Microstructural evolution and mechanical properties of eutectoid Ti–7Ni alloy produced by electron beam powder bed fusion

A eutectoid Ti–7Ni (wt.%) alloy was additively manufactured by electron beam powder bed fusion (E-PBF) in this work. The effects of electron beam current and preheating time on the microstructure and mechanical properties of the E-PBF-built Ti–7Ni (wt.%) alloy were investigated. The evolution of the eutectoid microstructure during the in-situ heat treatment of E-PBF was elucidated. The addition of nickel resulted in a microstructure consisting of pre-eutectoid α laths and lamellae of submicron α and Ti2Ni. The morphology of the Ti2Ni evolved from fine lamellae to discontinued ellipsoids with increasing electron energy density or preheating time. Defects such as pores and lack of fusion were observed in the alloy under insufficient electron energy density. Fine Ti2Ni lamellae and pre-eutectoid α laths were obtained in the alloy processed at an electron energy density of 54 J/mm³ and a preheating time of 6 s, contributing to a considerable ultimate tensile strength of 660 MPa and good elongation of 16.8%. In addition, a graded microstructure along the building direction was observed due to the coarsening of Ti2Ni during multiple thermal cycles. It was demonstrated that the microstructure of the eutectoid Ti–7Ni alloy can be controlled by the in-situ heat treatment during the layer-by-layer E-PBF process. The findings in this work provide understanding and guidance for microstructure regulation in other additively manufactured eutectoid titanium alloys.