Abstract: In this paper by means of TEM and HREM, we found that the presence of r-phase plays the most important role in enabling two way shape memory effect (TWME) to be obtained by the stabilization of stress induced martensite variants (SSIM). This idea is different from the traditional suggestion that strain fields developed by dislocation produced TWME. With prolonged isothermal ageing time, the bainitic tronsforma-tion product α1 precipitated, the amount of γ-phase increased, while the martensite was retained and became less ondered. These factors all reduced TWME. During age-ing, the stacking faults in bainitic product α1 disappeared, which indicated that the bainitic product α1 is a sub-equilibrium phase. The interfaces of α-plates also lost straightness step by step, some plates twisted, bronched and overlapped. Its formation is irreversible in tnorphology and reduces the quantities of martensite. Although the precipitation of r may stabilize some martensite or oppose its growth,α may hinder nucleation of martensite. Thus, it was considered that α-plates were the worst de- stroyed element. These suggestions agreed well with our performance test results in that the transformation temperntures and the amount of TWME changed in the same way with prolonged ageing, firstly increasing and then decreasing after reaching an optimal maximum value.

Key words: precipitate, stabilization, TWME, TEM and HREM