Inhomogeneous microstructures, micropillar compression and thermomechanical properties of additive manufactured NiTi with Ni-loss compensation

Ni-loss is an inevitable issue in the process of additive manufacturing for NiTi shape memory alloys (SMAs). Controlling the intensity of Ni-loss or compensating for the Ni-loss is crucial for achieving additive manufactured Ni-rich NiTi SMAs with the desired phase transformation temperatures and functional properties. In this work, the pre-alloyed NiTi powder is coated with nano-Ni particles evenly, compensating for the Ni-loss during the additive manufacturing for NiTi SMAs effectively. The formability of NiTi parts based on pre-alloyed NiTi powder coated with nano-Ni particles is acceptable, and the NiTi parts exhibits inhomogeneous microstructures. Specifically, NiTi B2 austenite matrix region occupied by uniform Ni 4 Ti 3 and Ti 2 Ni precipitates (Zone I), and NiTi B2 austenite matrix region with continuously Ti 2 Ni precipitates along the grain boundaries (Zone II) are obtained. Compressive superelasticity test of cylindrical NiTi samples (φ3 × 6 mm) shows that an ultrahigh recovery strain of 6.22% and a large recovery rate of 86.4% in the 1st cycle, and a stable recovery strain of 5.01% and a corresponding recovery rate of 85.1% in the 10th cycle are achieved. Nanoindentation and micropillar (φ3 × 6 μm) compression results indicate that the zone I had a higher Young's modulus and nanohardness, and more stable compressive stress-strain curves compared to zone II. This is attributed to the effective strengthening provided by the coherent/semi-coherent interfaces of uniformly distributed precipitates in zone I, as opposed to the stress concentration and interface decohesion promoted by the semi-coherent/incoherent Ti 2 Ni precipitates along grain boundaries in zone II. Furthermore, the semi-coherent or incoherent Ti 2 Ni distributed along the grain boundaries in zone II tends to cause large stress concentration, thereby resulting in micropillar fracture in zone II during the compression process. Geometric phase analysis shows that due to the heterogeneous precipitates and inhomogeneous microstructures, a gradient strain field exists within the additive manufactured NiTi matrix, which could significantly enhance the superelasticity of the NiTi SMAs. These results provide a novel strategy for tailoring the microstructure of additive manufactured NiTi to yield superior functional properties. • Nano-Ni coating on pre-alloyed NiTi powder could effectively compensate Ni-loss for NiTi SMAs during LPBF. • LPBF NiTi achieves 6.22% recovery strain and 86.4% rate in the 1st compression cycle. • Micropillars tests show surface exfoliation post-compression due to stress concentration induced by Ti 2 Ni along boundaries. • Gradient strain field from inhomogeneous microstructures enhances NiTi superelasticity significantly.