October_2021_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 1 4 7 FEATURE In simple terms, the shape memory effect is exhibited when a phase transformation to martensite is thermally in- duced, deformed while in the martensitic state, and is then heated to revert it to its parent phase and shape. Superelas- ticity refers to the reversion of martensite to its parent phase during the release of the deforming stress (without heating). The occurrence of these phenomena is dictated by the tem- peratures at which the phases in the material are stable, as both thermal and strain energy play a critical role in stabiliz- ing martensite. Fatigue fracture should always be considered as a pos- sible failure mechanism for Nitinol implants due to the cyclic nature of physiological loading. Nitinol fatigue fracture sur- faces generally exhibit similar features to most engineering alloys, including a flat fracture plane, ratchet marks, radial lines, beachmarks, and striations when possible. Specifical- ly, Nitinol fatigue fracture surfaces often exhibit radial lines and “feathering” more frequently than other engineering alloys. Also, due to Nitinol’s tension-compression asym- metry, Nitinol can experience fatigue crack initiation and arrest from residual tensile stresses because of high device crimp strains. FATIGUE FRACTURE AND STRIATIONS IN NITINOL A hallmark of fatigue in Nitinol, as with some other metals, is the presence of striations on the fracture surface. Fatigue striations are produced as a result of incremental sharp crack advancement and subsequent blunting. For a typical work-hardening metal, the initially sharp crack tip is blunted by the plastic zone ahead of it, but the crack tip will sharpen again during subsequent compression. In the case of Nitinol, the region ahead of the advancing crack tip con- tains deformed martensite, which also serves to repeatedly blunt the crack tip during each cycle [1] . Fatigue striations are often absent on Nitinol fatigue fracture surfaces due to a few factors. First, striation spac- ing may be too small to be resolved using common scanning electronmicroscopes due to small cyclic crack advancement during high-cycle fatigue, especially near crack initiation [1] . Second, fatigue striations may be worn away due to repeat- ed crack closure during cycling. Striations in Nitinol fatigue fractures are commonly observed late in the crack growth process. FRACTURES INITIATED BY NEAR-SURFACE NONMETALLIC INCLUSION Nitinol fatigue fractures often originate from near-sur- face nonmetallic inclusions [2-4] . These nonmetallic inclusions are natural byproducts of the physical melting process [5] and are further fractured and distributed by forging and draw- ing operations. Nevertheless, these nonmetallic inclusions, when co-located with critical high cyclic strains, can act as crack-initiating features despite being well within the spec- ifications of ASTM F2063 [6] . The existence of an inclusion at the fracture origin of a Nitinol device does not necessarily indicate that the inclusion was a defect (or that the device was defective). Any current Nitinol device will likely contain millions of inclusions. The chance of an inclusion, no matter how small, being near the critical cyclic strains (particularly in the absence of another, larger stress-concentrating fea- ture) is relatively high. Figures 1 through 3 are a collection of BSE micrographs of a Nitinol wire fatigue fracture surface that included a near-surface nonmetallic inclusion. The bent wire-formcom- ponent was being fatigue tested in a phosphate-buffered sa- line (PBS) bath at 37°C and 60 Hz. The sample was cycled at 1% mean strain and 0.45% strain amplitude (on the upper end of the elastic portion of austenite, just into the stress- induced martensite plateau). The strains for the fatigue test were determined by finite element analysis. The component fractured after approximately 15.6 million cycles and was re- moved from the test for investigation. From inspection of the fracture surface in Fig. 1, radial lines point to an origin at the bottomof the fracture surface. Higher magnification of these 1 1 Fig. 3 — SEM micrographs detailing the fast fracture overload area of the fractured wire-form component fromFig. 1. Microvoid coales- cence is observed on the fracture surface in this area.