Twofold role of dislocations in the relaxation behavior of Ti–Ni martensite

The relaxation peak in Ti–Ni-based shape memory alloys is attracting much attention as a high damping source, and its origin was recently revealed to be the interaction between twin boundaries and hydrogen. However, reports on the thermal cycling effect, which is characterized by the peak height decrease with cycling, seem to suggest that dislocations may also affect the relaxation behavior. To understand the role of dislocations in the twin boundary–hydrogen interaction peak, changes in the relaxation behavior with thermal cycling were systematically studied by dynamical mechanical analysis for both H-containing and dehydrogenated Ti50Ni50 samples. Our results show that although the origin of the relaxation peak is not related with dislocations, dislocations can affect the twin boundary–hydrogen interaction process in two opposite ways. First, tangled dislocations (created by thermal cycling) make twin boundary motion difficult. On the other hand, dislocations can also absorb hydrogen from twin boundaries and make twin boundary motion easier. All the thermal cycling effects in dynamic tests and dc tensile tests can be explained by the above dual role of dislocations in the twin boundary–hydrogen interaction process.