In-situ TEM cooling/heating experiments on deformed NiTi shape memory single crystalsT. Simon1, A. Kröger1, Ch. Somsen1, A. Dlouhy2 and G. Eggeler1
1 Ruhr-University Bochum, Institute for Materials, 44780 Bochum, Germany
2 Institue of Physics of Materials, ASCR, Ziskova 22, 61662 Brno, Czech Republic
Published online: 1 September 2009
In the present study we report about the influence of dislocations on martensitic transformations in NiTi single crystals. Microstructural investigations are performed on Ni50.4Ti49.6 (at.%) single crystals using in-situ cooling and heating transmission electron microscopy (TEM). Solution heat-treated Ni50.4Ti49.6 single crystals were oriented by electron backscatter diffraction (EBSD) and compressed in  B2-direction to different strain levels. DSC measurements on undeformed and deformed material states reveal a two step transformation from B2 to R-phase and then from R-phase to B19´ confirmed by TEM. The analysis of the dislocation structure of a 3.3% compressed single crystal shows that mainly screw dislocations with  type burgers vectors are present. During cooling, the martensitic R-phase grows homogeneously. On further cooling in some regions a burst like growth of B19'-neddles can be observed in addition to B19'-regions that nucleate and grow promoted by the stress-fields of dislocations. Dislocation analysis after back transformation suggests that dislocations form as a result of the martensitic transformation.
© Owned by the authors, published by EDP Sciences 2009