Study on the role of carbon in modifying second phase and improving tensile properties of NiTi shape memory alloys fabricated by electron beam directed energy deposition

Carbon has always been considered as a harmful impurity element in NiTi shape memory alloys and has been controlled as low as possible. Surprisingly, it is found in this work that a certain amount of carbon can be beneficial for the NiTi alloys. Electron beam directed energy deposition (EB-DED) technology is employed to fabricate NiTi alloy components by using two types of feeding wires with lower carbon content (0.009 wt%) and higher carbon content (0.037 wt%), respectively. Results show that when the feeding wire of lower carbon content is used, the dominant second phase in the as-built NiTi alloys are coarse Ti2Ni phase, which is brittle and harmful to tensile properties. When the feeding wire of higher carbon content is used, the second phase in the as-built NiTi alloys is changed to fiber-like TiC phase, with other microstructural characteristics basically unchanged. Accordingly, the tensile ductility is notably improved by the increase in carbon content, with elongation doubled from 6.1 % to 12.8 %. In addition, the cyclic life of the NiTi alloys in tensile superelasticity tests is also significantly improved. The NiTi alloys with lower carbon, no matter heat treated or not, prematurely fracture within 10 load cycles. In contrast, the NiTi alloys with higher carbon, after aging treatment, demonstrate an excellent superelastic recovery rate of 93.17 % after the 1st load cycle, and do not fracture after 50 load cycles. Such tensile property is the best among those achieved so far for the additively manufactured NiTi alloys. This study provides a new perspective on the role of carbon, which, instead of being a harmful element, could actually be used to improve the microstructures and properties of the additively manufactured NiTi shape memory alloys.