Effects of Crystallographic Texture and Applied Strain Rate on the Cyclic Behavior of Nickel-Titanium.

Abstract

Shape memory alloys (SMAs) are utilized in a wide range of applications due to their unique characteristics, most notably the shape memory effect and superelasticity. In spite of intensive research, much is still unknown about the solid-to-solid, diffusionless phase transformation from a cubic austenite phase to a monoclinic martensite phase that is responsible for these properties, and the complex thermo-mechanical interactions that accompany this transformation. The aim of this research was to characterize this phase transformation during displacement-controlled cyclic loading in superelastic nickel-titanium (also known as Nitinol or NiTi), which is the most commonly utilized SMA, with a focus on the effect of globally applied strain rate and crystallographic texture. Experimental studies of thin sheet specimens of polycrystalline NiTi under uniaxial tensile loading were conducted using a combination of digital image correlation (strain fields) and infrared thermography (thermal fields). Specimens were prepared along directions oriented 0° (RD), 45°, and 90° (TD) to the rolling direction of the sheet and subjected to fifty cycles at prescribed three different strain rates. A strong cycle-to-cycle strain similarity was found in the martensite, indicating that local elastic stress fields are driven by a dislocation structure and martensitic nuclei that largely stabilize during the first loading cycle. This cyclic similarity increased when the crystallographic orientation of the test specimen was less favorable for phase transformation. It was also found that on loading, these unfavorably oriented specimens accommodated less axial strain inside the martensitic deformation band and more axial strain outside of the band. Unfavorable textures also resulted in the nucleation of more martensitic bands, and this became increasingly apparent at faster applied strain rates. At fast applied strain rate, the cyclic behavior exhibited significantly greater transformation homogeneity and accumulated more latent heat, which affected the macroscopic response between cycles. Thus, testing at fast strain rate was performed with a 1800 second hold between cycles in order to examine the effect of accumulated latent heat. Other parameters including the evolution of martensite volume fraction and velocity of the bands were examined with respect to strain rate and specimen crystallographic texture.