Publication: Hydrogen infusion and diffusion of Ni-Ti shape memory alloys of austenite phase.
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Date
2022-07
Authors
Jarod O’neil, Q.J Chong
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Abstract
Ni-Ti alloy with its unique properties in shape memory effect and superelasticity has found its use in the application of many fields including medical, engineering and manufacturing industries. Ni-Ti alloy has always been susceptible to hydrogen embrittlement due to diffusion and absorption of hydrogen atoms into the NI-Ti shape memory alloy. This study investigates whether hydrogen charging the Ni-Ti wire in high temperature so that the wire is in austenite phase and subsequent aging has any effect on the mechanical and thermal properties of the alloy. The hydrogen infusion process was done via electrolytic charging at a constant current of 200mA for 12, 18 and 24 hours in 0.1 percent concentration of sodium sulphate solution. The charged specimens were then aged at room temperature for different duration in order for hydrogen to diffuse further into the core of the specimen. The results obtained show that the enthalpy of austenite and martensite phase transformation for Ni-Ti wire decrease from 14.12 to 0.4296 J/g, and 16.19 to 3.455 J/g, respectively for hydrogen charged specimen for 12 hours and subsequent aging for 4 days. The thermal transformation peaks were suppressed further until disappearance was observed as the aging duration increase for the charged specimen. The tensile deformation shows the transformation stress level increase to level as high as 380Mpa and experience non-flat
stress plateau after hydrogen charging and subsequent aging. The Ni-Ti alloy even experience fracture when the transformation stress of the alloy even exceeded the limit of strength of the Ni-Ti alloy. The bending deformation also shows the transformation stress level increase up to as much as 4 N after hydrogen charging and subsequent aging. The bending deformation also shows a non-flat stress plateau for the transformation stress level and an increase of the hysteresis loop increase as the aging duration increase.