Improving Fatigue: Role of the R-Phase

In recent years, significant efforts have been devoted to improving the durability of NiTi for biomedical applications. Optimizing for fatigue properties is rather complex. There are various aspects that are key to fatigue life, such as: inclusion size, count, and distribution, grain size and orientation, transformation temperatures, and surface finish.

The medical device community often uses the Austenite finish temperature to tune for loading and unloading stiffness. Hence during cycling, the material goes through a dominantly first-order cubic (B2) to monoclinic (B19') martensitic transformation. This study focuses on the effect of thermomechanical treatments on fatigue behavior. Specifically, the role of R-phase. We demonstrate that by stabilizing the R-phase at body temperature and replacing the Austenite parent phase with the R-phase, fatigue life can be improved. We will show that when the duty cycle is predominantly between the R-phase and B19' martensite, a softening in cyclic modulus will occur. We will present fatigue results in strain–controlled loading conditions over a span of 2% -5% mean strain, relevant to physiological loading conditions. Also, the mechanism responsible for the increase in fatigue life will be discussed.