In this talk, phase transformation kinetics of shape memory alloy materials are studied by DSC tests and electrical resistance (ER) tests. Data on polycrystalline NiTi specimens shows that the phase transformation temperature from detwinned martensite to austenite is nearly 20°C higher than that from twinned martensite to austenite. The ER tests of SMA materials during heating and cooling under different loads further demonstrate that stress has a significant influence on the transformation of detwinned martensite but a small influence on twinned martensite. The linearity between ER and strain during the tests is not affected by the stress, which has positive implications for devices and control algorithms. A transformation phase diagram is revised to reflect the nature of the results, which thus includes two zones for inverse transformation. Since previous constitutive models for SMA behavior do not account for this effect, the implications for modeling and device design are discussed.
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