Hydrogen Pick-up During Heat Treatments of Ni-Rich NiTi Shape Memory Alloys

Technologically relevant heat treatments of Ni-rich NiTi shape memory alloys are performed in air with a non-negligible content of moisture which may result in a flow of hydrogen into the workpiece. In the present study, we report results of differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and atom probe (AP) experiments in which martensitic transformations and microstructure of Ti-50.9at%Ni shape memory samples were investigated after heat treatments in controlled gaseous environments. The heat treatments were performed in a mixture consisting of hydrogen and helium with an overall filling pressure of 900 mbar. The cylindrical samples were heat treated in two regimes. In the Regime I, they were subjected to only annealing at 850°C for 1800 s and water quenched. In the Regime II, the cylinders received the Regime I treatment and an additional aging at 450°C for 3600 s terminated by water quenching. The DSC experiments clearly show that the B2 -> B19’ and R -> B19’ martensitic transformations diminish with increasing partial pressure of hydrogen applied during the Regime I and Regime II treatments, respectively. We relate the gradual suppression of the martensitic transformations to the modifications of the B2 austenite lattice by the hydrogen atoms. In parallel, we use DSC and AP techniques to investigate the hydrogen interstitial solid solution and its stability with increasing temperature. We conclude that hydrogen may function as a B2 stabilizer in a temperature regime from -150 up to 150°C.