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

Abstract

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 1mm 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 600s. 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 Ni4Ti3 precipitates were revealed. The results show that the nanograins grew rapidly from 34nm to 2.1μm within 12 A/0–25s and the Ni4Ti3 precipitates of 15–25nm were formed after 8 A/600s in coarse-grained samples, which respectively affected the functional properties of nanocrystalline and coarse-grained samples. In addition, 12 A/10s dissolved the preformed Ni4Ti3 nanoprecipitates in the aged samples, which also influenced their functional properties. Furthermore, DERHT induced precipitation and growth of Ni4Ti3 were suppressed in nanograins, leading to the less conspicuousness of aging effect in nanocrystalline samples.