Leaf-like dislocation substructures and the decrease of martensitic start temperatures: A new explanation for functional fatigue during thermally induced martensitic transformations in coarse-grained Ni-rich Ti-Ni shape memory alloys

During repeatedly imposed thermally induced martensitic transformations in Ti-Ni shape memory alloys, the martensite start temperature M _s decreases. This has been rationalized on the basis of a scenario where an increasing dislocation density makes it more and more difficult for martensite to form. However, it is not clear why dislocations which form because they accommodate the growth of martensite during the first cooling cycle should act as obstacles during subsequent transformation cycles. In the present work we use diffraction contrast transmission electron microscopy to monitor the formation of unique leaf-like dislocation substructures which form as the martensite start temperature decreases during thermal cycling. We interpret our microstructural results on the basis of a microstructural scenario where dislocations play different roles with respect to the propagation of a big martensite needle in one transformation cycle and the nucleation and growth of new martensite needles in the following cycles. As a consequence, chestnut-leaf-like dislocation arrays spread out in different crystallographic directions.