NiTi superelastic orthodontic wires with variable stress obtained by ageing treatments

Abstract

A new strategy to obtain NiTi orthodontic archwires with variable correcting stresses in different segments is presented. Different heat treatments are applied in selected segments, which produce variations in the superelastic properties. These allow to modulate the correcting stresses to the requirements of the specific teeth. This method avoids laser welding, which is associated with corrosion and brittleness problems. The aim of this study is to analyze the effect of the heat treatment by zones on transformation temperatures, and stresses in NiTi orthodontic archwires with identical chemical composition. A reduction of the Ti content is observed, in the matrix of the posterior segment, due to the formation of Ti2Ni precipitates. This produces a decrease of the transformation temperatures and consequently an increase of correction stresses. The changes are smaller in the lateral segment due to the lower temperature reached. No changes are observed in the anterior segment in relation to the original archwire. The titanium-rich precipitates undergo observation and characterization by Transmission Electron Microscopy (TEM) and electron diffraction.