Mohajeri, Mahdi (2020-04). Degradation of Phase Transforming SMA in Presence of a Corrosive Environment and Thermomechanical Loads. Doctoral Dissertation. Thesis uri icon


  • Shape memory alloys (SMAs) are in class of active materials with the capability to undergo and recover large deformations upon subjecting to certain external stimuli, i.e. temperature and mechanical load. This capability has roots in the martensitic transformation, a solid-to-solid, diffusion-less phase transformation between austenite and martensite. Studying the failure mechanisms in SMAs is rather challenging because of the effect of phase transformation/detwinning and the inherent thermo-mechanical coupling, and there has been a recent interest in investigating the effect of aforementioned phenomena on failure of SMAs. Newer challenges in environmental conditions leads the effort for corrosion assessment and study the parameters influencing metallic degradation. The purpose of these studies was to provide a detailed understanding of Nitinol phases and their effects on corrosion behavior of NiTi SMAs. In this study, corrosion behavior of NiTi SMAs undergoing detwinning/reorientation, martensitic phase transformation, and in presence of cyclic loadings are investigated using mechano-electrochemical techniques. Uniaxial tensile tests were conducted to determine the mechanical properties such as elongation, ultimate tensile strength, modulus, transformation strain, and plateau stress. Electrochemical corrosion behavior of each phase was investigated using cyclic potentiodyamic polarization, electrochemical impedance spectroscopy, Electrochemical frequency modulation techniques. The mechano-electrochemical experimental techniques coupled with imaging and high-resolution surface analysis helped to quantify parameters describing the passive-active state properties. The relationship of constant load vs. interfacial characterization gave important insight for understanding SMA performance in a corrosive environment.

publication date

  • April 2020