Effect of Annealing on the Microstructure, Phase Composition and Microhardness of the Ni–Ti Alloy Produced by Wire-Feed Electron-Beam Additive Manufacturing

The influence of annealing at a temperature of 980°C on the microstructure, phase composition, and microhardness of the additively manufactured TiNi-based material has been investigated. TiNi billets were produced using dual-wire electron beam additive manufacturing by simultaneous deposition of titanium and nickel wires. The typical morphology of dendritic microstructure changes from the bottom to the top of the asbuilt billet and is characterized by large dendritic blocks in the bottom part and thin dendrites in the central and upper parts. According to the data of scanning electron microscopy energy-dispersive X-ray (SEM EDS) analysis and the phase diagrams, dendrites and interdendritic regions consist of a mixture of TiNi + TiNi₃ and TiNi + Ti₂Ni phases, respectively, but the data of X-ray analysis confirm the presence of the Ti₃Ni₄ phase as well. Annealing contributes to the partial dissolution of large dendrites in the bottom part of the billet, increase of the thin dendrite sizes, and smoothing of their forms. The data of SEM EDS analysis and XRD both testify that the multiphase mixture of different phases tends to transform into a single-phase TiNi structure during annealing, but this transformation is incomplete. All these factors lead to the increase of microhardness of the annealed samples from ≈5 GPa for the as-built specimen to ≈7 GPa for the 8-h annealed specimen.