Pseudoelasticity and Transformation Features of some NiTi Orthodontic Commercial Wires

Abstract

Premartensitic and martensitic phase transitions have been investigated between -20 and +60 °C in two types of NiTi orthodontic wires subjected to constant bending strains. The electrical resistivity clearly shows a shift towards higher temperatures of the austenite/R-phase (A-R) transition, indicating pseudoelastic behaviour due to stress-induced R-phase. Pseudoelasticity associated with stress-induced martensite was not clearly displayed by resistometric measurements, due to the inhomogeneity of the applied strain field. Stress-strain tensile experiments, on the contrary, clearly show that pseudoelasticity mostly arises, as expected, from stress-induced martensite. The Clausius-Clapeyron relationships have been deduced for martensite induced by stress from both the austenite and R-phase and they appear to differ one another. Appreciable recoverable strains could be identified for both types of material at deformation levels beyond the plateau region.