Martensite/R-phase Superelasticity and its Implications to Nitinol Durability

While it is presumed that superelastic medical devices require an A_f temperature below body temperature, such is not the case.  By utilizing very low temperature aging, the R-phase can be stabilized to temperatures at or above body temperature, resulting in superelastic devices that stress induce Martensite from the R-phase and then revert to the R-phase upon unloading.  By ridding the superelastic process of Austenite, one is left with a more compliant device, affording profound improvements in fatigue performance.  The process to achieve this novel condition will be presented, along with both the monotonic and cyclic properties that result.  These will be demonstrated in both wire (with rotating bending fatigue) and in finished devices (with so-called diamond sample fatigue).