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The authors are grateful to the State Key Laboratory of Fatigue and Fracture for Materials, Institute of Metal Research, Academia Sinica, for providing financial support and testing facilities.

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REFERENCES

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is longer than that in pseudoelastic state regardless of the morphology and distribution of a phase. This result is coincidental with that obtained by Delaey et al.t4] in the single-phase CuZnAI alloy. However, in either the martensitic or pseudoelastic state, the effect of a phase on fatigue life is obvious. Compared with that of the singlephase alloy, the fatigue life of dual-phase alloys with homogenously distributed globular a phase is increased in both the martensitic and pseudoelastic state, and the network a phase along grain boundaries decreases fatigue life when the cycling stress is high but no evident effect is observed at low stress levels. Nevertheless, the blocky c~phase in grain boundaries shows no observable influence. Scanning electron microscopic observation suggests that c~ phase can modify the fracture features remarkedly. This implies that a phase plays an important role in fatigue crack nucleation and propagation. More detailed studies need to be conducted to clarify the effect of c~ phase on fracture mechanisms.

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1. S. Komatsu, S. Kuribayashi, T. Sugimoto, K. Sugimoto, and K. Kamei: J. Jpn. Inst. Met., 1987, vol. 51, pp. 599-607 (in Japanese). 2. M. Qi, M. Zhu, and D.Z. Yang: Chin. J. Met. Sci. Technol., 1992, in press. 3. J.S. Lee and C.M. Wayman: Metallography, 1986, vol. 19, pp. 401-19. 4. L. Delaey, J. Janssen, D. Van de Mosselaer, G. Dullenkopf, and A. Deruyttere: Scripta Metall., 1978, vol. 12, pp. 373-76.

Fig. 4--Fatigue life (N;) in relation to maximum stress (o-) for alloys tested in pseudoelastic state.

that the dual-phase alloy with globular a phase demonstrates the highest strength both in the martensitic and pseudoelastic states, while that with network a phase gives out the lowest fracture stress. The strength of the singleand dual-phase alloy with blocky a phase is medium. For all alloys, the fracture stress in martensitic state is higher than that in the pseudoelastic state. The high tensile strength of dual-phase alloys with globular a phase is contributed to the grain refinement, as indicated in Figure 1. This reduction in grain size is due to suppression of grain boundary migration of the/3 phase by the presence of a phase during the solution treatment. As the a phase is a soft phase compared with the parent and