From Fort Wayne Metals, Inc.
Homogeneous 177-lm diameter nanocrystalline Nitinol wires were produced and compared to microcrystalline Nitinol from equivalent ingot stock. Property measurement was carried out using cyclic tension testing, strain-controlled fatigue testing and bend and stress-free recovery testing (BFR). A B2 cubic structure of 5- to 60-nm grain size was confirmed by transmission electron microscopy (TEM). The fatigue strain capability at 107 cycles is 30% greater in the nanocrystalline versus microcrystalline annealed wire. Grain size and residual strain are positively correlated in cyclic uniaxial tension testing within the superelastic temperature and strain regime. A transformation-induced loading plateau that is 40% longer than in standard superelastic polycrystalline wire is also reported for the first time.
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