Relaxation mechanisms in martensitic NiTi(Cu): Internal friction measurements correlated to in situ TEM straining

The microstructural origin of the internal friction response in B19’ martensitic NiTiCu polycrystals is investigated by in situ TEM straining experiments. Internal friction measurements showed that the transient contribution Q_Tr⁻¹, determined as a function of temperature in martensitic state, vanishes when the heating or cooling is stopped. In situ TEM experiments evidence defects moving across the martensitic variants along the microtwin boundaries. They are identified as partial dislocations (PDs). They move under applied stress as well as under temperature gradients. In the present study a correlation between their movement and the changes of Q_Tr⁻¹ is proposed. The Q_Tr⁻¹ is found to be sensitive to the density of PDs depinned from the martensite variant boundaries. Their movement appears to be a reversible process, as when stress is released the PDs return into the variant interface and consequently Q_Tr⁻¹ value drops.