Degradation and fracture of Ni-Ti superelastic wire in an oral cavity

Superelastic Ni-Ti wire is widely used in orthodontic clinics, but delayed fracture in the oral cavity has been observed. Because hydrogen embrittlement is known to cause damage to Ti alloy systems, orthodontic wires were charged with hydrogen using an electro-chemical system in saline. Tensile tests were carried out, and fracture surfaces were observed after hydrogen charging. The strength of the Co-Cr alloy and stainless steel used in orthodontic treatment, was not affected by the hydrogen charging. However, Ni-Ti wire showed significant decreases in strength. The critical stress of martensite transformation was increased with increasing hydrogen charging, and the alloy was embrittled. The fractured surface of the alloys with severe hydrogen charging exhibited dimple patterns similar to those in the alloys from patients. In view of the galvanic current in the mouth, the fracture of the Ni-Ti alloys might be attributed to the degradation of the mechanical properties due to hydrogen absorption.