Unconventional precipitation and martensitic transformation behaviour of Ni-rich NiTi alloy fabricated via laser-directed energy deposition

Abstract

In this study, we report an unconventional precipitation and martensitic transformation behaviour of directly aged Ni-rich NiTi alloys fabricated via laser-directed energy deposition (LDED). Ni4Ti3 particles precipitate uniformly under all ageing conditions and no traditional multiple-step martensitic transformations are observed. We conclude this unique behaviour to the intrinsic characteristics of the LDED technique, which are metastable microstructures and high residual stresses. On the one hand, these features make grain boundaries no longer a fevered location for precipitation and, on the other hand, significantly suppress the martensitic transformation when ageing at low temperatures (300°C/400°C). As the aging temperature increase (500°C), residual stresses release significantly, accompanied by the growth of Ni4Ti3 precipitates from several nanometres to 452 ± 181 nm with increased interparticle spacing. At the same time, reverse martensitic transformations change from two-step (B19′ → R → B2) to single-step (B19′ → B2).