Effect of hydrogen on the tensile strength of aged Ni-Ti superelastic alloy

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 toward hydrogen embrittlement has been examined with respect to the current densities and aging in air at room temperature. Orthodontic wires have been cathodically hydrogen charged using a different current density of 5, 10, and 20 A/m(2) from 2 to 24 h in 0.9% NaCl aqueous solution at room temperature. The critical stress for the martensite transformation under a monotonous tensile test has been 20-90 MPa higher than that without hydrogen charging. In addition, embrittlement takes place in the austenite-martensite transformation plateau. For a short period of charging, the Ni-Ti alloy conserves its superelastic behavior. However, after 24 h of aging in air at room temperature, fracture at the austenite-martensite transformation plateau takes place earlier.