Microstructure and shape memory properties due to the formation of a rhombohedral phase (R-phase) formation in nickel rich binary Ni-Ti alloy containing 50.23 atom% Ni were investigated in this research. After annealing, the samples were aged for 1, 5 and 7 hours at 773K. Microscopic studies revealed that the rhombohedral phase heterogeneously nucleated and then grew within the regions near the grain boundaries of the alloy. It was also observed that the amount of this phase increased with the ageing time. Microstructural TEM observations indicated that a considerable number of dislocations were simultaneously produced during the R-phase formation as a result of the ageing process. It was, therefore, suggested that the formation of the R-phase regions was related to the presence of the stress fields produced around the dislocations. With increasing of these strain fields, possibility of direct austenite to martensite transformation apparently reduced; while the formation of the intermediate R-phase regions grew. A diverse new set of twins and self accommodated twin envelops were observed in martensitic microstructure of the samples together with the pseudo-stable R-phase regions.
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