Effect of Hydrogen on the Transformation Ratcheting of NiTi SMA Wires: Experiment and Modeling

Abstract

This paper investigates the effect of hydrogen on the transformation ratcheting of NiTi shape memory alloy (SMA) wires in the experimental and theoretical aspects. In the aspect of experiments, the NiTi SMA orthodontic wires are hydrogen charged by the electrochemical charging method at room temperature with varying charging durations and charging lengths. After that, the ex-situ cyclic tension-unloading experiments are performed for the charged and non-charged wires. Experimental results reveal that the two transformation platforms (two-step MT) occur during the forward MT at the beginning and end of cyclic deformation for hydrogen-charged wires, which can be regarded as a global response of the non-charged and charged regions. Furthermore, this two-step MT and transformation ratcheting aggravate with the increase of the charging duration. In the aspect of the theoretical model, a diffusional-mechanically coupled constitutive model is developed. In this constitutive model, the strain is considered as four components: elasticity, transformation (MT), hydrogen expansion and transformation-induced plasticity (TRIP). Combining Helmholtz free energy and Clausius–Duhem inequality, the thermodynamic driving forces of MT and TRIP are obtained. Fick’s law and the mass conservation equation are incorporated to derive the evolution of hydrogen concentration. A transition from material points to the whole wire is employed to extend the model from a material point to the entire wire, and the overall response with a heterogeneous hydrogen concentration field is obtained. The proposed model's ability to predict the transformation ratcheting of the non-charged and charged NiTi SMA wires is verified by contrasting predictions and experimental results.