Abstract: The modification of the electron diffraction pattern(EDP) of B2 in an as—rapidly quenched then short—time aged Ti—Ni shape memory alloy(SMA) has been studied systematically using TEM. It was found that the diffusion scattering rings in EDP of an as—rapidly quenched Ti-Ni SMA is associated with short range order (SRO) of vancancies in parent B2. After aging at 450℃ for 5 min the diffusion scattering rings are replaced by 1/2 {001} and 1 / 4 {012} extra—reflections. It indicates that the SRO of vacancies has transformed into long range order (LRO) of vacancies,moreover,the LRO of vacancies exists in microdomains.

Key words: Ti—Ni shape memory alloy, vacancy order, electron diffraction