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Crystal structure, composition and morphology of the precipitate in an aged Ti-51 at%Ni shape memory alloy have been examined by means of high voltage and high resolution analytical electron microscopy. As a result, the following have been clarified. The precipitate possesses a rhombohedral ratio in the unit cell is deduced to be Ni4Ti3, from consideration of space group. The composition is well consistent with Ni56Ti44 experimentally obtained from energy dispersive X-ray spectroscopy, within experimental error. The crystal structure is a slight modification of the B2 structure of matrix. The calculated intensity distribution in reciprocal space based on the structure model is in qualitative agreement with the observed one. Lenticular morphology of the precipitate, internal {154}R, twins and antiphase domains are successfully explained by the structure model. On the basis of this knowledge, a relation between the existence of such precipitates and the appearance of all-round shape memory effect is briefly discussed.

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