In-situ strain- and temperature-control X-ray micro-diffraction analysis of nickel-titanium knitted architectures

The reversible phase transformation between B2 austenite phase and B19' martensite phase is the governing mechanism behind the exciting active and passive capabilities of Nickel-Titanium (NiTi) knitted architectures. NiTi knitted architectures are manufactured from a monofilament fiber of originally-straight NiTi wire that is bent into an interlocking network of adjacent loops. Depending on the thermo-mechanical load path, NiTi knitted architectures can provide excellent isothermal energy-absorption using the superelastic effect (SE) or function as large-deformation actuators that respond to thermal inputs using the shape memory effect (SME). The magnitude of NiTi knitted architecture characteristic performance metrics is dependent on the ability of the knitted architecture to undergo the reversible phase transformation, which is a function of the material stresses, strains, and temperature. This research quantifies the NiTi knitted architecture austenite phase fraction of the highly stressed filament surface in X-ray diffraction experiments as a function of the measurement position on the knitted loop and applied thermo-mechanical loading conditions. A Bruker D8 Discover 2D micro-diffractometer was equipped with a custom tensile straining- and temperature-control device. The Direct Comparison Method was employed to derive the austenite phase fraction from the X-ray diffraction patterns. This research establishes novel in-situ strain- and temperature-control X-ray micro-diffraction experiments and provides understanding of the governing deformation modes in NiTi knitted architectures.