Phase transition features of NiTi orthodontic wires subjected to constant bending strains

Abstract

A resistometric investigation of premartensitic and martensitic phase transitions in NiTi orthodontic wires has been carried out as a function of temperature between −20 and +60°C, during cooling and heating. Transformation features of two types of commercial wires were examined in either their undeformed state or under application of a constant bending strain. For the undeformed case the transformation behavior was also characterized by differential scanning calorimetry (DSC). The start temperature (Ms) for the formation of martensite was appreciably different for the two types of material, while that (Rs) for the formation of the R-phase was about the same. For one material (type 1) the enthalpies ΔH∗ for the A → R and R → M transitions were deduced from DSC data and used for calculating the Clausius-Clapeyron relationships Rs(σ) and Ms(σ). For the other material (type 2), due to the exceedingly low value of Ms, the Ms(σ) relationship could not be determined. Above room temperature electrical resistivity data taken under bending did not give clear indications of stress-induced martensite for material of type 2, which, however, displayed good superelastic behavior. This lack of clarity arises from complex alterations taking place in the structure (lamellar) and proportions of the different phases (A, R and M) present under inhomogeneous bending strains when the temperature is changed. To fully clarify this point stress-strain tensile experiments were carried out, preliminary results of which are also presented in this paper.