Electron beam freeform fabrication of NiTi shape memory alloys: Crystallography, martensitic transformation, and functional response

In this work, NiTi shape memory alloys parts were additively manufactured using electron beam freeform fabrication (EBF³) under different processing parameters, including the beam current, travel speed, and wire feeding speed. The forming quality, phase composition, microstructure change, crystallography, martensitic transformation, shape memory and superelastic responses were systematically investigated. All deposits mainly consisted of B2-austenite at room temperature, and a handful of B19′-martensite and submicron-sized Ti₄Ni₂O_x precipitates were also detected. The martensitic transformation of NiTi alloys prepared by EBF³-technique possessed an individual reversible path between B2 and B19′ upon heating/cooling. The optimized deposit possessed the best comprehensive properties, where the values of the relative density, shape memory recovery and superelastic recovery ratios were 99.6%, 98.95%, and 55.78%, respectively. Furthermore, the dependence of the martensitic transformation behavior on the thermomechanical condition and the relationship between plastic deformation and phase transformation during superelastic deformation are discussed in detail. Our work details that the EBF³ provides a suitable way for the complex fabrication of large-scaled parts based on shape memory alloys.