Influence of annealing treatments on superelasticity of NiTiTaHf shape memory alloy

To develop a high temperature shape memory alloy with high transformation temperature and good cold-working ability，NiTiTaHf wires with a diameter of 0.2 mm are fabricated through cold drawing and subsequent annealing.The influence of annealing temperature on microstructure，martensitic transformation and superelasitic behavior of NiTiTaHf wire are investigated. The results show that the superelasticity of NiTiTaHf wire is quite sensitive to the annealing temperature. The sample annealed at 500 ℃ displays the substantial superelasticity featuring a recoverable strain of 7.6% over a broad temperature range from 20 ℃ to 200 ℃. In contrast，the sample annealed at 700 ℃ only shows a small superelastic recoverable strain （less than 3.3%）over a narrow temperature range （140-200 ℃）.This difference in superelastic behavior is caused by the change of grain size resulting from the different annealing temperature，the grain size of the sample annealed at 500 ℃ is about 12 nm. This leads to the occurrence of strain glass state in the sample and the resultant superelasticity over a wide temperature range.