On formation mechanism of Ti2Ni particles in melt-spun TiNi shape memory ribbons

In the present work, a melt spinning technique was employed to produce Ti51·5Ni48·5 shape memory ribbons. Two types of Ti2Ni particles were observed using high resolution transmission electron microscopy, and a model is proposed based on the microstructural observations. For the large Ti2Ni particles (∼300–500 nm), it is suggested that they are the product of an L32(Ti)+TiNi→Ti2Ni peritectic reaction, where the subscript stands for the Ni content. However, in the case of small particles, novel structures composed of a Ti-rich core and an amorphous shell layer are observed, which are the result of the rapidly solidified L(Ti)→β(Ti)+L24(Ti) and L24(Ti)→β(Ti)+Ti2Ni divorced eutectic process.