Nitinol, a nearly equiatomic alloy of nickel-titanium, can "remember" a previous shape and can recover strains as high as 10% by deformation (superelasticity) or temperature change (shape memory). These properties result from a reversible first-order phase transition between austenite (cubic, B2) and martensite (monoclinic, B19'). As such, deformation mechanisms of Nitinol are more complex than the conventional modes of plastic deformation in traditional alloys. Consequently, the mechanical behavior of Nitinol under multiaxial conditions remains poorly understood.

Nevertheless, because of these unique mechanical characteristics, in combination with excellent biocompatibility, Nitinol is used as self-expanding endovascular stents to scaffold diseased peripheral arteries. First-generation Nitinol stents were designed to provide sufficient scaffolding forces to hold open vessels, yet provide enough elasticity to "breathe" with pulsatile pressure differentials from the cardiac cycle. A variety of clinical studies indicate that these stents perform this primary function quite well. More recent in-depth studies, however, reveal that superficial femoral arteries (SFAs) are subjected to complex in vivo multiaxial deformation with up to 60% rotation and ~20% contraction in the SFA as the leg is bent from an extended position. Correspondingly, during a walking cycle, a stent deployed in the SFA undergoes severe multiaxial displacements from pulsatile motion (~4x10⁷ cycles annually) plus bending, torsion, and axial motions (at a rate of ~1x10⁶ cycles annually). Although there are ~40 times more cardiac displacement cycles, the combined non-pulsatile motions result in far greater cyclic strain magnitudes, and therefore, have the possibility of inducing greater fatigue damage, and sometimes fracture.

We investigated deformation of a stent-like component under moderate to high deformation conditions in an in situ apparatus at ALS BL 7.3.3, using a 1x1 micron white x-ray beam. The sample (the whole loading apparatus) was rastered to cover a 400x600 microns region on a 7x7 micron grid. A white beam Laue diffraction pattern was collected at each point on the grid. We were able to index the Laue pattern to obtain the all the 6 elements of the 2nd rank deviatoric strain tensor. Local strain maps thus obtained at several deformation levels were compared with a strain map obtained from the state-of-art finite element (FE) model used for Nitinol stent design.

Besides local granularities the strain maps from microdiffraction agreed well with FE model at low deformations, but difference between them increased dramatically at high deformation values. For example, contrary to the model predictions, there remains a spine of retained austenite in the middle of the strut even at the highest deformation studied here. Accordingly, it is observed that the transformation front moves down the length of the strut rather than moving towards the middle (3 - 5 mm).

The experimental observation of redirection of the transformation front has very important implications for strut fractures in Nitinol stents. At relatively low displacements, if the local strain field encounters a sufficiently large flaw, the defect will induce the strain field to grow such that the material may fracture at the location predicted by the FE model. Therefore, in this regime fracture of Nitinol stents is very well predicted by the current models. However, in the absence of flaws near the initial high-strain region predicted by FE model, the transformation front, which has now changed direction, will begin to propagate down the length of the strut. As the propagating strain front runs down the length of the strut, fracture may occur at locations significantly different from those suggested by the finite-element model, making fracture of SFA stents under non-pulsatile overload conditions unpredictable.

These results show that a much better understanding of crack propagation and modes by which superelastic Nitinol accommodated high deformation is needed. We are in process of publishing a series of investigations which sheds some light on these fundamental deformation and fracture processes in superelastic materials.

Primary Citation
A. Mehta, X.-Y. Gong, V. Imbeni, A. R. Pelton and R. O. Ritchie, "Understanding the Deformation and Fracture of Nitinol Endovascular Stents Using In Situ Synchrotron X-ray Microdiffraction", Adv. Mater. 19, 1183 (2007)