The Effect of Texture On Martensitic Transformation in Nickel-Titanium

This talk presents an experimental study on the effect of texture on stress-induced martensitic phase transformation in Nickel-Titanium (Nitinol). Thin sheet specimens of Nitinol were examined under uniaxial tensile loading using simultaneous three-dimensional digital image correlation (3D-DIC) and infrared thermography in order to spatially and temporally track strain localization and released/absorbed latent heat resulting from transformation. After characterization of the sheet texture, tensile specimens were fabricated along directions oriented 0° (RD), 45°, and 90° (TD) to the rolling direction of the sheet, where RD and 45° specimens were found to have similarly favorable textures for transformation, and the TD specimen had a relatively unfavorable transformation texture. The effect of texture on the mesoscopic formation of martensite, and its relation to the globally observed stress-strain behavior, was examined during cycling tests of up to 50 cycles at prescribed strain rates of 10^-4, 10^-3, and 10^-2 s^-1. It was found that specimens less favorably oriented for transformation (TD specimens) exhibited a greater non-linearity in the mechanical response prior to the nucleation of a martensitic band, higher transformation onset stresses, smaller transformation strains, and an increased number of martensitic fronts than more favorably oriented specimens. Interestingly, the TD specimens also exhibited more cycle-to-cycle similarity in the local strains accommodated by the martensite. All of these differences became more pronounced at higher strain rates. The effect of specimen texture on the in-situ strains (indicative of the extent of martensitic transformation), and its connections to the globally observed stress-strain behavior, will be compared and discussed.