The Competition between R and M Martensites in Nitinol Medical Devices

The austenite phase in Nitinol becomes unstable when cooled and seeks to find a successor structure of lesser entropy.   There are two candidate successors, a monoclinic martensite (M), and a rhombohedrally distorted martensite (R) often referred to as the R-phase.  While both are thermoelastic, the stability of R is far less dependent upon stress than is M.  Thus in medical devices it is common to observe both M and R during thermal cycling (in DSC or BFR tests, for example) yet only observe M in isothermal environments, such as tensile testing or in body temperature service.   Critical to the performance of many Nitinol medical implants is the lower plateau stress at body temperature, caused by the reversion of M.  Yet for many medical devices, the A_f temperature, as defined by ASTM and others, is the temperature at which R reverts to austenite upon heating, not the temperature at which M reverts. 

This paper will highlight several aspects of the competition between R and M from the perspective of medical device performance, clarifying why the temperature at which M reverts is important rather than the A_f temperature.  The impact of this on how materials should be specified will be discussed.