Publication: Hydrogen absorption and diffusion in niti shape memory alloy
Date
2023-05-01
Authors
Ng Ching Wei
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Abstract
Nickel titanium (NiTi) alloy is a popular biomaterial in biomedical implants and orthodontic applications. In orthodontic treatment, the usage of NiTi archwire in combination with dental bracket is capable of delivering constant force on the treated tooth, to induce tooth movement. The contact between NiTi archwire and stainless steel brackets could form a galvanic couple in the oral cavity, while giving rise to hydrogen absorption into archwire. The absorption of hydrogen atoms impairs its shape memory behavior and functionality in orthodontic treatment. The deterioration of shape memory behavior of NiTi archwire may persist and worsen over the treatment duration, owing to the inner diffusion of hydrogen interstitial atoms into deeper core of the matrix.
This study investigated the effect of hydrogen absorption and diffusion over time toward shape memory and load-deflection behavior of NiTi alloy wires. Two different NiTi wires were use, the industrial round wire, and the commercial rectangular orthodontic wire. At room temperature, the round wire was at martensitic phase, and the rectangular wire was at austenite phase. The absorption of hydrogen into wire specimens was induced via electrolytic charging. The inward diffusion of hydrogen over time was achieved via aging at room temperature.
The differential scanning calorimetry showed that the absorption and diffusion of hydrogen in martensitic NiTi round wire suppressed the size of thermal martensite phase transformation peaks. Furthermore, the additional hydrogen-related phase transformation peaks were also observed after hydrogenation, while its peak size and enthalpies increased over the aging duration. The tensile deformation behavior was also largely affected, of which the stress-induced martensitic phase transformation exhibited a non-flat plateau with the formation of force curvatures at the onset and ending stages, respectively. The loading force, as deduced from its load-deflection behavior via three-point bending deformation also increased slightly. This impaired its strain recovery.
The effect of hydrogen charging towards load-deflection behavior of rectangular austenitic NiTi wire via three-point bending test was only detected after charging for 16 and 24 hours. The strain recovery of the wire deteriorated over aging time in the unloading stage of load-deflection curve. The largest residual deflection was 0.65 mm for the 24-hour-charged and 7-day-aged wire specimen, as compared to 0.03 mm for the as-received specimen. In a three-bracket bending test, the hydrogen charging caused the wire to fracture during the loading stage. Likewise, the hydrogen diffusion via aging led to the large residual deflections upon complete unloading, of which its magnitude increased from 0.12 mm for the as-received specimen, to 0.70 mm after charging for 24 hours and aging for 7 days, at a maximum deflection of 4 mm. After prolonged aging, the effect of inner diffusion of hydrogen has caused obstruction towards the reverse stress-induced martensite phase transformation, thus causing the unloading force curve to occur at lower force levels.