Influence of hydrogen on hydrogen embrittlement, superelasticity, and thermal hysteresis of additive manufactured NiTi shape memory alloy

Understanding of hydrogen embrittlement (HE), superelasticity (SE), and thermal hysteresis in additively manufactured NiTi shape memory alloys (SMAs) is limited. The functional properties of Wire Laser-Directed Energy Deposition (WL-DED) NiTi are examined: (1) HE, (2) SE, and (3) thermal hysteresis. Samples were hydrogen charged for 6-72 h. The degradation of functional properties was investigated by comprehensive characterization. Results indicated a suppression of reversible strain, a decrease in transformation temperatures (24 h), and hysteresis after 48 h. The intermetallics (Ni 3 Ti/Ti 2 Ni), R-phase suppression, and the B2 austenite to B19’ martensite transformation were discovered. Hydrogen charging (72 h) induces SE partial recovery and hydride formation/hydrogen redistribution. We proposed a dual-stage time-dependent HE model; Stage I (0-48 h): the synergy of HELP + HEDE mechanisms of HE, and Stage II (48–72 h): hydride embrittlement/HE mitigation. The proposed pre-stabilization treatment intentionally shifts hydrogen into deep traps/hydrides, trading a little initial functional performance for improved hydrogen/HE resistance. • Hydrogen charging degrades the superelasticity of AM NiTi shape memory alloy. • Thermal hysteresis narrows non-monotonically due to hydrogen trapping. • Hydrogen peaks at 2840 ppm (48 h), causing max functional properties loss in AM NiTi. • Microstructure shows martensite variation, plate refinement, and microcracks. • Dual-stage hydrogen embrittlement/damage model, pre-stabilization strategy proposed.