Effects of Room Temperature Aging on Hydrogen-Charged Nitinol

Previously unreported room temperature aging effects in hydrogen-charged Nitinol have been observed. Nitinol wire dogbone tensile samples were cathodically charged with hydrogen levels ranging from 15wppm to 55wppm then allowed to age under ambient conditions for times ranging from 10 to 1000min. The average fracture strain for uncharged specimens was 20%. While no embrittlement was acutely found after charging, profound embrittlement was observed after room temperature aging from 1000 minutes; for example, aging wires with 35 wppm resulted in a reduction in average fracture strain to 15% after 30 minutes and 10% after 1000 minutes. No significant changes in plateau heights or transformation temperatures were observed between uncharged and hydrogen-charged samples at any time point. The XRD peak structure and d-spacings for hydrogen-charged samples were consistent with prior research indicating the formation of a hydride with a yet unidentified structure. It is suggested that the room temperature embrittlement phenomenon is connected with the isothermal precipitation of hydrides.  The time-dependent nature of hydrogen embrittlement in Nitinol has far reaching implications to both the manufacturing and application of Nitinol.