The microstructure and martensitic transformation behaviors in Ti-Ni-Hf -X (Ag, Sn) high temperature shape memory alloys

The microstructures and martensitic transformation behaviors of the quaternary Ti-Ni-Hf-X (Ag, Sn) high temperature shape memory alloys were investigated systematically. The results showed that Ag and Sn were soluble in Ti-Ni-Hf matrices with a limited solubility of 0.28 at.% and 1.78 at.%, respectively. In addition to the martensite and (Ti,Hf)2Ni phase, the pure elemental Ag particle was also observed in the Ti-Ni-Hf-Ag alloys. However, the excessive Sn addition resulted in the formation of the intermetallics such as Ti3Sn type and TiNi2Sn type phase in the Ti-Ni-Hf-Sn alloys. Moreover, with the increasing of Sn content, the amounts of (Ti,Hf)2Ni phases decreased owing to the dissipation reaction of 12(Ti,Hf)2Ni +8Sn → 6TiNiHf + 5(Ti,Hf)3Sn + 3(Ti,Hf)Ni2Sn. Single martensitic transformation corresponding to B2⇋B19′ was observed in Ti-Ni-Hf-Ag alloys. While the Ti-Ni-Hf-Sn alloys with the higher Sn content (≥2 at.%) were characterized by two-steps martensitic transformation during the heating process. Both Sn and Ag addition reduced the martensitic transformation temperatures.