Inclusions and surface defects are the primary drivers of fatigue fractures for Nitinol. Surface defects can take the form of die marks, surface roughness, or scratches, and when present, these will typically be the sites for fatigue fractures. When such surface defects are not present, however, surface-exposed inclusions become the primary sites of fatigue fractures. Surface-exposed inclusions are either fully embedded (no surrounding voids) or particle-void assemblies (PVAs). PVAs are up to 600 times more likely to be the sites of crack nucleation compared to fully embedded particles, so they are of primary interest. PVAs typically take the form of a single, small, and unbroken inclusion with one or two associated voids (coined a "teardrop"), or a longer, drawn-out, and broken-up inclusion with many voids (coined a "stringer").

Failure in device-grade NiTi is often due to nonmetallic inclusions in the NiTi, along with phase transformation in the surrounding NiTi. While progress has been made through metallurgy for high-purity NiTi, along with fatigue life experiments (e.g., total life with respect to mean strain), there remains a lack of understanding about how the fatigue life of NiTi devices is determined in the presence of inclusion defects. To better understand inclusions, this work is probing the initiation and growth of fatigue cracks in and around particle- void assemblies. The approach utilizes a combination of fatigue testing, scanning electron microscopy (SEM), both optical and SEM digital image correlation (DIC), and focused ion beam milling (FIB). Initial results have revealed that cracks tend to form first around smaller "teardrop" inclusions instead of larger "stringer" inclusions. The crack-forming inclusions tend to have nonmetallic inclusions that remain intact. This is in contrast with the larger, "stringer" inclusion that has nonmetallic particles that are not intact. Additional work is being done to better understand these findings. Additionally, ongoing studies with energy-dispersive X- ray spectroscopy (EDX) are identifying the composition of the various inclusion types.