Because of its biocompatibility, superelastic Ni-Ti wire alloys have been successfully used in orthodontic clinics. The susceptibility of Ni-Ti shape memory alloys toward hydrogen embrittlement has been examined with respect to the residual stress after a few number of cycles in air at room temperature. Orthodontic wires have been cycled until having an imposed deformation of 2.1%, 4%, and 7.7% between 1 and 50 cycles and then have cathodically been charged by hydrogen with a current density of 10 A/m2 for 4 h in a 0.9% NaCl aqueous solution at room temperature. Throughout cycling, a residual strain has been formed and has increased by the number of cycles and the value of the imposed deformation. After hydrogen charging, the critical stress enhances when the number of cycles is great and the value of the imposed deformation is high. In addition, an embrittlement occurs for the specimen submitted to 50 and 30 cycles with an imposed strain of 2.1% and 4%, respectively. Nevertheless, no embrittlement has been detected after 50 cycles until 7.7% of the imposed deformation. The results of this study imply that the embrittlement could be related to the discontinuity in the distribution of defects created by partial superelastic cycling.

Because of its good corrosion resistance and biocompatibility, superelastic Ni-Ti wire alloys have been successfully used in orthodontic clinics. However, delayed fracture in the oral cavity has been observed. The susceptibility of a Ni–Ti shape memory alloy towards hydrogen embrittlement has been examined with respect to the residual stress after a few numbers of cycles and ageing in air at room temperature. Orthodontic wires have been cathodically charged by hydrogen with a current density of 10 A/m2 from 4h in 0.9% NaCl aqueous solution at room temperature. The critical stress for the martensite transformation under a monotonous tensile test of the as-charged specimen has been 30 MPa higher than that without hydrogen charging. However, after 1 to 50 cycles followed by hydrogen charging, the austenite-martensite plateau is decreased outstandingly compared to the monotonous tensile test of the as-charged by hydrogen specimen. Moreover, compared to the non-cycled and hydrogen charged material, the cyclic deformed and charged by hydrogen specimens present an increase of the initial stress for inducing martensite structure. This increase is about 110 MPa after 50 cycles. In addition, an embrittlement has been detected for the specimen submitted to 50 cycles and hydrogen charging for 4h. This behavior is attributed to the generated dislocations during cyclic deformation which act as trapping sites of hydrogen and a barrier for the austenite-martensite transformation.