Abstract
It has been previously suggested that the fatigue lifetime of superelastic Ni–Ti might be improved if the R-phase were the parent to martensite rather than austenite. This body of work tests that hypothesis in two separate side-by-side fatigue tests both carefully constructed to match the superelastic properties in the two study arms. Both experiments show the R-phase parent to be more durable than the more commonly considered austenitic parent phase. The first experiment considers straight wire specimens fabricated from standard purity material, in a tension–tension fatigue test to 107 cycles, at mean strain ranging of 0.5–5.8% and strain amplitudes of 0.15–0.45%. The second experiment considers formed wire specimens in bending fatigue, more representative of realistic medical components, with a maximum mean strain of 1.2%, and maximum strain amplitudes ranging from 0.72 to 1.64%. Compared with the austenitic parent material, the R-phase material tolerated 0.1–0.3% higher strain amplitudes.
Highlights
Nickel titanium (Ni–Ti, or nitinol) has become the material of choice for numerous medical applications, including peripheral vascular and venous stents, endovascular aneurysm repair grafts, heart valve frames, and filters [1, 2]
These results confirm that the upper plateau stress (UPS) and lower plateau stress (LPS) are equivalent for both groups, at 500 MPa and 300 MPa, respectively
With a duty cycle operating between LPS and the UPS, the AMA material cycles between A and M, while the RMR material cycles strictly between R and M with no appearance of A
Summary
Nickel titanium (Ni–Ti, or nitinol) has become the material of choice for numerous medical applications, including peripheral vascular and venous stents, endovascular aneurysm repair grafts, heart valve frames, and filters [1, 2]. Superelastic recovery strains of over 10% can be realized by the reversion of stress-induced monoclinic B19’ martensite (M) to a simple cubic B2 austenite parent phase (A). The R-phase is trigonal, intermediate to A and R in terms of entropy, and is very similar to A from a transformational strain perspective. This three-phase competition is akin to the vapor–liquid–ice system of water in that the three phases create a triple point in stress (or pressure)–temperature space. The kinetic barriers between the A and R-phase are far smaller than between either A and M or between R and M. This, in turn, creates an asymmetry, making the R-phase far more prevalent in cooling than heating
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
