Microstructural analyses of thermal or mechanical fatigued Nitinol show remarkable similarities and are characterized by an increase in dislocation density with increasing number of cycles. Dislocation bands, which are thought to be due to the effects of moving martensite interfaces, align with the martensite lattice invariant plane. These microstructural effects result in modification of transformation temperatures, strain (under stress-control) and stress (under strain-control). Processing has a major effect on fatigue properties, whereby optimized thermomechanically treated microstructures provide more stable (and predictable) behavior than the annealed microstructures.
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