Influence of thermocycling on the functional properties of the NiTi alloy produced by wire arc additive manufacturing

The influence of the thermocycling under a stress of 200 and 300 MPa on the martensitic transformation temperatures, recoverable and irreversible strain was study in the NiTi samples produced by wire arc additive manufacturing (WAAM). Two types of samples were used: the T1 sample including the Ti-rich NiTi and Ni-rich NiTi layers in the working length and the T2 sample including only the Ni-rich NiTi layers. It was found that the variation in the martensitic transformation temperatures on thermal cycling depended on the chemical composition of the layer. The transformation temperatures decreased on thermal cycling in the Ti-rich NiTi layer (in T1 sample), whereas they were constant in the Ni-rich NiTi layers (in T1 and T2 samples). The recoverable strain in both samples did not change during thermal cycling. The irreversible strain was found in the T1 sample regardless of stress acting on thermal cycling, while T2 sample showed the plastic strain only in the first cycle under 300 MPa. It was shown that the difference in a functional behavior of the T1 and T2 samples on thermocycling was due to that the T1 sample contained the Ti-rich NiTi layer, that was characterized by a low dislocation slip limit contrary to the Ni-rich layers which were hardened by the Ni4Ti3 precipitates.