Usually a multi-stage martensitic transformation is observed in Ni-rich TiNi alloys after heat treatment at 350-500 °C. It is due to the internal stresses created by the Ni4Ti3 participate. In the present work it was found that the multi-stage martensitic transformation appeared in Ti-50.0 at.% Ni alloy after thermal cycles through the temperature range of the phase transitions. Annealed sample undergoing one-stage phase transition was subjected to 32 thermal cycles in the DSC apparatus. The results had shown that three-stage forward martensitic transformation observed after 32 thermal cycle was due to the B2 → R, B2 → B19′ and R → B19′ phase transitions. It was found that the B19′ phase obtained from the B2 phase underwent the reverse transformation at higher temperatures than the B19′ phase obtained from the R phase. After annealing the cycled sample at 400 °C the transformation behavior was similar to the non-thermal cycled alloy. It was concluded that the main reason for the multi-stage phase transition induced by the thermal cycles was the phase hardening.
Scopus subject areas
- Metals and Alloys