PSI - Issue 66

A.R. Pelton et al. / Procedia Structural Integrity 66 (2024) 265–281 Pelton/ Structural Integrity Procedia 00 (2025) 000–000

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allowed phases, strains, and orientations to be determined as a function of deformation. High-resolution surface strain measurements were obtained by optical digital image correlation (DIC) as well as by scanning electron microscope DIC (SEM-DIC). Key findings from these investigations include demonstration of non-uniform strain and phase transformation distributions as a function of deformation mode and direction. We will demonstrate the effects of Nitinol texture with the work from (Barney, Xu et al. 2011) that characterized Nitinol tube sections that were deformed in situ with synchrotron µdiffraction along longitudinal, circumferential, and transverse orientations. Tube sections that were deformed along the drawing direction indicated a strong preference for <111>-oriented grains; in contrast, sections in the 45  direction had a strong preference for <100> orientation. There was a large variation in the superelastic response of Nitinol in these three tube directions that was strongly influenced by the path that the martensitic transformation follows through the microstructure. Specifically, in severely worked Nitinol, bands of <100> grains occur whose orientation deviates markedly from the surrounding matrix; these bands have an unusually large impact on the initiation and the propagation of martensite, and hence on the mechanical response. These <100> grains appear to resist transformation and bear progressively higher elastic stresses than neighboring grains during deformation. These effects are shown in Figure 3. In Figure 3a, the texture of Nitinol tubes along the drawing direction, <111>, compared with the 45  orientation, <100>, is shown. Figure 3b shows the uniaxial stress-displacement data of these two orientations along with the transformation pathways. The onset of the stress-induced transformation occurred at ~ 350MPa in the longitudinal direction and > 500MPa in the 45  direction. Furthermore, the Austenite-to-Martensite transformation progresses more uniformly along the longitudinal direction than in the 45  direction as shown in the accompanying phase maps.

Fig. 3: (a) The local texture variations versus the global averaged textures. Longitudinal orientations (0  ) have an averaged texture of <111>, whereas transverse (45  ) orientations have an averaged texture of <100>. (b) The strain evolution of 0 ◦ and 45 ◦ samples as a function of increasing deformation. Both orientations develop regions of transformed martensite (black), however the morphology of transformed region is different. 0  samples have flat, nearly uniform region, whereas 45  samples develop highly heterogeneous diagonal bands that cuts across the specimens. Furthermore, both sets of samples feature regions of austenite that are highly strained, beyond the typical 1.2% threshold for martensite transformation. After (Barney, Xu et al. 2011). A series of studies with optical DIC illustrated the non-uniform strain distribution and phase transformation pathways of superelastic Nitinol tubing under pure tension, pure compression, bending, and torsion (Reedlunn, Churchill et al. 2014, Reedlunn, LePage et al. 2020). Figure 4(a) shows the moment-curvature mechanical data as a function of deformation that is in accord with the known effect of a decrease in unloading plateau with increasing curvature (strain). Figure 4(b) shows the corresponding strain distribution of the tube at these five bending deformations. Note that under tension the strains, and likely the martensite formation, is very nonuniform on the tension side. In contrast, under compression, the strain distribution is much more uniform. It is also worth stating that the absolute magnitude of the strain in compression is less than that in tension; this is in line with the known tension-compression asymmetry in Nitinol (Gall, Sehitoglu et al. 1999, Reedlunn, Churchill et al. 2014, Bucsek, Paranjape et al. 2016).