Rapid fabrication of NiTi alloys with different functional properties by electron beam freeform fabrication using Ni and Ti wires

NiTi shape memory alloys with different functional properties at room temperature were successfully prepared by dual-wire electron beam freeform fabrication technology using Ni and Ti wires. The results indicated that the chemical composition, phase transition temperatures, and functional properties could be controlled effectively by adjusting the wire feeding ratio. With the increase of Ni wire feeding speed, the phase transition temperatures of NiTi alloys decreased monotonously, and the acicular martensites in the samples changed to coarse columnar austenites. The depositions exhibited a single reversible martensitic transformation with Ni content ranging from 50 at. % to 51.2 at. %. The mechanical properties of the samples varied with Ni contents and the test directions. In addition, the finely dispersed Ni₄Ti₃ phases in the Ni_51.2 sample significantly increased the dislocation slip resistance of the matrix, resulting in a superelastic recovery rate of 75.4 % at 5 % pre-strain. However, with the further increase of Ni content, the critical stress of martensitic transformation at room temperature was also enhanced, leading to premature fracture under high-stress conditions. This work provides an effective method for rapid preparation of NiTi alloys with different phase transition temperatures and functional properties.