spun ribbons of a Ti50Ni25Cu25 (at.%) shape memory alloy is investigated. Cycled specimens and uncycled ones as a reference are characterized by various methods like differential scanning calorimetry, X-ray diffraction, transmission electron microscopy, and electrical resistance. After thermal cycling DSC curves during cooling exhibit two transformation peaks whereas the reverse transformation proceeds in only one step. The separation of the two peaks and also their maximum remains stable during cycling but the peak shapes become broader. The reason for the two-stage transformation is found in a crystalline surface layer already present in the as-spun state. A heat treatment between 700°C and 800°C for 600s is able to recrystallize the surface layer. Only minor changes of the microstructure due to current-driven thermal cycling could be observed. The average grain size is about 500 nm in both cycled and uncycled specimens. A few plate-like precipitates are present created during the crystallization process. Twins are frequently observed. Dislocations, however, are never observed not even in the cycled specimens.