NiTi合金コイルばねの耐久性に及ぼす記憶処理の影響

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In order to clarify the effects of shape memory treatment on durability of helical spring properties in NiTi alloy, spring force-temperature curves at constant strain were measured before and after thermal cycling test (between 0°C and 90°C) and isothermal retention test (at 120°C, 150°C and 180°C).The results were summarized as follows:(1) Spring force of the R phase (martensite) and transformation temperatures (Ms, Mf) linearly decreased with rise in shape memory treatment temperature before practice of the cycling test and the retention test.(2) The transformation temperatures were unaffected by the cycling test up to 2×104 times in spite of the shape memory treatment temperature and the loading strain (0.8 and 1.0%). Whereas, the spring force of the parent and R phase decreased with rise in thermal cycling, markedly in the case of higher shape memory temperature and larger loading strain.(3) Degradation of the spring force was saturated with thermal cycling of 1×104 times at 1.0% loading strain, but continued to decrease up to thermal cycling of 2×104 times at 0.8% loading strain.(4) Isothermal retention test at 0.6% loading strain leaded to increase in transformation temperatures and degradation of the spring force, effectively depending on retention temperature and time, with the exception that the transformation temperatures were unaffected only by retention temperature of 120°C.
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