Hydrogen effect on the austenite-martensite transformation of the cycled Ni-Ti alloy

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/m(2) 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.