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/m2 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.
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