Structure formation of NiTi alloys for superelastic medical implants

The basic assumption of this work was to achieve superelastic behaviour of implants used for bone distraction for correction maxillofacial and craniofacial deformities. This paper describes two possibilities of inducing superelastic behaviour of spring implants used for children cranial correction. The first one is based on the usage of straight wires for [omega] and U-shape spring formation. The superelasticity is induced to the wires by a proper cold deformation and annealing below the recrystallization temperature. Dislocation cells and low angle grain boundaries are characteristic for these wires. The second possibility of the superelasticity induction into ring springs is based on precipitation hardening. For this purpose an alloy with higher amount of nickel (51 %at.) was used at which precipitation of coherent particles of Ni4Ti3 leads to the matrix hardening and allows to achieve a distinguish stress plateau on the stress-strain curve. The obtained implants have shown their usefulness in clinical applications.