A Practitioner’s Perspective of Hydrogen in Ni-Ti Alloys

Nearly all studies of hydrogen in Ni-Ti (Nitinol) report bulk or average hydrogen content, thereby assuming that hydrogen is homogeneously distributed. Yet hydrogen is invariably locally introduced, and thus its distribution is far from homogeneous: immediately after charging, bulk hydrogen might measure 50 wppm, while localized levels may exceed 3000 ppm. This paper treats several aspects of hydrogen in Ni-Ti, recognizing that its effects are localized and highly dynamic. With this in mind, the process of hydrogen uptake is discussed, along with how quickly it migrates and where it resides on the lattice. It is shown that as little as 100 wppm hydrogen can completely suppress the austenite–martensite transformation, and that suppression is a kinetic rather than a thermodynamic issue. Ductility is profoundly affected as well, but in a time-dependent manner based on the diffusion of hydrogen—tensile samples are fully ductile after charging, but become fully brittle hours later. Fatigue is affected to a lesser extent, although again, there are pronounced room-temperature aging considerations. Finally, the “safe limit” for hydrogen is discussed, concluding that the existing 50 wppm ASTM threshold is reasonable, but with the very important caveat that the hydrogen has been allowed to homogenize.