The influence of crystallographic texture on fracture of the shape-memory alloys

Shape-memory alloys (SMAs) have been the materials of choice for decades in applications that include blood vessel stents, root canal files, and Joule-heated actuator wires. Although SMAs exhibit superior elastic recovery and extended component life over typical metals, fracture and failure still occur in SMA devices such as cardiovascular stents. As SMAs gain new applications, such as adaptive aerospace and automotive components and structural vibration damping cables, the understanding of SMA fracture becomes increasingly relevant. In this study, the effect of strain rate on martensitic phase transformation and crack propagation in the superelastic SMA NiTi is examined during fracture. Expanding upon earlier pilot experiments at quasi-static strain rates, this study presents new results with consideration to grain sizes and crystallographic texture. The complex thermo-mechanical interactions of the local phase transformation induced by high stress at the crack tip, and its impact on the observed microscopic and macroscopic fracture behavior, are discussed. Prior experimental study by others has explored the gross macroscopic behavior of SMAs during fracture, but the adaptive characteristics of SMAs necessitate an area-specific experimental technique to clarify crack tip behavior. For example, the macroscopic response of NiTi across a wide range of temperatures with both quasi-static and dynamic strain rates was explored in 2006 [Adharapurapu, Jiang, Vecchio, and Gray, 2006] and with specific focus on fracture toughness across strain rates in 2007 [Jiang and Vecchio, 2007]. It was later noted that the grossly averaged fracture characteristics and fracture surfaces of SMAs are similar to that of typical ductile metals, but that on the local scale the crack growth rate and presence of phase transformations distinguish SMAs from typical metals [Robertson, Pelton, and Ritchie, 2012]. Because specific areas of phase transformation cannot be discerned with traditional load cells and extensometers, a combination of high-speed digital image correlation and infrared thermography was used to investigate the local thermo-mechanical interactions that arise from stress-induced martensitic phase transformation during fracture. Further metallography with electron backscatter detection grain mapping was performe