Microscopic strain and traction-separation around particle void assemblies in Nitinol

The micromechanical understanding of particle void assemblies (PVAs) is limited, largely due to their small size (less than 10 μm). To improve our knowledge of the accumulation of damage around these small PVAs, this work utilizes in-SEM testing and scanning electron microscope digital image correlation (SEM-DIC) to capture in-situ strain maps by applying fatigue cycles via uniaxial, constant amplitude testing to produce crack initiation, propagation, and, ultimately, failure at small PVAs present in high-purity Nitinol dogbones. Starting at zero cycles and continuing at pre-determined cycle intervals until sample failure has occurred, SEM images of crack initiation and propagation are captured to determine crack growth rate, with in-SEM fatigue rates of 30+ Hz. SEM-DIC strain maps track the crack path. When matched with backscatter images, SEM-DIC also indicates if cracks propagate around or away from inclusions. Furthermore, crack tip opening displacements and traction-separation curves at the inclusion/matrix interface are quantified from SEM-DIC maps to compare the separation and adhesion of the PVAs from the surrounding NiTi matrix. After fatigue cracking, the 3D shape and elemental composition of the inclusions are gathered with focused ion beam (FIB) milling, fractography, and SEM-EDS. These results provide new insights into the micromechanical behavior of PVAs in NiTi and may lead to improvements in defect and damage modeling for finite element analysis and cohesive zone modeling of Nitinol durability.