The effect of texture on stress-induced martensite formation in nickel-titanium

摘要

An experimental study was performed to investigate the effect of texture on stress-induced martensitic phase transformation in the shape memory alloy (SMA) nickel-titanium (Nitinol). Thin sheet specimens of Nitinol were examined under uniaxial tensile loading using three-dimensional digital image correlation in order to spatially and temporally track strain localization indicative of martensitic transformation. Tensile specimens were fabricated along directions oriented 0° (RD), 45°, and 90° (TD) to the rolling direction of the sheet and subjected to 50 cycles at prescribed strain rates of ε_g D 10~(-4); 10~(-3), and 10~(-2) s~(-1). It was found that upon loading, specimens unfavorably oriented for transformation (TD specimens) nucleated a greater number of deformation bands due to a smaller difference between nucleation and propagation stresses, and also accommodated less axial strain inside the band and more axial strain outside the band. The unfavorable (TD) specimens also exhibited a stronger cycle-to-cycle similarity in the strain accommodated inside the deformation band, which has important implications for the design of SMA structures for fatigue applications. Finally, the (primarily martensite) region of the deformation band(s) consistently showed significantly stronger cycle-to-cycle similarity than the (primarily austenite) region outside the band(s), regardless of specimen texture.