Because of its good corrosion resistance and biocompatibility, superelastic NiāTi wire alloys have been successfully used in orthodontic clinics. However, delayed fracture in the oral cavity has been observed. The susceptibility of a NiāTi 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 20āA/m2 from 2 to 24āh in 0.9% NaCl aqueous solution at room temperature. The critical stress for the martensite transformation under a monotonous tensile test has been 20ā90āMPa higher than that without hydrogen charging. In addition, embrittlement takes place in the austeniteāmartensite transformation plateau. For a short period of charging, the NiāTi alloy conserves its superelastic behavior. However, after 24āh of aging in air at room temperature, fracture at the austeniteāmartensite transformation plateau takes place earlier.
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