Effect of hydrogen on super-elastic behavior of NiTi shape memory alloy wires: Experimental observation and diffusional-mechanically coupled constitutive model.

NiTi shape memory alloys (SMAs) are inevitably in contact with hydrogen in specific service environments, which can degrade their mechanical behaviors. In this work, the effect of hydrogen on the super-elasticity of NiTi SMA orthodontic wires is investigated experimentally and theoretically. Firstly, cathodic hydrogen charging was performed for the wires at a current density of 10A/m with various charging times (2.5min, 5min, 7.5min and 10min) and charging lengths (20 mm, 40 mm, 60 mm and 80 mm) in 0.5 mol/L H2SO4+2 g/L CH4N2S electrolyte solution at room temperature. Then, ex-situ tension-unloading tests were carried out shortly after the hydrogen charging. The stress-strain responses showed a two-step martensite transformation (MT), i.e., the start stress of MT for the region with hydrogen charging is much larger than that without hydrogen charging. Based on the experimental observations, a diffusional-mechanically coupled constitutive model is constructed. Elastic strain, transformation strain, transformation-induced plasticity (TRIP) and hydrogen swelling deformation are considered. The effect of hydrogen on the thermo-mechanical behavior of NiTi SMA is taken into account by introducing the hydrogen concentration (HC)-dependent critical temperatures of MT and slip resistance of TRIP. The thermodynamic driving forces of MT and TRIP are derived from the constructed Helmholtz free energy and dissipation inequality. The balance equation of hydrogen diffusion is obtained by the chemical potential and Fick's diffusion law. To obtain the overall response of the wire with a heterogeneous HC field, a scale transition rule is proposed. The capability of the proposed model to describe the super-elasticity of NiTi SMA with various hydrogen charging times and charging lengths is validated by comparing the predicted results with the experimental ones.