Effects of Hydrogen on the Phases and Transition Temperatures of NiTi

Austenitic NiTi samples were cathodically charged with hydrogen to produce concentrations ranging from a baseline value of 9 wppm to 650 wppm. The effect of hydrogen on the phases and transition temperatures of NiTi was studied with X-ray diffraction (XRD), differential scanning calorimetry (DSC), tensile testing, and scanning electron microscopy (SEM). XRD analyses showed the B2 peaks shifting to lower 2θ angles with increased hydrogen content. The shift in 2θ angles represented an increase in the lattice parameter from 3.011 to 3.018 Å (0.7% volume increase) between 9 and 647 wppm hydrogen. DSC analyses showed a decrease in the transition temperatures and enthalpies for both the austenite and martensite phases. In the austenite phase the As was lowered by 8oC, the Af by 9oC, and the enthalpy by 13 mJ/mg between 9 and 240 wppm hydrogen. The impact was more significant in the martensite phase where the Ms was lowered by 80oC, the Mf by 110oC, and the enthalpy by 11 mJ/mg between 9 and 200 wppm. Tensile testing of baseline material and hydrogen charged samples showed an increase of approximately 60 MPa in the martensite stress plateau. SEM analyses showed a transition from a ductile to brittle fracture mode with increasing hydrogen content.