Bending properties and transformation temperatures of heat treated Ni-Ti alloy wire for orthodontic appliances.

The effect of heat treatment temperature on bending properties and transformation temperatures of a Ni-Ti alloy wire, 1.0 mm in diameter, was investigated so that superelasticity could be used in orthodontic appliances needing shape memory processes. The heat treatment process was at 713 K for 1.8 ks and between 673 K and 813 K for 1.8 ks. A three-point bending test and differential scanning calorimetry were performed. The transformation temperatures of the wires were lowered with increasing heat treatment temperature. The reverse transformation finishing temperature was below the body temperature with the treatment above 753 K. Residual deflection of the Ni-Ti wire after bending was small with the secondary heat treatment above 733 K. The load in the unloading process was less changeable and increased with the treatment temperature between 733-813 K. Secondary heat treatment in this range was suitable for using superelasticity in expansion arch appliances.