Dependence of processing window and microstructural evolution on initial material state in direct electric resistance heat treatment of NiTi alloy

Direct electric resistance heat treatment (DERHT) is especially suitable for the rapid functional property tailoring of NiTi alloys. However, the dependence of processing window and microstructural evolution of DERHT on the initial material state remains unclear so far. To clarify this issue, NiTi wires of 1 mm in diameter with aged, non-aged, nanocrystalline and coarse-grained material states were respectively undergone DERHT with electric currents of 12 and 8 A within the heating time from 0 to 600 s. The variations of phase transformation and superelasticity were studied and the corresponding DERHT processing windows were obtained. The underlying microstructural evolutions were identified and the variations of grain size and Ni ₄Ti ₃ precipitates were revealed. The results show that the nanograins grew rapidly from 34 nm to 2.1 μm within 12 A/0–25 s and the Ni ₄Ti ₃ precipitates of 15–25 nm were formed after 8 A/600 s in coarse-grained samples, which respectively affected the functional properties of nanocrystalline and coarse-grained samples. In addition, 12 A/10 s dissolved the preformed Ni ₄Ti ₃ nanoprecipitates in the aged samples, which also influenced their functional properties. Furthermore, DERHT induced precipitation and growth of Ni ₄Ti ₃ were suppressed in nanograins, leading to the less conspicuousness of aging effect in nanocrystalline samples.