Improvement of tensile superelasticity by aging treatment of NiTi shape memory alloys fabricated by electron beam wire-feed additive manufacturing

For the first time, this work comprehensively studied the effectiveness of precipitation hardening achieved by aging treatment in improving the tensile superelasticity of NiTi alloys fabricated by electron beam wire-feed additive manufacturing (EBAM), which possesses inherent advantages in producing dense and oxidation-free structures. Aging treatments under three temperatures (450, 350, and 250 °C) and different durations were conducted, and the resultant performance of tensile superelasticity, together with the corresponding evolution of precipitation and phase transformation behavior were investigated for the EBAM-fabricated NiTi alloys. Results showed that by appropriate aging treatment, EBAM fabricated NiTi alloys could achieve excellent recovery rates of approximately 95% and 90% after the 1st and 10th load/unload cycle for a maximum tensile strain of 6%, which were almost the highest achieved so far by AM processed NiTi alloys and close to those of some conventional NiTi alloys. The improvement of tensile superelasticity benefited from the fine and dispersive Ni4Ti3 precipitates, which could be introduced by aging at 350 °C for 4 h or at 250 °C for 200 h. Moreover, the large amount of Ni4Ti3 precipitates would promote the intermediate R-phase transformation and bring a two-stage or three-stage transformation sequence, which depended on whether the distribution of the precipitation was homogeneous or not. This work could provide guidance for the production of NiTi alloys with good tensile superelasticity by EBAM or other additive manufacturing processes.