Bending property and phase transformation of Ti-Ni-Cu alloy dental castings for orthodontic application.

Bending property of Ti-Ni-Cu alloy castings was investigated in a three-point bending test for orthodontic application in relation to the phase transformation. The compositions of the alloys were Ti-50.8Ni and Ti-40.8Ni-10.0Cu (mol %), and four cross-sectional shapes of the specimens were selected. Heat treatment was performed at 713, 753 or 793 K for 1.8 ks. The bending load changed by the cross-sectional size and shape mainly because of the difference in the moment of inertia of area, but the load-deflection relation did not differ proportionally in the unloading process. The difference between the load values in the loading and the unloading processes was relatively small for Ti-Ni-Cu alloy. With respect to the residual deflection, there was no significant difference between Ti-Ni and Ti-Ni-Cu alloys with the same treatment condition. The load values in the loading and the unloading processes decreased by each heat treatment for Ti-Ni alloy; however, the decrease in the load values for Ti-Ni-Cu alloy was not distinct. It is proved that Ti-Ni-Cu alloy castings produce effective orthodontic force as well as stable low residual deflection, which is likely to be caused by the high and sharp thermal peaks during phase transformation.