M. Barney, Chevron Energy Technology Company, Richmond, CA; D. Xu, University of California, Berkeley, Berkeley, CA; S. W. Robertson, Nitinol Devices and Components, Fremont, CA; V. Schroeder, A. Pelton, Nitinol Devices & Components, Fremont, CA; R. O. Ritchie, University of California, Berkeley, CA; A. Mehta, Stanford Synchrotron Radiation Laboratory, Menlo Park, CA
Nitinol undergoes a reversible martensitic phase transformation under the application of stress, giving rise to the property of superelasticity useful in many biomedical applications. This effect is seen in the stress-strain curve as two linear elastic regions separated by a nearly flat “plateau” region, where the cubic austenite phase transforms to monoclinic martensite. However, the causes of the variation seen in the onset and degree of superelasticity are still poorly understood, making accurate prediction of the response deep in the superelastic region difficult. Herein, we show results of an in-situ microdiffraction study of tensile loaded “dogbones” cut with different orientations from flattened tubes. We find that the onset and degree of superelasticity changes dramatically as the load axis is rotated from the tube axis. Correlating bulk stress-strain measurements with local strain and orientation maps gives us insight into the local phenomena that are causing these variations. For example, we observe that the end of the plateau seldom signifies the completion of transformation, contrary to common belief. Though the transformation begins at one end of the gauge and progresses to the other, the transformation band is not contiguous and the degree of contiguity depends on misalignment. Upon transformation, stress redistributes such that the untransformed ‘islands’ within a transformed band bear a greater degree of load, seen mainly in the most misaligned samples. In conclusion, we find not only do the onset and degree of superelasticity depend strongly on the average crystallographic orientation (texture), but actual transformation pathways appear to be governed by the very local grain misorientations. Our results, therefore, suggest a fuller understanding of the superelasticity in Nitinol requires a complete investigation of the both the crystallographic and transformation anisotropy, as well as an understanding of the inter-grain interactions and the role they play in determining the transformation pathway.
