PURPOSE: To use a Ti-Ni alloy having excellent reversibility and to extremely increase the release energy at the time of relieving of deformation by using the alloy consisting of a prescribed ratio of Ni, a specified ratio or below of at least one kind among Cr, V, Al, and Nb and the balance substantially Ti.
CONSTITUTION: A remarkable improvement effect is not admitted in spite of addition of the 3rd elements to the alloy as to the recovery of superelasticity at 37°C if the concn. of the Ni is below 49.5at.% as regards the material of the wire to be used for the orthodontic appliance. On the other hand, the effect increases but the workability is poor if the concn. of Ni exceeds 0.5at.% and, therefore, the concn. of the Ni is specified to 49.5 to 50.5%. Cr, V, Al, and Nb are extremely highly effective as the 3rd elements in enhancing the recovering stress at 37°C. The above-mentioned effect decreases if the amt. of the 3rd elements to be added is below 0.25at.%. The workability is poor in the range exceeding 2.0at.% and, therefore, the content of at least one kind among the 3rd elements is specified to 0.25 to 2.0%.