Effects of Hydrogen-charging on the Phase Transformation of Martensitic NiTi Shape Memory Wires

A better understanding of the hydrogen interactions with shape memory alloys (SMAs) is needed for applications, often in extreme environments, in the aerospace, oil & gas, medical, and technological industries. Martensitic NiTi SMA wires (Ni_49.8Ti_50.2) were tested under accelerated hydrogen-charging conditions to reveal the effects on phase transformation. Hydrogen-charging was performed by immersion in a H₃PO₄ solution (85 vol.%) at 65°C for different durations of time (0, 5, 15, 30, 60, 145 and, 240 minutes). The SMA wires were characterized by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and laboratory and synchrotron radiation X-ray diffraction (SR-XRD). DSC was performed in the temperature range of -150°C to 500°C to examine phase transformation behavior in the as-received and hydrogen-charged condition. Heating/cooling cycles were performed to observe phase evolution with heating and cooling. The martensite/austenite transformation behavior of hydrogen-charged samples is markedly different from that of the as-received samples: 1) Three additional peaks are observed in hydrogen-charged samples. These peaks only appear in the first heating cycle and are associated with hydrogen interactions, e.g. hydride formation, with NiTi SMA. After the first heating cycle, hydrogen has been desorbed and released from the NiTi SMA wire. 2) All subsequent heating/cooling cycles show suppressed martensite/austenite transformation peaks. These peaks exhibit an increasing shift downward in transformation temperature with each cycle. In-situ SR-XRD measurements will be presented, which complement the DSC and SEM results.